cassiterite, topaz, apatite and minor amounts of Fe, Cu, Mo and Bi sulphides (Hall,. 1971; 1974; Alderton ...... with this phase have very different signatures to those responsible for earlier phases of ...... Digital photograph by Charles Winpenny.
The Tectonics of Variscan Magmatism and Mineralisation in South West England Volume II of II
Nicholas Gerald LeBoutillier
Submitted by Nicholas Gerald LeBoutillier to the University of Exeter as a thesis for the degree of Doctor of Philosophy in Mining Geology, September 2002. This thesis is available for Library use on the understanding that it is copyright material and that no quotation from the thesis may be published without proper acknowledgement. I certify that all material in this thesis which is not my own work has been identified and that no material is included for which a degree has been previously conferred upon me.
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Contents. Volume II Title Page…………………………………………………………………….….Page 321 Contents………………………………………………………………………....Page 322 List of Figures in text………………………………………………..…………..Page 326 List of Plates in text……………………………………………………………..Page 330 List of Tables in text…………………………………………………………….Page 333 Chapter 6 – The Cornubian Ore Province – a review……………………….Page 334 6.1 Introduction………………………………………………………………….Page 334 6.2 History of research…………………………………………………………..Page 338 6.3 Overview of mineralisation…………………………………………………Page 343 6.3.1 Introduction……………………………………………………..……..Page 343 6.3.2 Pre-granite mineralisation……………………………………….…….Page343 6.3.3 Skarns and pegmatites……………………………………………..….Page 345 6.3.4 Greisens, sheeted vein complexes and breccia pipes……………...…..Page 350 6.3.5 Main-stage lode mineralisation……………………………….……….Page 355 6.3.6 Crosscourse mineralisation……………………………………………Page 365 6.3.7 The nature of the mineralising fluids……………………………...…..Page 368 6.4 Lode formation and development………………………………………...…Page 373 6.4.1 Lode geometry………………………………………………………...Page 373 6.4.2 Lode development……………………………………………………..Page 376 6.4.3 Structural controls on mineralisation………………………………….Page 385
Chapter 7 - The Tectonics and Paragenesis of Mineralisation in SW Cornwall…………………………………………………………………...Page 395 7.1 Introduction………………………………………………………………….Page 395 7.2 The Land’s End Granite………………………………………………….….Page 396 7.2.1 Introduction……………………………………………………………Page 396 7.2.2 The Towednack District………………………………………...……..Page 397 7.2.2.1 Paragenesis: Carnelloe Mine……………………………………Page 399 7.2.2.2 Paragenesis: Zennor Head Mine………………………….……..Page 406 7.2.2.3 Rosewall Hill to Wicca Pool…………………………………….Page 408 7.2.3 The Trencrom District………………………………………………...Page 410 7.2.3.1 Trencrom Hill……………………………………………….…..Page 411 322
7.2.3.2 Castle An Dinas Quarry………………………………….…..….Page 411 7.2.3.3 Paragenesis: Giew Mine………………………………...………Page 415 7.2.4 The Penzance-Ludgvan District………………………………….……Page 418 7.2.5 The Lamorna District………………………………………………….Page 420 7.2.5.1 Mousehole……………………………………………………….Page 421 7.2.5.2 Lamorna Point…………………………………………..……….Page 423 7.2.5.3 Carn Barges………………………………………………….…..Page 426 7.2.5.4 Tater Du………………………………………………………....Page 426 7.2.5.5 St Loy’s Cove……………………………………………..…….Page 427 7.2.5.6 Porthguarnon…………………………………………………….Page 428 7.2.6 The St Levan District…………………………………………...……..Page 428 7.2.6.1 Penberth Cove…………………………………………...………Page 429 7.2.6.2 Treen Head………………………………………………………Page 430 7.2.6.3 Porth Curno………………………………………………...……Page 432 7.2.6.4 Porth Chapel……………………………………………….……Page 434 7.2.6.5 Porthgwarra……………………………………………..………Page 435 7.2.6.6 Paragenesis: Nanjizal Mine.…………………………….………Page 436 7.2.7 The Morvah-Sancreed District.………………………………….……Page 453 7.2.7.1 Gurnards Head.……………………………………………….…Page 454 7.2.7.2 Carn Galver.………………………………………………..……Page 456 7.2.7.3 Watch Croft……….…………………………………..…………Page 457 7.2.7.4 Portheras Cove………..……………………………….…………Page458 7.2.7.5 Land’s End and Mayon Cliff…………..………………..………Page 459 7.2.7.6 Sennen Cove………………..……………………………...……Page 461 7.2.8 The St Just and Pendeen District………..…………………….………Page 461 7.2.8.1 Nanven…………………………..………………………………Page 463 7.2.8.2 Cape Cornwall…………………………….………….…………Page 468 7.2.8.3 Botallack Mine………………….……………………….………Page 473 7.2.8.4. Geevor Mine………………………..………………..…………Page 475 7.3 St Michael’s Mount…………………………………………………………Page 481 7.3.1 Introduction……………………….………………………………..…Page 481 7.3.2 The sheeted vein system……………….……………………...………Page 482 7.4 The Tregonning-Godolphin Granite………………….………………..……Page 484 7.4.1 Introduction…………………………..……………………………..…Page 484 7.4.2 The St Erth District……………..…………………………………..…Page 485 323
7.4.3 The Gwinear District…………………..……………………...………Page 486 7.4.3 The Leedstown District……………….………………………………Page 487 7.4.5 The St Hilary District………………………………………………….Page 488 7.4.5.1 Perranuthnoe and Trevean Cove……………………………...…Page 490 7.4.5.2 Kenneggy Sand……………………………………………….…Page 491 7.4.6 The Breage District……………………………………………………Page 492 7.4.6.1 Lesceave Rocks………….………………………………………Page 493 7.4.6.2 Rinsey Cove………….……………………………………….…Page 495 7.4.6.3 Megiliggar Rocks…………………………………..……………Page 495 7.4.6.4 Paragenesis: Trewavas Mine……………………....……………Page 496 7.5 The Carnmenellis Granite………………..…………………….……………Page 500 7.5.1 Introduction……………………………………………………………Page 500 7.5.2 The Falmouth District……………..………………………………..…Page 501 7.5.3 The Carnmenellis District………………………………….…………Page 502 7.5.3.1 Paragenesis: Wheal Roots……………………………………….Page 503 7.5.4 The Troon District………………………………………………….....Page 509 7.5.4.1 Paragenesis: Great Condurrow Mine (1)………………………..Page 510 7.5.4.2 Paragenesis: Great Condurrow Mine (2)………….…...………..Page 513 7.5.4.3 Paragenesis: South Tolcarne Mine………………………..…….Page 514 7.5.4.4 Holman’s Test Mine………………………………………….…Page 516 7.5.5 The Tuckingmill District……………………………………………...Page 519 7.5.5.1 Paragenesis: North Pool Mine…………………………….…….Page 521 7.5.6 The Carn Brea District…………………………………………….…..Page 525 7.5.6.1 Paragenesis: South Crofty Mine…………………………...……Page 526 7.5.6.2 Paragenesis: New Cook’s Kitchen Mine………………………..Page 537 7.5.7 The Gwennap District……………………………………………..…..Page 563 7.5.7.1 Paragenesis: Wheal Gorland…………………………...………..Page 564 7.5.8 The Scorrier District………………………………………….……….Page 572 7.6 Further paragenetic data………………………………………………….….Page 573 7.6.1 Introduction………………………………………………………...….Page 573 7.6.2 Craddock Moor Mine………………………………………….…..Page 573 7.6.3 South Caradon Mine……………………………………..………..Page 575 7.6.4 South Terras Mine……………………………………..………….Page 579 7.7 Synthesis……………………………………………………………….……Page 582 7.7.1 The mineralisation of the Land’s End Granite……………...…………Page 582 324
7.7.2 The mineralisation of The Tregonning-Godolphin Granite……….…..Page 591 7.7.3 The mineralisation of the Carnmenellis Granite………………………Page 594 7.7.4 Conclusions………………………………………………………...….Page 597 Chapter 8 – Conclusions……………………………………………………….Page 601 8.1 Granites and mineralisation……………………………………………..…..Page 601 8.2. Suggestions for further work……………………………………………….Page 606 References………………………………………………………………………Page 608 Appendix (mineralisation data sheets), comprising Pages 640 – 712. These pages are present, in Microsoft Word format, on the CD on the rear cover of Volume II.
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List of Figures in text. Volume II CHAPTER 6. Figure 6.1. A map of the orefield of South-west England…………………………………………….p.336 Figure 6.2. A comparison of the zonation models of Davison and Dines…………………………….p.339 Figure 6.3. The pattern of mineral zoning in the St Agnes District……………………………….…..p.340 Figure 6.4. Major types of primary tin deposits in Cornwall……………………………………….....p.354 Figure 6.5. A section through a typical Sn-Cu lode……………………………………………..…….p.356 Figure 6.6. Section through New Cook’s Kitchen Shaft, South Crofty Mine………………………...p.375 Figure 6.7. The effect of fault movements on fracture width……………………………………..…..p.376 Figure 6.8. Moore’s fracture geometry models………………………………………………………..p.377 Figure 6.9. The evolution of vapour pressure relative to confining pressure and declining temperature in a crystallising volatile-rich granite…………………………………………………….p.379 Figure 6.10. The formation of parallel greisen veins………………………………………………….p.380 Figure 6.11. The effect of increasing fluid pressure on the state of compressive stress in a rock…….p.381 Figure 6.12. The formation of extensional fractures……………………………………………….….p.382 Figure 6.13. The formation of extensional hydraulic fractures by autogenous pulsation………...…...p.383 Figure 6.14. The formation of hydrothermal breccias by explosive decompression……………….....p.384 Figure 6.15. The relationship between the granite/killas contact and the economic tin zone at Geevor Mine, Pendeen………………………………………………………………………...p.386 Figure 6.16. Lithological control on mineralisation at the Chiverton Mines………………………….p.387 Figure 6.17 Relationships between main-stage lodes and elvan dykes……………………………….p.388 Figure 6.18. A section through the B Lode of Wheal Jane Mine…………………………………..…p.389 Figure 6.19. The wrench fault-bound lode system of Wheal Vor………………………………….…p.390 Figure 6.20 The influence of crosscourse fracture sets on the formation of the main-stage lodes at Geevor Mine………………………………………………………………….p.391 Figure 6.21. A long section through South Crofty Mine……………………………………………...p.392 Figure 6.22. A plan of the 400 fathom level, South Crofty Mine……………………………………..p.393
CHAPTER 7. Figure 7.1. The mineralised districts of the Land’s End Peninsula……………………………...……p.396 Figure 7.2. Stereodata and rose diagram for all lodes and minor veins in the Towednack District.….p.398 Figure 7.3. A sketch map of mine setts and lodes on the coast around Zennor…………………….....p.399 Figure 7.4. A rose diagram of vein and lode orientations at Carnelloe Mine………………………....p.400 Figure 7.5. A rose diagram of schorl vein orientations at Wicca Pool and Tregerthen Cliff………....p.409 Figure 7.6. Stereodata and rose diagram for all lodes and minor veins in the Trencrom District…….p.410 Figure 7.7. A rose diagram of vein orientations on Trencrom Hill……………………………….…..p.411 Figure 7.8. The location of Castle An Dinas Quarry………………………………………...………..p.412 Figure 7.9. A rose diagram of vein orientations at Castle An Dinas Quarry………………………….p.412
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Figure 7.10. Stereodata and rose diagram for all lodes in the Penzance-Ludgvan District………..….p.419 Figure 7.11. A sketch map of the southern part of the Penwith Peninsula…………………………....p.420 Figure 7.12. A contoured pole plot and rose diagram for all veins in the Lamorna District………….p.420 Figure 7.13. A rose diagram of vein orientations at Mousehole……………………………………....p.421 Figure 7.14. Rose diagrams of vein orientations around Lamorna Point……………………………..p.424 Figure 7.15. A rose diagram of vein orientations at Carn Barges………………………………….…p.426 Figure 7.16. A rose diagram of vein orientations at Tater Du……………………………………...…p.427 Figure 7.17. A rose diagram of vein orientations at St Loy’s Cove……………………………..……p.427 Figure 7.18. A rose diagram of vein orientations at Porthguarnon………………………………...….p.428 Figure 7.19. A contoured pole plot and rose diagram for all lodes and veins in the St Levan District…………………………………………………………………………………….....p.429 Figure 7.20. A rose diagram of vein orientations at Penberth Cove………………………………..…p.429 Figure 7.21. A rose diagram of vein orientations at Treen Cliff and Treen Head………………….…p.430 Figure 7.22. A rose diagram of vein orientations at Porth Curno……………………………………..p.432 Figure 7.23. A rose diagram of vein orientations at Porth Chapel…………………………………….p.434 Figure 7.24. A rose diagram of vein orientations at Porthgwarra………………………………….….p.435 Figure 7.25. A sketch map of Nanjizal, showing the location of the mine workings………………....p.436 Figure 7.26. Rose diagrams of schorl vein orientations at Nanjizal……………………………..……p.438 Figure 7.27. A rose diagram of quartz/haematite vein orientations at Nanjizal………………..……..p.439 Figure 7.28. A contoured pole plot and rose diagram for all lodes and veins in the Morvah-Sancreed District……………………………………………………………………………...p.453 Figure 7.29. A rose diagram of lodes in the Morvah District with all minor veins removed…………p.454 Figure 7.30. A geological sketch map of the area around Gurnards Head……………………………p.455 Figure 7.31. A rose diagram of vein orientations around Gurnards Head…………………………….p.455 Figure 7.32. A rose diagram of vein orientations at Carn Galver………………………………….….p.457 Figure 7.33. A rose diagram of vein orientations at Watch Croft……………………………….…….p.458 Figure 7.34. A rose diagram of vein orientations at Portheras Cove……………………………….…p.459 Figure 7.35. A rose diagram of vein orientations at Land’s End………………………………...……p.459 Figure 7.36. A rose diagram of vein orientations at Sennen Cove……………………………..……..p.461 Figure 7.37. A contoured pole plot and rose diagram for all lodes and veins in the St Just & Pendeen District……………………………………………………………………………..p.462 Figure 7.38. A rose diagram of lodes in the St Just & Pendeen District, with all minor veins removed………………………………………………………………………….……p.462 Figure 7.39. A rose diagram of lode and vein orientations at Nanven…………………………..……p.463 Figure 7.40. A rose diagram of lode and vein orientations around Cape Cornwall…………………..p.469 Figure 7.41. A section along Saveall’s Lode, St Just United Mine…………………………………...p.470 Figure 7.42. The workings of the mines of the St Just-Pendeen District………………………….….p.476 Figure 7.43. A section along the Pendeen coast, showing the position of the economic tin zone……………………………………………………………………………………...p.478 Figure 7.44. Typical sections through North Lode of Geevor Mine………………………….………p.480 Figure 7.45. A contoured pole plot and rose diagram for greisen and quartz veins on St Michael’s Mount…………………………………………………………………………………….p.482
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Figure 7.46. The mineralised districts of the Mounts Bay/Hayle Bay area……………………...……p.484 Figure 7.47. A contoured pole plot and rose diagram for all lodes in the St Erth District…………....p.485 Figure 7.48. A contoured pole plot and rose diagram for all lodes in the Gwinear District………….p.486 Figure 7.49. A contoured pole plot and rose diagram for all lodes in the Leedstown District……….p.488 Figure 7.50. A contoured pole plot and rose diagram for all lodes and veins in the St Hilary District……………………………………………………………………………………....p.489 Figure 7.51. A rose diagram of lodes in the St Hilary District, with all minor veins removed……....p.489 Figure 7.52. The geology of Mounts Bay……………………………………………………….……p.490 Figure 7.53. A rose diagram of vein orientations between Perranuthnoe and Trevean Cove………..p.491 Figure 7.54. A rose diagram of vein orientations at Kenneggy Sand………………………………...p.491 Figure 7.55. A contoured pole plot and rose diagram for all lodes and veins in the Breage District…………………………………………………………………………………….…..p.493 Figure 7.56. A rose diagram of lodes in the Breage District, with all minor veins removed……..…..p.493 Figure 7.57. A rose diagram of vein orientations at Lesceave Rocks…………………………….…..p.494 Figure 7.58. A rose diagram of vein orientations around Rinsey Cove……………………….....……p.495 Figure 7.59. A rose diagram of vein orientations at Megiliggar Rocks……………………….…..…..p.496 Figure 7.60. The mineralised districts of the Carnmenellis area………………………………..…….p.500 Figure 7.61. A contoured pole plot and rose diagram for all lodes in the Falmouth District……........p.501 Figure 7.62. A contoured pole plot and rose diagram for all lodes and veins in the Carnmenellis District…………………………………………………………………………….….....p.502 Figure 7.63. A sketch map and plan of the location and workings of Wheal Roots………..……....…p.503 Figure 7.64. A rose diagram of lode and vein orientations at Wheal Roots…………………….….....p.504 Figure 7.65. A contoured pole plot and rose diagram for all lodes and veins in the Troon-Lanner District………………………………………………………………………………….p.509 Figure 7.66. A rose diagram of lode and vein orientations at Great Condurrow Mine ……………....p.510 Figure 7.67. A rose diagram of vein orientations at Holman’s Test Mine…………………………....p.517 Figure 7.68. A contoured pole plot and rose diagram for all lodes in the Tuckingmill District……...p.520 Figure 7.69. A contoured pole plot and rose diagram for all lodes in the Carn Brea District………...p.525 Figure 7.70. A geological sketch map showing the location of South Crofty Mine…………………..p.526 T
Figure 7.71. A plan of the 340 fm level of South Crofty Mine, showing the major lodes worked……………………………………………………………………....p.527 Figure 7.72. The paragenetic sequence seen in the lodes of the deeper workings of South Crofty Mine………………………………………………...…………..p.529 Figure 7.73. A sketch plan of the workings on North Tincroft Lode…………………………….……p.538 Figure 7.74. A section through North Tincroft Lode…………………………………………….……p.539 Figure 7.75. The development of the North Tincroft Lode……………………………………..…….p.542 Figure 7.76. The paragenesis of phase 2 mineralisation in the North Tincroft Lode…………………p.551 Figure 7.77. The paragenesis of phase 3 mineralisation in the North Tincroft Lode…………………p.558 Figure 7.78. A contoured pole plot and rose diagram for all lodes in the Gwennap District…………p.564 Figure 7.79. A survey plan of the adit level workings of Wheal Gorland………………………….…p.566 Figure 7.80. A sketch section and plan of Davey’s Lode, Wheal Gorland……………………………p.567 Figure 7.81. A contoured pole plot and rose diagram for all lodes in the Scorrier District…..……….p.572
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Figure 7.82. Vein orientation data for the Land’s End Granite……………………………………….p.583 Figure 7.83. Relative displacements on the major vein sets across Penwith………………………….p.586 Figure 7.84. A reworking of Moore’s fluid pressure cell model (1975)……………………………....p.590 Figure 7.85. Vein orientation data for the Tregonning-Godolphin Granite…………………………...p.591 Figure 7.86. Relative displacements on the major vein sets around Mounts Bay…………………….p.593 Figure 7.87. Vein orientation data for the Carnmenellis Granite………………………………...……p.595
CHAPTER 8. Figure 8.1. A timescale of post-Variscan events in SW Cornwall…………………………………….p.602
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List of Plates in text. Volume II CHAPTER 6. Plate 6.1. Wolframite-bearing pegmatite, South Crofty Mine………………………………………...p.348 Plate 6.2. ‘Quartz floor’ pegmatite veins, South Crofty Mine………………………………………...p.349 Plate 6.3. Greisen veins, Cligga Head…………………………………………………………………p.352 Plate 6.4. The NPQ Lode, South Crofty Mine………………………………………………………...p.359 Plate 6.5. The Dolcoath North Lode, South Crofty Mine……………………………………………..p.360 Plate 6.6. The NPB2 Lode, South Crofty Mine…………………………………………………….…p.361 Plate 6.7. A sub-vertical crosscourse, South Crofty Mine………………………………………….…p.367
CHAPTER 7. Plate 7.1. One of the lodes of Carnelloe Mine……………………………………………………...…p.400 Plate 7.2. Carnelloe Mine. Photomicrograph of the Sn ore at ×10 PL………………………………...p.401 Plate 7.3. Carnelloe Mine. Photomicrograph of zoned cassiterite at ×4 PL…………………………..p.402 Plate 7.4. Veor Cove and Carnelloe Head from Zennor Head………………………………………...p.403 Plate 7.5. A haematite vein within the Veor Cove shear Belt……………………………………...….p.404 Plate 7.6. Exploratory adit workings on the haematite-bearing replacement ‘veins’ of the Veor Cove shear belt………………………………………………………………………………...…p.405 Plate 7.7. An SEM backscatter micrograph of a lode sample from Zennor Head Mine…….………...p.407 Plate 7.8. A composite tourmaline-quartz vein, Castle An Dinas Quarry………………………...…..p.413 Plate 7.9. A composite tourmaline-quartz vein, Castle An Dinas Quarry………………………...…..p.414 Plate 7.10. An SEM backscatter micrograph of a lode sample from Giew Mine……….………….…p.416 Plate 7.11. An SEM backscatter micrograph of a lode sample from Giew Mine……….………....….p.416 Plate 7.12. An SEM backscatter micrograph of a lode sample from Giew Mine……….……….……p.417 Plate 7.13. Quartz veins in granite at Mousehole……………………………………………….……..p.422 Plate 7.14. Quartz veins in granite at Lamorna Point……………………………………………….…p.424 Plate 7.15. A banded quartz-schorl vein in granite at Lamorna Point……………………………...….p.425 Plate 7.16. A coarse-grained schorl vein cutting granite on Treen Head…………………………..….p.431 Plate 7.17. Schorl veins in granite at Porth Curno………………………………………………….....p.433 Plate 7.18. Late haematite/quartz veins in granite at Nanjizal………………………………………...p.437 Plate 7.19. An elliptical pod of stanniferous tourmalinite; Nanjizal Mine……………………...…….p.440 Plate 7.20. The entrance to the underground workings of Nanjizal Mine……………………….……p.441 Plate 7.21. The ‘main lode’ of Nanjizal Mine, exposed adjacent to an open gunnis……………...…..p.442 Plate 7.22. The main overhand stope at Nanjizal Mine……………………………………………….p.443 Plate 7.23. Zoned dravite crystals overgrown by later schorl, Nanjizal Mine……………………..…p.445 Plate 7.24. Zoned schorl crystals, Nanjizal Mine………………………………………………….….p.445 Plate 7.25. Two generations of schorl, Nanjizal Mine…………………………………………..……p.446 Plate 7.26. Euhedral apatite, Nanjizal Mine………………………………………………………..…p.446
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Plate 7.27. Subhedral apatite, Nanjizal Mine………………………………………………………….p.447 Plate 7.28. Subhedral apatite, intergrown with orthoclase and schorl, Nanjizal Mine………………..p.447 Plate 7.29. Euhedral cassiterite, Nanjizal Mine……………………………………………………….p.448 Plate 7.30. Euhedral cassiterite, Nanjizal Mine………………………………………………...……..p.448 Plate 7.31. An SEM backscatter micrograph of a lode sample from Nanjizal Mine…….…….……...p.449 Plate 7.32. An SEM backscatter micrograph of a lode sample from Nanjizal Mine…….….……..….p.449 Plate 7.33. A banded schorl-blue peach-quartz vein, Mayon Cliff………………………………...….p.460 Plate 7.34. Overlapping alteration haloes around a vein at Nanven………………………………..….p.464 Plate 7.35. A late-stage quartz vein in granite at Nanven………………………………………….….p.465 Plate 7.36. A ferroan jasper guide, Nanven…………………………………………………………...p.465 Plate 7.37. The surface workings of Wheal Hermon, Nanven………………………………………...p.466 Plate 7.38. A blue peach tin-bearing lode, Nanven…………………………………………………....p.468 Plate 7.39. Saveall’s Lode and St Just United Mine, Priest’s Cove…………………………………...p.471 Plate 7.40. Quartz veins in granite, Priest’s Cove……………………………………………………..p.472 Plate 7.41. The Crowns, Botallack Mine………………………………………………………..…….p.473 Plate 7.42. Greisen veins, St Michael’s Mount…………………………………………..……………p.483 Plate 7.43. Trewavas Mine on Trewavas Head…………………………………………….………….p.497 Plate 7.44. An SEM backscatter micrograph of a lode sample from Trewavas Mine….….………….p.498 Plate 7.45. An SEM backscatter micrograph of a lode sample from Trewavas Mine……..………….p.498 Plate 7.46. An SEM backscatter micrograph of a lode sample from Trewavas Mine….….……....….p.499 Plate 7.47. A Blue peach lode, Wheal Roots………………………………………………………….p.505 Plate 7.48. A lode exposed on pillars within a stope at Wheal Roots………………………...……….p.506 Plate 7.49. Copper-bearing lode, Great Condurrow Mine ……………………………………………p.511 Plate 7.50. A chlorite-quartz-fluorite vein, Great Condurrow Mine ………………………………….p.511 Plate 7.51. Relict biotite crystal in chlorite, South Tolcarne Mine……………………………..……..p.515 Plate 7.52. A copper-bearing schorl vein, Holman’s Test Mine………………………………..……..p.518 Plate 7.53. A copper-bearing schorl vein, Holman’s Test Mine………………………………………p.518 Plate 7.54. A copper-bearing schorl vein, Holman’s Test Mine………………………………………p.519 Plate 7.55. An SEM backscatter micrograph of a lode sample from North Pool Mine….…….…..….p.522 Plate 7.56. An SEM backscatter micrograph of a lode sample from North Pool Mine…….….…..….p.522 Plate 7.57. An SEM backscatter micrograph of a lode sample from North Pool Mine…….….….…..p.523 Plate 7.58. An SEM backscatter micrograph of a lode sample from North Pool Mine…….…....……p.523 Plate 7.59. The 3B Pegmatite Lode, 380 fathom level, South Crofty Mine……………………….…..p.530 Plate 7.60. The No:10 Lode, 340 Fm level, South Crofty Mine………………………………………p.532 Plate 7.61. An SEM backscatter micrograph of a lode sample from South Crofty Mine….….……....p.534 Plate 7.62. An SEM backscatter micrograph of a lode sample from South Crofty Mine….…….…....p.534 Plate 7.63 A stope on North Tincroft Lode, above deep adit level…………………………………....p.540 Plate 7.64. An SEM backscatter micrograph of a lode sample from North Tincroft Lode……...…….p.543 Plate 7.65. An SEM backscatter micrograph of a lode sample from North Tincroft Lode……...…….p.543 Plate 7.66. An SEM backscatter micrograph of a lode sample from North Tincroft Lode…..……….p.544 Plate 7.67. An SEM backscatter micrograph of a lode sample from North Tincroft Lode…..…….....p.544 Plate 7.68. An SEM backscatter micrograph of a lode sample from North Tincroft Lode……..….....p.545
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Plate 7.69. An SEM backscatter micrograph of a lode sample from North Tincroft Lode……….…...p.545 Plate 7.70. An SEM backscatter micrograph of a lode sample from North Tincroft Lode………........p.546 Plate 7.71. An SEM backscatter micrograph of a lode sample from North Tincroft Lode………........p.546 Plate 7.72. Phase 2 mineralisation, North Tincroft Lode…………………………………………...…p.547 Plate 7.73. Phase 2 mineralisation, North Tincroft Lode…………………………………………...…p.548 Plate 7.74. Phase 2 mineralisation, North Tincroft Lode……………………………………………...p.548 Plate 7.75. An SEM backscatter micrograph of a lode sample from North Tincroft Lode….………...p.554 Plate 7.76. Phase 3 mineralisation, North Tincroft Lode……………………………………………...p.555 Plate 7.77. An SEM backscatter micrograph of a lode sample from North Tincroft Lode………...….p.556 Plate 7.78. An SEM backscatter micrograph of a lode sample from North Tincroft Lode……...…….p.557 Plate 7.79. An SEM backscatter micrograph of a lode sample from Davey’s Lode, Wheal Gorland...p.568 Plate 7.80. An SEM backscatter micrograph of a lode sample from Davey’s Lode, Wheal Gorland...p.569 Plate 7.81. An SEM backscatter micrograph of a lode sample from Davey’s Lode, Wheal Gorland...p.569 Plate 7.82. Lode sample. Craddock Moor Mine……………………………………………………….p.574 Plate 7.83. Lode sample. South Caradon Mine………………………………………………………..p.577 Plate 7.84. Lode sample. South Caradon Mine………………………………………………………..p.577 Plate 7.85. An SEM backscatter micrograph of a lode sample from South Caradon Mine………..….p.578 Plate 7.86. An SEM backscatter micrograph of a lode sample from South Caradon Mine………..….p.578 Plate 7.87. Lode sample. South Terras Mine………………………………………………………….p.581
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List of Tables in text. Volume II CHAPTER 6. Table 6.1. Estimated total mineral and metal production from South-west England…………………p.337 Table 6.2. Hosking’s model of the paragenetic sequence within the Cornubian Orefield……………p.342
CHAPTER 7. Table 7.1. The results of XRF analysis of a sample of haematised greenstone from the Veor Cove Shear belt…………………………………………………………………………………..p.405 Table 7.2. The results of XRF analysis of a sample of lode material from Zennor Head Mine…...….p.408 Table 7.3. The results of XRF analysis of a sample of lode material from Giew Mine…………...….p.418 Table 7.4. The results of XRF analysis of a sample of lode material and host granite from Nanjizal Mine………………………………………………………………………………….……….p.450 Table 7.5. A generalised paragenesis for the lodes of Geevor Mine……………………………….….p.479 Table 7.6. The results of XRF analysis of two samples of lode material from Wheal Roots…………p.508 Table 7.7. The results of XRF analysis of two samples of lode material from Great Condurrow Mine……………………………………………………………………………...…p.513 Table 7.8. The results of XRF analysis of a sample of lode material from Great Condurrow Mine……………………………………………………………………………..….p.514 Table 7.9. The results of XRF analysis of a sample of lode material from South Tolcarne Mine………………………………………………………………………………...…p.516 Table 7.10. The results of XRF analysis of a sample of lode material from North Pool Mine……….p.524 Table 7.11a. The results of XRF analysis of samples of lode material from North Tincroft Lode of New Cook’s Kitchen Mine………………………………………………..….p.559 Table 7.11b. The results of XRF analysis of samples of lode material from North Tincroft Lode of New Cook’s Kitchen Mine…………………………………………………...p.560 Table 7.12. The results of XRF analysis of two samples of lode material from Wheal Gorland…………………………………………………………………………………………p.571 Table 7.13. The results of XRF analysis of a sample of lode material from Craddock Moor Mine……………………………………………………………………………….….p.575 Table 7.14. The results of XRF analysis of a sample of lode material from South Caradon Mine………………………………………………………………………………..….p.579 Table 7.15. The results of XRF analysis of a sample of lode material from South Terras Mine………………………………………………………………………………….….p.582
333
Chapter 6: The Cornubian Ore Province – a Review. 6.1 Introduction. The Cornubian Orefield is the most intensely mineralised belt in the British Isles and it has been exploited continuously for over 3000 years (Penhallurick, 1986). Early legends of visits by Phoenician traders remain unsubstantiated, but later Greek accounts of trading for tin at the ‘White Hill of Ictis’ (St Michael’s Mount, which 2500 years ago would have stood out in a flat-lying wooded plain close to the coast) in the ‘Cassiterides’ (Tin Isles) are generally accepted. Julius Caesar, writing in the first century B.C, speaks of tin production in Britain and it is likely that the mineral wealth of the island (and its strategic position on the Irish gold trade route) was an added incentive for the Roman invasion in 44 A.D. Most of this early tin production came from placer deposits (Penhallurick, 1986). Surface exposures (particularly on the coast) of lodes were also worked, principally by opencast methods or by driving on lode into cliffs. These ‘coffin’ workings are still visible on the coast around St Just [SW370313], close to the workings of Geevor Mine [SW375345] and at Botallack [SW363335] (Noall, 1993; 1999). Underground mining seems to have started around the 12th century. The granting of royal charters in 1201 P
P
and 1305 (setting up the Stannary Parliament, with its independent taxation, legal and control systems) was of major importance to the tin trade, granting miners special privileges with regard to land access, prospecting and mineral extraction. The granting of these rights saw in a major phase of prospecting across the south-west, initially from the alluvial workings on the moorlands, out into the lowland valley floors and the discovery of lode outcrops from steam exposures and exploratory trenches. From perhaps the Roman period until the early 13th century, Dartmoor was the principal tin P
P
producing area in the orefield (Scrivener, 1982), and during the latter part of the 12th P
P
century it became the main source of the metal in Western Europe. During the early part of the 13th century Cornwall took over as the major producer (Cornish tin production P
P
rose to double that of Dartmoor, at around 500 tonnes per annum), a position it maintained until the closure of South Crofty Mine [SW668412] in 1998. Dartmoor’s production steadily declined (reaching a peak of 285 tonnes in 1515) until the mid 18th P
century saw a revival of its fortunes (Scrivener, 1982). Early workings were chiefly of alluvial and eluvial placer deposits, known as ‘tin streaming’. The tin-bearing sands and gravels were dug out from the riverbed and banks 334
P
and the heavy cassiterite separated by sluices and crude sediment traps. Underground or opencast workings, prior to the introduction of gunpowder, were worked by a combination of firesetting and manual extraction with picks and chisels. The cassiterite concentrate or rough ore was then taken to a smelter (known as a ‘blowing house’) to be further refined, in the case of the rough ore, by crushing (using water-powered sets of stamps) and hydraulic separation, and was then smelted. The molten metal was often poured into granite ingot moulds, some of which still survive (Penhallurick, 1986). The introduction of gunpowder blasting by Bohemian miners (where they worked the tin deposits of the Erzgebirge) during the reign of Elizabeth I (Penhallurick, 1986), saw the rapid development of underground mining in Cornwall. Initially this was for tin, but the manufacture of brass and the use of copper in the national coinage during the 18th P
P
century saw this metal assume prime importance in the orefield. The discovery of large deposits of copper in the Camborne-Redruth and Gwennap districts in the late 1600’s spurred on a further phase of exploration throughout the county, the chief focus of which was now the discovery of lodes, as the alluvial deposits were becoming increasingly exhausted. As production increased, the main barrier to extending the mine workings at depth was the position of the water table and the need to pump out excess water to keep the workings dry. This was overcome by the development of horsedriven, water-powered, and later, steam-powered pumping engines (Pryce, 1778). The use of steam power (building on the work of Newcomen, Boulton and Watt and Trevithick) revolutionised the mining industry and allowed deep mining to expand rapidly during the 19th century (Buckley, 1997). Steam engines were used to pump P
P
water, haul up ore (and, later, men), transport materials and drive sets of stamps. Cornwall became not only a major mining centre, but also a test-bed of new industrial and engineering ideas that were exported across the globe. The 19th century was the heyday of Cornish mining. After a period of closure in the P
P
1790’s (when cheap copper ore from Parys Mountain on Anglesey almost wiped out the Cornish copper industry) the mines proliferated and during the century over 2500 mines were operated in the orefield as a whole (Alderton, 1993). Copper and tin were the main products of these mines, but considerable tonnages of other metals and minerals were produced (see Table 6.1 and Figure 6.1), particularly iron, lead (Douch, 1964), arsenic (Earl, 1993), manganese, zinc and tungsten. During the 1860’s copper mining reached its peak (Dines, 1956) with production reaching 15,500 tons of metal; Britain supplied 335
Figure 6.1. A map of the orefield of South-west England (after Dunham et al., 1978).
336
around 40% of world consumption (and was the largest producer). Production of tin reached a peak in 1870 with a little over 10,000 tons of metal (Burt et al., 1987). A ruinous fall in metal prices in 1866 (brought about by the discovery of new copper deposits in Michigan (U.S.A) and Chile; and tin deposits in Malaya) saw the mining industry go into a rapid decline with the closure of many mines and the emigration of thousands of miners and their families to the opening mining fields of Australia, South Africa, Mexico, North and South America and the Far East (Noall, 1999).
MINERAL OR METAL
TONNES (approximate)
Sn metal
2,770,000
Cu metal
2,000,000
Fe ore
2,000,000
Pb metal
250,000
As (as As2O3)
250,000
Pyrite
150,000
Mn ores
100,000
Zn metal
70,000
W (as WO3)
5,600
U (+Ra, At, Po) ore
2000
Ag ore
2000
Ag metal (from sulphide ore)
250
Co-Ni-Bi ores
500
Sb ores
300
Mo metal
very small
Au metal
very small
Barite
500,000
Fluorite
10,000
Ochre/umber
20,000
Kaolinite (china clay)
150,000,000
Table 6.1. Estimated total mineral and metal production from South-west England. After Dines (1956), Alderton (1993) and South Crofty PLC (1988-1998).
World metal prices became increasingly volatile and the industry in Cornwall and Devon became caught in a cycle of ‘boom’ and ‘bust’ with fewer mines surviving each crash in prices (Morrison, 1980; 1983). Relatively few mines survived until 1900 (when 337
tin production had fallen to 2000 tons metal per annum; and copper to around 50 tons metal per annum; Burt et al., 1987) and after a brief rise in metal prices in the early 1900’s saw prospects improve, the First World War and the loss of labour brought many of the surviving mines to the brink of ruin. Casualties in the immediate post-war years included the famous Dolcoath [SW660401] in 1920, and Carn Brea and Tincroft mines [SW667405] in 1921. By the Second World War only South Crofty Mine (Buckley, 1997) and East Pool Mine [SW673415] remained in the Camborne-Redruth District with Geevor Mine at St Just, Castle-An-Dinas wolfram mine [SW946623] north of St Austell and Cligga Mine [SW738538] at Perranporth. East Pool Mine (Heffer, 1985) and Cligga Mine closed in 1945 and Castle-An-Dinas closed in 1959 (Brooks, 2001). A rise in metal prices during the 1970’s (which saw tin eventually reaching over £10,000 per tonne) saw renewed prospecting in the SouthWest and the reopening of Wheal Jane Mine [SW771427] near Truro and the opening of Wheal Concord [SW723458] at Blackwater and Wheal Pendarves [SW645383] near Camborne. During this period the tin price was stabilised by the International Tin Council (ITC), formed by the main tin-producing nations, buying and selling metal on the London Metal Exchange to keep the price as high as possible. When Brazil and China (non members) refused to be bound by any quota agreements and flooded the market with tin metal in October 1985, the ITC Buffer Stock Manager was unable to buy all the metal and ran out of money. Its trading was suspended on October 24th and P
P
the price fell overnight from £8,140 per tonne to £3,300 per tonne (Down, 1986). Wheal Concord and Wheal Pendarves closed in 1986. Geevor mine managed to survive, in a much reduced form, until 1991 and also in that year Wheal Jane closed. This left South Crofty Mine as the sole surviving mine in the Cornubian Orefield. It was hoped that the tin price would rise, but it fluctuated between £2,900 and £4,300 per tonne (averaging £3,400); with production costs of around £4000 per tonne the mine was continuously losing money, despite every effort to minimise costs. The mine eventually closed on March 6th 1998, bringing to an end some 3000 years of mining history. The mine was P
P
purchased and unabandoned in 2001; at the time of writing (2002) mining has not yet resumed and the venture faces an uncertain future.
6.2 History of Research. The early works of Carew (1602), Borlase (1758), Pryce (1778), Phillips (1814), Thomas (1819) and Carne (1822) are excellent first hand accounts of individual mines 338
and deposits. By the early part of the 19th century scientific descriptions of lodes and P
P
deposits were beginning to be made. The works of Henwood (1843) and, later, Collins (1912) provide excellent descriptions of lodes, relationships between lodes of different orientations and the nature and variable mineralisation of later faults (crosscourses), in many famous mines of the time that were inaccessible by the early 20th century. P
P
MacAlister (1908) recognised that the granite intrusions and their accompanying mineralisation took place in three stages: (1) intrusion of granitic magma with accompanying thermal metamorphism of the country rocks close to the contact; (2) intrusion of quartz porphyry dykes along fractures and cleavage planes in the metasediments; (3) deposition of ores in both sedimentary and igneous host rocks. With various refinements this statement remains valid today. The 1920’s and 1930’s saw the Cornubian Orefield used to test new theories of ore genesis and zonation (Halls et al., 1985). Dewey (1925) and Davison (1921, 1925a, 1927) developed a model of zonation (with tin/tungsten/arsenic mineralisation passing out into copper, then lead/zinc, and finally antimony/manganese/iron mineralisation) based on mono-ascendant single-pass hydrothermal fluids emanating from the Cornubian Batholith, giving rise to a concentric arrangement of mineralisation radiating out from the exposed granite cupolas (Figure 6.2).
Figure 6.2. A comparison of the zonation models of Davison and Dines (from Hosking, 1979).
Davison’s model suggested that the focal points of this mineralisation were grouped around original high spots in the batholith roof and that the mineral zones formed a series of ‘shells’ parallel to the granite contact with a copper zone overlying and overlapping a central tin zone and itself overlain and overlapped by a lead/zinc zone. Davison cited the concentric zonal pattern of mineralisation around St Agnes [SW723507] as an example (though this is not as simple as it first appears, Sn lodes in 339
the district dip mostly to the north, while Cu lodes dip south and are of a later phase of mineralisation), modified by denudation, of this original pattern (Figure 6.3).
Figure 6.3. The pattern of mineral zoning in the St Agnes District, used by Davison in the formulation of his theory of zonation (after Bromley and Holl, 1986).
Dines (1934) provided field evidence against Davison’s model. He held the view that the various zones were considerably flatter than the granite contact (Geevor mine remains the classic example of this phenomenon) and that the higher the zone, the greater its lateral extent. Dines also suggested that the appearance of certain minerals in particular zones was temperature dependent and determined by the temperature gradient between the granite and the surface. He also noted the irregular distribution (particularly of tin) of mineralisation related to the position of cusps in the granite roof and coined the term ‘emanative centres’ to describe these focal points of mineralisation. His model, however, was still based on a ‘single pulse’ mono-ascendant premise, using the cooling granite as the only source of heat. In 1956 Dines’ exhaustive memoir The Metalliferous Mining Region Of South West England was published, it remains perhaps the single most important account of the geology of mining in South-West England published to date.
340
Hosking (1964, 1979) refined and expanded on Dines’ model and took into account earlier phases of mineralisation that straddled the magmatic/hydrothermal boundary and pre-dated the main stage lodes. He also saw localised temperature gradients and wallrock interactions as more important than a regional temperature gradient related to magmatic
emplacement.
Hosking
recognised
seven
depth/temperature
zones
characterised by distinctive assemblages of ore and gangue minerals and also noted characteristic wallrock assemblages associated with each zone (Table 6.2). In applying a temperature framework to his model he was greatly aided by the work of Sawkins (1966) and Bradshaw and Stoyel (1968) who, through the study of fluid inclusions, found that not only were there significant differences in the depositional ranges of tin, copper and lead/zinc mineralisation, but that each mineral species has its own, fairly restricted, temperature zone.
He also tried to apply a time frame to the span of
mineralisation, envisaging a 200 million year protracted episode running from the Permian to the Eocene. During the 1960’s structural studies at Geevor Mine (Garnett, 1962) and South Crofty Mine (Taylor, 1965) made important contributions to our understanding of the mechanisms of lode formation, as did a landmark paper by Moore (1975) on the origin of the lode-bearing fracture systems across the Cornubian Orefield. Further studies at Mount Wellington Mine (Cotton, 1972), Wheal Jane (Walters, 1988; Holl, 1990), Wheal Pendarves (Alderton, 1976), the St Just District (Jackson, 1977), the Tavistock District (Bull, 1982), Dartmoor (Scrivener, 1982), the Wadebridge District (Clayton, 1992) and South Crofty Mine (Farmer, 1991) looked at the individual deposits from mineralogical, geochemical and structural perspectives. Moore (1982) argued that the pattern of W-Sn-Cu-Zn zoning above emanative centres could be explained in terms of a pattern of ‘hot spot’ geothermal circulation similar to that seen at Wairaki in New Zealand. While this model explained some of the pervasive alteration patterns seen in parts of the batholith (e.g., the St Austell Granite), it failed to take into account the textural evidence used later by Halls (1987, 1994) in his mechanistic approach to the formation of the lode system. Complex models of polyascendant fluid phases and structural reactivation had now replaced the earlier models of Davison and Dewey. The focus of research shifted to the geochemistry of the ore fluids (Rankin and Alderton, 1983, 1985; Jackson et al., 1982) 341
Table 6.2. Hosking’s model of the paragenetic sequence within the Cornubian Orefield (from Hosking, 1964).
342
and the building of a geochronological framework for the timing and duration of mineralisation. The works of Halliday (1980), Darbyshire and Shepherd (1985, 1987, 1994) and Chesley et al. (1991,1993) favoured the view of a protracted history of mineralisation, spanning over 200 million years, that was advanced in major reviews of the orefield by Dunham et al. (1978), Bromley and Holl (1986), Bromley (1989), Jackson et al. (1989) and Willis-Richards and Jackson (1989). Similar studies by Chen et al. (1993) and Clark et al. (1993) point to a much narrower timeframe and a diachronous pattern of mineralisation across the batholith, reflected in reviews by Alderton (1993), and Scrivener and Shepherd (1998).
6.3 Overview of Mineralisation. 6.3.1 Introduction. Most of the mineralisation present in the orefield can be directly linked to the granite batholith in some way, although some deposits clearly pre-date the granite and a variety of syngenetic sedimentary and SEDEX origins have been ascribed to these (Clayton et al., 1990; Clayton, 1992). Deposits falling into this category include the manganese deposits of East Cornwall and West Devon and the stratiform Pb-Sb-Cu deposits of the Wadebridge district in North Cornwall. The Sn-W-As-Cu mineralisation for which the region is famous occurs in a variety of forms, but principally in high-angle fissure veins (lodes) in or close to the granites (Garnett, 1962; Farmer, 1991). Mineralisation across the Cornubian Orefield can be divided into the following, chronologically arranged, groups: (1) pre-granite orebodies of sedimentary/sedimentaryexhalative type; (2) syn-granite intrusion orebodies – skarns and pegmatites; (3) early post-granite intrusion orebodies – greisens and sheeted vein complexes; (4) main stage polymetallic orebodies – Sn-Cu-As-Zn-Pb lodes and carbonas, etc; (5) late post-granite mineralised (Zn-Pb-Ag-Co) and unmineralised fissure veins – crosscourses.
6.3.2 Pre-Granite Mineralisation. Deposits of this type fall into two main groups: (1) manganese/iron oxide and hydroxide deposits; (2) massive and vein sulphide deposits. The manganese deposits occur primarily in the Lower Carboniferous chert beds to the west of Dartmoor, which extend into the Callington area of Cornwall, close to Bodmin Moor. A large belt of similar chert-bearing rocks extends across the north side of the St 343
Austell pluton. The ore occurs as irregular, ramifying lensoid masses, intimately associated with chert and, sometimes, jasper; and consists primarily of pyrolusite, psilomelane and wad, often with a central core of the Mn silicate, rhodonite. Rhodochrosite and tephroite are also noted from some deposits, particularly that of Treburland Mine [SX218814] near Altarnun (Golley and Williams, 1995). Deposits within the metamorphic aureoles of the granites are dominated by silicates, suggesting silicification/metasomatism of the original oxides and hydroxides. The deposits, spatially and genetically related to lavas and tuffs and occurring as replacement bodies in the cherts (as at Greystones Quarry [SX344784] near Launceston), are irregular and patchy and none of them are very large. Between 1800 and 1907 58,660 tonnes of Mn ore was produced, of which 90% was raised by Chillaton and Hogstor mines [SX429812] northwest of Tavistock (Dunham et al., 1978). A number of small ochre deposits have been exploited to the southeast of Bodmin Moor, and also in South Devon. These deposits appear to be chemical exhalative horizons within basic lavas and tuffs that have undergone hydrothermal alteration (Jackson et al., 1989). Various other stratiform exhalative bodies have recently been discovered, particularly in South Devon and eastern Cornwall (R. Scrivener, pers. comm.). These include quartzbarite-ankerite bodies carrying disseminated and massive sulphides of Zn, Fe, Cu, Sb, Co, Ni and Hg. They are of interest as potential carriers of precious metals (Alderton, 1993) and may be more common than previously thought, acting as potential sulphur reservoirs for later stages of mineralisation. Quartz-barite bodies are also known to occur. Vein deposits of argentiferous lead are known from the Combe Martin [SS564466] area of north Devon. The source of the lead and associated metals (Zn, Ni, Cu, Sb) is unknown, but U-Pb ages of 360 ± 30 m.y (Dunham et al., 1978) confirm a pre-granite origin for the deposits. Copper veins found in the slates to the south around North Molton [SS742295] may be of a similar age. Massive and vein sulphides occur in the strongly deformed Middle Devonian to Lower Carboniferous metasediment/volcanic sequences of the Wadebridge-Camelford area (Trevone Basin) of North Cornwall. Two distinct metallogenic events have been 344
identified (Clayton et al., 1990; Clayton, 1992), an early suite of stratiform/stratabound Fe-Cu-Zn deposits occurring in black, carbonaceous slates and Fe-Cu-Zn-Sb-Pb mineralisation occurring in brecciated and carbonatised metabasites. These are augmented by later vein-hosted Sb-As-Au and Pb-Zn-Ag mineralisation. The biogenic sulphides (Clayton and Spiro, 2000) of the stratiform/stratabound deposits were reworked during the Variscan Orogeny and underwent a series of metasomatic events. The later veins formed under conditions of brittle shear and uplift during the Late Variscan utilising locally derived high temperature metamorphic fluids (and using the existing deposits as a sulphur reservoir) and formed at depths of between ~5 km and ~2 km and temperatures of ~380oC and ~280oC. P
P
P
P
The copper deposits within the Lizard Ophiolite also pre-date granite intrusion. These consist of irregular discontinuous masses of native copper lining joints and fractures in, sometimes brecciated, serpentinite on the Lizard Peninsula of South Cornwall. The copper is associated with minor chalcocite, malachite and cuprite, sometimes with a steatite/talc gangue (Dines, 1956). The deposits are irregular and small and were worked briefly between 1820 and 1845.
6.3.3 Skarns and Pegmatites. Contact metamorphosed and metasomatised calcareous sediments are known as ‘calcflintas’ in the Cornubian Orefield. Although of considerable areal extent (particularly between the Bodmin Moor and Dartmoor granites) these rocks rarely contain large enough concentrations of metallic elements to be economically worked. Exceptions to this rule are found around the northern flank of Dartmoor, close to Okehampton [SX590950], where Belstone and Ramsley mines worked three sulphide-rich (Cu with As-Co) horizons, interbedded with cherts, up to 30 metres thick, in calc-silicate hornfelses (Dines, 1956). The rocks also carry up to 7 wt% Sn, but this is locked in silicates (malayaite and andradite garnet, etc) rather than as a sulphide or oxide (Alderton, 1993). The nearby quarry at Meldon, where Carboniferous cherts, slates, limestones and dolerite meet the granite contact, has a metasomatic assemblage (internally derived) dominated by garnet, vesuvianite, wollastonite and diopside (Edmonds et al., 1975). A second wave of metasomatism (granite derived) produced axinite and datolite with subordinate hedenbergite, andradite and Fe-wollastonite. Minor amounts of Cu, Bi, As, Co and Ni sulphides occur along with rhodonite-tephroitespessartine-bustamite
assemblages
in
Mn-rich 345
horizons.
Metasomatism
and
metamorphism of iron-bearing sands has lead to the development of magnetite-bearing ore, on the southeast side of Dartmoor, which was commercially exploited at Haytor Mine [SX772764] (Dines, 1956). Skarns are also developed on the coast of Penwith around St Just where the granite meets the Devonian cover rocks. These are predominantly doleritic rocks with subordinate metabasic volcanics (Floyd et al., 1993) and interleaved/faulted sections of hornfelsed slates. Metamorphism, shearing and metasomatism, with redistribution of Ca and Fe, has lead to the development of complex garnet-magnetite-dipside-epidoteactinolite-tremolite skarns (Jackson et al., 1982). These are associated with axinite, spinel, tourmaline, hornblende, chlorite, apatite, calcite and sulphides (Van Marcke de Lummen, 1985) and a number of rare tin-bearing silicates such as stokesite (CaSnSi3O9.2H2O), wickmanite (MnSn(OH)6) and malayaite (CaSnSiO5). These B
B
B
B
B
B
B
B
B
B
sheared belts are found within 100 metres of the granite contact and may reach 15 metres thick and extend downwards beyond 400 metres below sea level (Jackson et al., 1982; Alderton, 1993). A second fissure controlled stage of metasomatism has been recognised giving rise to veins of garnet and other minerals up to 1 metre in thickness. These are particularly prominent in the area around Botallack (where some of the layered metasomatic rocks have been impregnated with cassiterite, e.g., Grylls Bunny Mine above The Crowns) Th data from Grylls Bunny gives temperatures in the range of B
B
280oC – 380oC. An Ar-Ar determination of the age of the skarns at St Just gave a result P
P
P
P
of 274 Ma (Jackson et al., 1982). The skarns were formed by the expulsion of Ca, Mg and Fe from the basic volcanics, followed by enrichment in Sn, Fe, B, Be, U, W, Ta, F and Cl, transported by fluids of magmatic departure (Alderton, 1993). Beer and Ball (1986), examining the Sn and W contents of the killas around the granite plutons found consistent evidence of enrichment in these metals in metasomatic fronts radiating out from the granites. They commonly found a narrow exo-contact zone (some 20-50 metres from the contact) of moderate Sn and W values, succeeded outwards by a zone of high metal values (up to120 ppm+ for Sn) that may reach 500-800 metres from the contact. Beyond this point the values gradually fall away to background (4 ppm for W, 3 ppm for Sn). This metasomatic phase appears to post date extensive alkali metal metasomatism, which was responsible for the growth of feldspar in the hornfelses 346
closest to the granite, with biotite forming further out. The feldspar zone (up to 50 metres from the contact) appears to have resisted the later metal-bearing metasomatism far better than the biotite hornfelses or spotted slates further out, thus explaining the dip in metal values close to the contact. This metasomatic dispersal of Sn and W is unrelated to the vein mineralisation that post-dates (and sometimes modifies) it and marks the expulsion of metallic pneumatolytic vapours and fluids possibly before the granite (carapace, at least) was fully below the solidus. The dispersion haloes formed during this event (and also for the pathfinder element bismuth; Ball et al., 1982) have also been observed above concealed cusps and ridges (e.g. Bosworgy [SW573331], near St Erth; Beer et al., 1975; Rollin et al., 1982) in the granite and have been used as a tool in mineral exploration. Within the Lands End Granite metasomatic albitisation has produced areas that were extensively replaced by later phases of mineralisation to form large pods of disseminated ore, particularly at junctions between lodes, where the highly fractured ground is extensively replaced by cassiterite with copper sulphides, similar in morphology to the carbonas associated with the main-stage lodes (Thorne and Edwards, 1985; Jackson et al., 1982; Jackson, 1979). Pegmatites occur in all the major plutons, often as small, discontinuous pods and lenses within the granites. Occasionally they will form larger continuous veins and schlieren or banded pegmatite/aplite/microgranite (Badham, 1980) veins (as at Tremearne [SW613267] and Megiliggar Rocks [SW610267] on the margin of the TregonningGodolphin Granite). Some show evidence of forming in localised fluid-rich sections of the magma and are contemporaneous with the host rock, while others cut across the host granite and out into the country rock. Other pegmatites of stockschieder type lie along the contacts of adjacent granite intrusions within the main plutons (best displayed in parts of the St Austell granite). Many of the pegmatites carry no metallic minerals, but may occasionally be worked for industrial minerals, as at Trelavour Downs [SW960571], west of St Austell (lithian siderophyllite mica; Hawkes et al., 1987) and at Tresayes [SW995585] (a 45 metre wide pegmatite, worked for potash feldspar) north of St Austell (Dunham et al., 1978). Metalliferous pegmatites (Plate 6.1) often carry arsenopyrite, löllingite and wolframite with occasional minor cassiterite.
347
Plate 6.1. Wolframite-bearing, quartz-feldspar-tourmaline pegmatite. No:4 lode footwall drive, 400 fathom level, South Crofty Mine (metre rule for scale).
They may be associated with tourmaline and can carry an extensive suite of accessory minerals including zinnwaldite, apatite, chlorite, stilbite, stokesite, topaz and triplite (iron phosphate, found at Megiliggar Rocks). They may occur as isolated pods, e.g. Rinsey Cove [SW593269] (Hall, 1974), or as larger sets of veins that may be economically worked, e.g. Buttern Hill [SX188822] W-bearing pegmatites on the NE edge of Bodmin Moor and the W-bearing pegmatites at Cligga Head (Dines, 1956). Pegmatites are fairly rare in the Cornubian Orefield and this may be due to extensive fracturing towards the end of crystallisation, which saw fluids escape into the lode fracture system, rather than accumulate within the granite. Th data from mineralised B
B
pegmatites falls into the range 250oC to 460oC (Alderton, 1993), whilst the K-Ar date P
P
P
P
for the Halvosso Pegmatite on Carnmenellis is >285 Ma (Halliday, 1980), which would place it in the latter stages of the cooling of the host granite, post-dating the outer Carnmenellis Granite and its mineralisation (Clark et al., 1993). Within South Crofty Mine (Plate 6.2) a large number of sub-horizontal stacked veins (inclined from 0o to 20o and varying from a few centimetres to 1 metre in thickness), P
P
P
P
termed ‘quartz floors’ are found in localised ‘swarms’. These veins carry an assemblage of quartz ± feldspar ± fluorite ± wolframite ± arsenopyrite with rare stannite and 348
occasional later Cu and Fe sulphides in internal vein networks (Taylor, 1965; Farmer, 1991; LeBoutillier, 1996). The quartz floors occur in cylindrical zones extending for over two hundred metres in dip height and although not true pegmatites (they appear to be tensional features infilled by locally-derived magmatic fluids, formed in response to internal shearing within the still-plastic granite) they carry a mineralogical assemblage similar to that seen in true pegmatites elsewhere (Dines, 1956).
Plate 6.2. Stacked ‘quartz floor’ pegmatite veins. Main Lode drive, 290 fathom level, South Crofty Mine (metre rule for scale).
They have in the past been considered as potential economic targets (Bratley, 1978) for their tungsten content, but were not worked for such in the latter part of the history of the mine. Steeply-dipping lodes with an identical mineralogy occur within the mine (Roskear Complex Lode, Complex Lode, Care’s Lode and North Pool Quartz Lode), 349
which pre-date the later tourmaline-bearing tin lodes. A similar W-bearing lode occurs at Castle-An-Dinas [SW945623], at Goss Moor, near St Austell, which is cut by a granitic intrusion at depth (Beer et al., 1986; Brooks, 2001).
6.3.4 Greisens, Sheeted Vein Complexes and Breccia Pipes. Greisen-associated mineralisation is well represented in the Cornubian Orefield and can be broken down into three main groups: 1) Endogranitic sheeted vein systems. 2) Exogranitic vein systems. 3) Pervasive bodies. The build up of magmato-hydrothermal fluids, in the apical sections and cusps of the individual granite plutons, during the final stages of magmatic crystallisation eventually lead to hydraulic fracturing of the granite overlying these fluid reservoirs. The resulting metasomatic reactions, along the walls of the fractures, with the pneumatolytic fluids resulted in the formation of greisen bordered vein systems within the granite and sheeted vein systems in the overlying and adjacent killas. Endogranitic vein systems are found throughout the province. The best examples are those of Cligga Head, St Michael’s Mount [SW514299], Hemerdon [SX572580] (Dartmoor), Goonbarrow Clay Pit [SX007585] (St Austell) and Bostraze Clay Pit [SW383317] (Lands End). These systems are characterised by presence of numerous sub-parallel quartz veins, typically 1-10 cm in width with strike lengths that may reach in excess of 100 metres (as at St Michael’s Mount; Wheeler et al., 2001), with distinct greisen (essentially muscovite, sericite and quartz (both primary and secondary ‘globular silica’) ± minor tourmaline) selvedges. These tend to have very sharp boundaries with the host granites and typically reach from 8-30 cm in width. The quartz veins may carry an assemblage consisting of tourmaline, muscovite, orthoclase, chlorite, wolframite, arsenopyrite, stannite (kësterite, Zn-stannite and stannoidite), cassiterite, topaz, apatite and minor amounts of Fe, Cu, Mo and Bi sulphides (Hall, 1971; 1974; Alderton, 1993). The depositional textures seen in some of these quartz veins show that crystallisation took place in a remarkably static and stable environment (Halls et al., 1999). The quartz crystals form interlocked and bridging arrays with frequent vugs. Within these vugs ore minerals have nucleated without showing any tendency to bilateral symmetry; bunchy 350
aggregates of wolframite (occasionally bridging some of the veins) and prismatic (occasionally twinned) zoned cassiterite are typical. The textures show that the rate of fracture opening was greater than the rate of mineral growth and that nucleation was governed by diffusion in a more or less static fluid at near-equilibrium saturation (Halls et al., 2000). Other veins show quartz arrays bridging and, often, filling the fractures. The associated ore and gangue minerals behave in a similar fashion. Growth may be normal to the walls of the fracture (simple extensional dilation) or oblique to the walls (a combination of shear and extension). In this case mineral growth took place at a greater rate than fracture opening. Occasionally veins of this type also show more than one phase of opening, due to renewed extension after crystallisation of the first phase of infill. The greisen veins formed as arrays of parallel extensional fractures in the upper parts of their respective cupolas where the pressure of segregated magmatic fluids exceeded the tensile strength (and minimum principal stress) of the rock. Each fracture was propagated laterally and vertically for as long as those conditions were met, and each fracture acted as a separate semi-closed section of the system as a whole. There was no large flow of water through the vein system, the mineralisation in the fractures appears to have been supplied by the immediately surrounding granite; water:rock ratios prevailing during mineralisation were low and physiochemical conditions were stable (Halls et al., 1999, 2000). Fluid inclusion data (Jackson et al., 1989) give a temperature range for the greisenising fluids of 250oC – 500oC, variable salinities (5-40 wt% NaCl) P
P
P
P
and high pH (3.4 to 4.9). Of these deposits only that of Cligga Head (see Plate 6.3) has been commercially worked, producing 300 tons of wolfram concentrates and 200 tons of black tin during World War II (Dines, 1956), though the Hemerdon deposit (SW Dartmoor) was almost brought into production (reserves of 40 million tonnes at 0.18% WO3 and 0.026% Sn; B
B
Thorne and Edwards, 1985) prior to the tin crash of 1985. Several exogranitic sheeted-vein systems have similar characteristics to those of the granite-hosted deposits, both in terms of their chemistry (both have micas with high Li, Rb and F contents; Alderton, 1976) and mineralogy. These deposits consist of numerous, closely spaced, sub-parallel, narrow (1-2 cm) quartz veins. These veins carry an assemblage of muscovite, chlorite, cassiterite, topaz, tourmaline, stannite, 351
wolframite, chalcopyrite and rare native copper. Wallrock alteration is of limited width and consists primarily of haematisation with the development of muscovite and tourmaline selvedges.
Plate 6.3. Parallel greisen veins, Cligga Head. The granite between the veins is now extensively kaolinised. The veins consist primarily of quartz ± tourmaline, with a range of accessory minerals that may include cassiterite, wolframite, scorodite and rare blue topaz (hammer, 40 cm, long, for scale).
Good examples of this style of mineralisation include Wheal Prosper [SX030642] and Mulberry Openwork [SX020658] (both north of St Austell; Foster, 1878; Bennett et al., 1981) and Redmoor [SX355710] (near Callington). It is thought that other, more poorly-mineralised, deposits (for example, Magow Rocks [SW581422] at Godrevey, near Hayle; Bromley and Holl, 1986) overlie endogranitic systems in the granite at depth and point to cuspate areas in the granite roof. Deposits like Mulberry and Wheal Prosper were worked as bulk tonnage – low grade deposits; Mulberry alone produced over 1 million tonnes of ore, at ~0.4% Sn. Investigation by the BGS has shown that these two deposits still contain significant reserves (Bennett et al., 1981), though in the current economic climate they are very unlikely to resume operation.
352
The Redmoor deposit, near Callington was investigated in the early 1980’s and also came close to being brought into production. It is situated in pelitic sediments and crossed by a number of main-stage lodes that form enrichment zones at the intersection with the sheeted veins. The deposit contains significant amounts of stannite, along with cassiterite, wolframite and arsenopyrite, with a host of accessory minerals including bismuthinite, pyrrhotite, chlorite and fluorite. The sulphide assemblage overprints an earlier Sn-W-As assemblage associated with the emplacement of the complex (Thorne and Edwards, 1985). In common with the endogranitic systems, these deposits have all undergone pervasive tourmalinisation of variable intensity (Bromley and Holl, 1986), affecting the wallrock between the veins. Pervasive greisenisation is best exemplified by the apical outcrop of the St Agnes granite porphyry at Cameron Quarry [SW704507] (Hosking and Camm, 1985; Bromley, 1989), close to St Agnes Beacon. This exposure shows the roof zone of the granite (marked by aplite-pegmatite and leucogranite ‘sheets’) and the killas contact. The porphyry (consisting of large phenocrysts of alkali feldspar and quartz; smaller phenocrysts of plagioclase and biotite; set in a fine-grained groundmass of quartz, feldspar, biotite, muscovite and tourmaline) is progressively greisenised away from the contact and converted into a mass of quartz and fine-grained white mica. Voids left by dissolved feldspars were later infilled with pseudomorphs of cassiterite, wolframite and various sulphides. In this instance fluid pressures were not high enough to cause fracturing, the fluids passing along grain boundaries and metasomatising the rock en masse. Breccia pipes are an expression of this early phase of mineralisation (see Figure 6.4) formed under unstable conditions, unlike the greisen systems described above. A number of breccia pipes are known to exist (Goode and Taylor, 1980) throughout the orefield, but the best example is that of Wheal Remfrey [SW925576], in the western lobe of the St Austell Granite (Allman-Ward et al., 1982) close to Fraddon Down. The complex is hosted within coarse megacrystic granite and consists of a tourmalinite bordered sheeted vein system and an elongate N-S trending tourmaline-dominated hydrothermal breccia body, measuring 500 metres by 40-100 metres (Bromley, 1989). The breccia body is broadly pipe-like, with steep contacts. It consists of irregular disordered clasts of granite, porphyry, elvan, tourmalinite and pelitic (it is estimated that 353
Figure 6.4. Major types of primary tin deposits in Cornwall (after Hosking, 1969).
354
the current exposure level is only 200 metres below the original granite roof) material that show evidence of transport and rotation. Towards the margins of the body the clasts are not rotated and appear to have travelled very little distance; beyond the breccia body, the host rock is cut by anastomosing vein and veinlets of tourmaline and tourmalinite. It is thought that the pipe formed when fluid overpressures initiated a fracture system, similar to that in the formation of greisen veins, however, instead of the fracture ending at a point determined by the stress conditions, it intersected a lower pressure regime and the sudden pressure loss to the system as a whole resulted in explosive decompression. This resulted in violent decrepitation under positive pore fluid pressures and the development of a chaotic fluidised system. It is thought that the trigger for this mechanism was the crystallisation of tourmaline. Boron-rich melts can contain high levels of dissolved water; with the crystallisation of tourmaline the water vapour pressure sharply increased and contributed to the increased stress that lead to fracture propagation (Pichavant, 1981). The breccia body shows evidence of very rapid nucleation and crystallisation, with clasts close to the pipe walls trapped in position, adjacent to their point of origin. Some clasts appear to have collapsed into the body from above (slates, etc from the contact), while others have been brought up from depth. The main sense of movement appears to be in from the side walls due to the shock wave on decompression. There is evidence of more than one decompressive event, with rapid crack-sealing of the fracture followed by progressive pressure build-up and repeated failure along the fracture zone once the tensile strength of the rock is exceeded (Allman-Ward et al., 1982; Bromley, 1989). There is little metallic mineralisation associated with the Wheal Remfrey breccia pipe, although other tourmaline-dominated bodies may carry cassiterite and rutile. They tend to be depleted in chalcophile elements, even with respect to background values (Bromley and Holl, 1986) and appear to be wholly derived from magmatic fluids.
6.3.5 Main-Stage Lode Mineralisation. The typical lodes of the province are steeply dipping (most >70o) fracture-infill veins, P
P
which are concentrated along the axis of the batholith and are closely associated with elvan dykes (Jackson et al., 1989). The lode system as a whole has produced almost all 355
the metallic output of the orefield and has produced not only tin and copper, but a range of metals including tungsten, iron, lead, zinc, silver, etc (see table 6.1). The origin of these metals is still in debate; the tin (and tungsten) is likely to have been derived by fractionation, from tin-rich sediments or protolith at the point of anatexis (Lehmann, 1987), though some authors point to the possibility of derivation from the mantle (Hutchison and Chakraborty, 1979). Though this seems less likely than a crustal source, Shail et al. (1998) have found traces of mantle helium in fluid inclusions from the orefield, attesting to some mantle involvement in mineralisation. The origin of the CuZn-Pb mineralisation is thought to be due to a combination of xenolith assimilation and hydrothermal leaching of basic rocks (and pelites); it has been calculated that the volume of basic rocks and their copper content could easily supply the amount of copper extracted in the province (Jackson, 1979). Though on the scale of the orefield, the lode system is extremely complex; within localised areas a number of fairly simple relationships can be established. Mineralogically (as a rule) the lodes show decreasing complexity with depth. Close to surface they are truly polymetallic and may show a mixed oxide/sulphide assemblage that, in many cases, has been modified by supergene activity (see Figure 6.5).to give a
Figure 6.5. A section through a typical Sn-Cu lode, showing the relative position of the gossan, supergene and primary sections and the zoning seen in some of the major structures in the CamborneRedruth District (from Hosking, 1988).
356
large potential list of secondary minerals. These include secondary sulphides, hydroxides, oxides, sulphates, arsenates, carbonates and native metals; many of these were first described in Cornwall and the area has been the focus of professional and amateur mineral collectors for centuries (Embrey and Symes, 1987). Within this near-surface zone the ores of tin, arsenic, copper and zinc were worked (though often in separate stages, depending on the economics of the time); many mines also produced minor amounts of lead and silver (Dolcoath) and occasional U, Fe, Bi, Mn, Ni and Co (Wherry Mine [SW470294], Penzance) ores. In the Camborne-Redruth and Gwennap areas (as elsewhere) the lodes close to surface were dominated by copper mineralisation (Dines, 1956). In the supergene zone original simple sulphides (chalcopyrite, pyrite) were replaced by malachite, azurite, tenorite, cuprite and native copper. Rare secondaries, such as olivenite and liroconite, etc, are known from the Gwennap area and Porthleven area (where Cu and Pb ores occur together). Below the oxidised zone secondary chalcocite, bornite, enargite and covellite were deposited before passing back into primary sulphides below the water table. These very shallow (often < 50 metres) rich deposits were mined at an early date (1700 onwards) and the high financial returns gained were responsible for the proliferation of mining activity across the orefield during the 18th century. P
P
With increasing depth the zinc and lead mineralisation died away leaving a zone of simple sulphides dominated by copper and arsenic (Collins, 1912; Dines, 1956). As the granite/killas contact was approached tungsten became locally important, reaching its greatest development immediately below the contact (e.g. Rogers Lode, East Pool Mine; Dines, 1956). Below the contact copper declined and tin became increasingly important, and at depth (~500 metres from surface) cassiterite is often the sole ore mineral present. This change from a simple oxide-dominated assemblage at depth, passing into mixed oxide/sulphide assemblages close to the contact and complex polymetallic assemblages at surface was the foundation for the theories of hydrothermal zonation formulated during the early 20th century. However, the relationship between the various phases was P
P
not always as clear cut and within a single lode there is often evidence of a protracted history of mineralisation, brecciation, shearing and further mineralisation, that negates the idea of mono-ascendant fluids. Most lodes do not show a single continuum of 357
pressure/temperature-controlled mineralisation, they show a series of punctuated events, with the later sections of the lodes showing lower temperature assemblages. In this way some lodes initially worked for copper may later have had the walls of the existing stopes reworked for their tin or tungsten content (e.g. the North Tincroft Lode of South Crofty Mine; LeBoutillier et al., 2000a; 2000b; 2001). The gangue minerals associated with the ores also vary with depth and are also temperature dependent (see Table 6.2). At depth (associated with tin ore) the main gangue minerals are tourmaline (fine-grained, powdery to flinty, Prussian blue to dark blue) and quartz. At higher elevations, lower temperatures and in lower energy environments (in areas reactivated by further fracturing) this gives way to a chloritedominated (Taylor, 1965; Farmer, 1991) assemblage (though initially in places still retaining a proportion of tourmaline) with quartz and fluorite. Lower temperature phases are dominated by quartz, siderite, fluorite, marcasite and rare calcite. While this trend to lower temperature mineral assemblages and lower energy environments over time and proximity to the surface is broadly correct, recent studies have shown that some shallow deposits can also be tourmaline-dominated and that some of the chlorite assemblages record violent brecciation events with clasts transported considerable distances (LeBoutillier et al., 2000a; 2000b; 2001). This again suggests a series of punctuated mineralisation events, utilising fluids from a variety of sources and under a variety of physiochemical conditions. Many lodes show a complex interplay between tectonically-driven episodes of mineralisation and remobilisation of constituents by convecting hydrothermal fluids. This last particularly applies to copper and uranium mineralisation (pitchblende and coffinite are themselves a late-stage infill in some lodes, e.g., No4 Lode at South Crofty Mine; Cosgrove and Tidy, 1954), which, in many secondary phases, are highly mobile and
readily
dissolved.
An
environment
in
which
a
cyclical
system
of
pressure/temperature changes occurs may see the deposition of the rare fibrous form of cassiterite known as ‘wood tin’ (Hosking et al., 1987), if the fluids are supersaturated with respect to tin. Often in contrast to the mineralogy (particularly at depth) the structural history of many lodes is complex and shows a series of brecciation and shearing events responsible for depositing a variety of individual assemblages in the lode over time (Plate 6.4). 358
Plate 6.4. The NPQ Lode, 390 fathom sublevel east, South Crofty Mine. The lode is one of an en echelon series of quartz-feldspar ± wolframite ± arsenopyrite ± fluorite ± chlorite early ‘pegmatitic’ lodes (showing dilational ‘banded’ and mylonitic shear textures) that has been reactivated during main-stage mineralisation with a central (dark) brecciated core of tourmaline, massive cassiterite and quartz (hammer, ~30 cm, for scale).
Brecciation textures are common in lodes in the deeper workings of many mines (Dines, 1956) and occasionally at surface (Dunham et al., 1978; Goode and Taylor, 1980). In the deeper workings of South Crofty Mine many lodes showed an early tourmaline (‘blue peach’)/quartz ± cassiterite breccia with a cassiterite/quartz cement (Plates 6.5 and 6.6). This was sometimes followed by other brecciation events, but was more often followed by further lower energy dilational episodes, giving the lode a banded appearance (some of these bands were, occasionally, microbreccias, emplaced within the lode), particularly along the hangingwall. Some of the reactivation episodes lead to 359
the deposition of later chlorite-dominated assemblages, while other events lead to fine fracturing across the lode and the deposition of low temperature chalcedony-marcasitesiderite assemblages.
Plate 6.5. The Dolcoath North Lode, 380 fathom level, South Crofty Mine. The lode is predominantly composed of brecciated blue peach and quartz with minor (100 metres) and often have dextral throws from a few metres to tens of metres. At South Crofty Mine a large number of chalcedony-filled crosscourses (Dominy et al., 1993,1994a, 1994b) faulted the main-stage lodes (Plate 6.7). These were typically subvertical structures with a banded appearance, (due to repeated infilling over a protracted period) typically 0.5 metres in width (but commonly ranging from 10 metres in width across Carnmenellis, occasionally reaching (e.g. Dolcoath Main Lode) widths of over 15 metres (Taylor, 1965; Farmer, 1991; LeBoutillier, 1996). Lodes in the St Just area are generally much narrower, with widths between 0.45 – 0.60 metres being the average (Garnett, 1962). Individual lodes may have strike lengths of several kilometres (the Great Flat Lode has a strike length of over 5 km) and dip lengths of many hundreds of metres (Dolcoath Main Lode was worked to a vertical depth of around 1000 metres from surface), though such structures tend to be exceptional. Examination of mine sections and plans (Taylor, 1965) reveals the relationship that the structures with the longest strike length also have the greatest dip height (and also width and, often, payability); thus lodes such as Dolcoath Main Lode, Highburrow Lode, Pryce’s Lode, and Reeve’s Lode (all in the Camborne-Redruth District) have strike lengths in excess of a kilometre and can be traced from surface for vertical depths of over 800 metres; they also lie both within killas and granite. These major structures contrast with other lodes that fall into two major groups: (1) lodes hosted within the 373
granite that peter out just beyond the granite/killas contact (though this was never put to the test at South Crofty Mine, as it was policy to halt development once the contact was reached. Lodes at the contact were reduced in width, were uneconomic and were structurally poor, but were never developed to see if this was a temporary condition or if the lode returned to pay in the killas after the example of Geevor Mine); (2) those lodes within the killas that fail before the granite contact is reached. The former produced mainly tin ores (with tungsten and arsenic around the granite/killas contact) and the latter mostly copper ores, with some tin at depth and, occasionally, close to surface (Dines, 1956). The majority of lodes are not simple planar features; in section (see Figure 6.6) they can be seen to be curved bodies occupying listric faults, which tend to flatten in dip (often from near vertical at surface or highest point up dip) with increasing depth (e.g. Dolcoath Main Lode, the No8 and No4 lodes of South Crofty Mine). Many lodes also branch up and down dip, forming horsetail geometries, particularly in the granite. On a smaller scale, irregularities such as rapid changes in dip and strike (or both) were responsible for the selective opening of lode segments during normal or reverse movements on the fault plane. This resulted in the formation of low pressure zones (Garnett, 1961, 1966a; Taylor, 1966) in the open spaces created, where rapid deposition of minerals could take place (see Figure 6.7). Collins (1912) noted that the richest sections of a lode were usually its steepest. This was also seen on lodes, such as Dolcoath South Lode, at South Crofty Mine, where steep sections of the lode were very rich and values died away rapidly following a change to more gentle dips (which often took place at a sharp, angular bend in the lode). This scenario is consistent with open space formation during normal (extensional) faulting; if the movement is strike-slip or oblique dip-slip, then changes in strike can also be important as sites of open space creation. The position of these open space sites on irregular fault planes was critical in the economic deposition of cassiterite and other metallic minerals (Taylor, 1966) during the various phases of mineralisation.
374
Figure 6.6. Long section through New Cook’s Kitchen Shaft, South Crofty Mine.
375
Figure 6.7. The effect of fault movements on fracture width (after Garnett, 1966).
6.4.2 Lode Development. A number of structural studies of lode formation were produced in the 1960’s (Garnett, 1961, 1962, 1966a, 1966b; Taylor, 1965, 1966), but it was Moore (1975) who created the first integrated account of the province as a whole. He argued that the lode and elvan dyke fissures were propagated in a regional stress field, with considerable local horizontal anisotropy, by internal fluid overpressures, generated in the still-fluid cores of the major plutons, during the later stages of magmatic emplacement and crystallisation. In his model (see Figure 6.8) he used pre-granite faults, joints and fractures in the granite carapace and cover rocks (generated by fluid ‘pressure cells’) as channels for both magma and mineralising fluids. The principal stress axes remained in 376
Figure 6.8. Moore’s fracture geometry models for (A) the porphyry dykes, and (B) main-stage lodes, based on his ‘fluid pressure cell’ model (after Moore, 1975).
approximately the same orientation during successive episodes of brittle deformation, but the principal stresses were interchanged many times during the evolution of the lode-dyke system. In his model the regional stress field controlled batholith emplacement and the generation of the E-W and NW-SW fracture systems; he explained the ‘radial pattern’ of lodes in the St Just area of the Lands End Granite in terms of the internal hydraulic pressures overcoming the regional stress field.
377
Each major pluton was mechanically independent, with some overlap of individual stress fields, leading to the development of superimposed and curvilinear structures. Moore envisaged failure taking place on the flanks of the plutons, rather than across the roof (thus explaining the ‘emanative centres’ of the northern flank of Carnmenellis and Land’s End) and also the contemporaneous development of normal faults at higher levels and oblique and strike-slip faults on the flanks of the plutons. Halls (1987, 1994) studied the mechanics of lode formation and was able to distinguish two distinct lode systems on the basis of structural, textural and mineralogical evidence. In his findings he took the opposite view to Collins (1912), who thought that lode formation was a passive process, with gravitational collapse and relaxation of pressure the main forces responsible. Halls looked at the early W-As veins and the main-stage Sn-Cu lodes, which had previously been described together as a single group in the paragenetic frameworks of previous authors. He was able to show, on the basis of crosscutting relationships, that the W-As ‘greisen’ veins were of an earlier generation, but was also able to show that they formed under very different conditions. During the later stages of crystallisation of the granite magma, incompatible elements were selectively partitioned into the residual fluids (see Figure 6.9). These fluids (water, halogens, metallic elements, boron, CO2, etc) eventually formed reservoirs under areas B
B
of positive relief in the pluton roof, beneath a carapace of already sub-solidus granite. The chemical potential energy stored in the water-saturated magma is converted to P∆V expansive work as the hydrous fluid exolves and fluid pressures in the rock increase. The fluids, as they begin to collect and silicate minerals crystallise, exert an increasing vapour pressure on the system. If the granite body is emplaced at a depth where the confining pressure is never exceeded by the vapour pressure (or vapour pressure is low), then the fluids will eventually be dissipated by diffusion (Niggli, 1929; Halls, 1987) into the surrounding rocks, giving rise to pervasive deuteric alteration (e.g. Cameron Quarry [SW701498], St Agnes; Hosking and Camm, 1985). If, however, the granite body is emplaced at shallower levels, then the vapour pressure may exceed the confining pressure (lowest principal stress) and the tensile strength of the rock (Pint >= σ3+T) B
giving rise to fracture propagation and fluid release (Niggli, 1929).
378
B
B
B
This release of volatiles took place into swarms of autogenously generated tensile hydraulic fractures (Halls et al., 1999, 2000). The segregating magmato-hydrothermal fluids were saturated with the ore and gangue minerals that form the typical greisen (quartz-muscovite ± wolframite ± cassiterite ± stannite ± feldspar ± tourmaline, etc) veins of the province. These veins are characterised by their parallel arrangement (see Figure 6.10) and distinctive envelopes of alteration (pneumatolytic reactions, at temperatures between the granite solidus and 280°C, outside the field of feldspar stability, lead to the replacement of plagioclase and K-feldspar by characteristic muscovite-quartz ± tourmaline ± topaz assemblages).
Figure 6.9. The evolution of vapour pressure relative to confining pressure and declining temperature in a crystallising volatile-rich granite. The path B-C1-D1 is that followed by a plutonic body in which the confining pressure remains greater than the evolved vapour pressure. Deuteric alteration will occur, pervasively altering the host rock, but the fluids will eventually be dissipated by diffusion. The path B-C2D2 is that taken when the granite body is emplaced at shallower levels and the fluids are released in pulses to form pneumatolytic veins of greisen type (after Niggli, 1929 and Halls, 1987).
379
Figure 6.10. The formation of parallel greisen veins where Pint >= σ3+T and Pfluid wallrock < Pfluid fracture B
B
B
B
B
B
B
B
(after Halls, 1994)
Textural evidence (Halls, 1987) shows that most of these greisen veins opened in a tensile mode. The conditions necessary for the formation of fractures of this type are that the internal fluid pressure must exceed the least effective principal stress and tensile strength of the rock (Halls et al., 1999, 2000) and that the differential stress must be small enough for Griffith extensional failure to occur. This can only take place (forces in the upper crust are typically compressive, with σ1-σ3 increasing with depth) where B
B
B
B
pore fluid pressures, acting against the lithostatic stress, reduce the compressive stresses to a point where the failure point is reached and hydraulic fracturing occurs (see Figure 6.11). When the fractures form, they will be aligned normal to σ3 (the preferential direction of B
B
opening) and form a series of parallel veins, providing that the differential stress >0. If the differential stress =0, the stress is hydrostatic and stress across all planes is equal; with no preferred direction of opening, a randomly-oriented set of tensile veins will result, which may give rise to a stockwork and/or brecciation (see Figure 6.12). If the differential stress increases to the point where a mohr circle would intersect the Navier/Coulomb section of the failure envelope, then shear failure will occur. Some
380
greisen veins show a hybrid extensional-shear mode of opening (Halls et al., 2000) as this condition is gradually approached.
Figure 6.11. The effect of increasing fluid pressure on the state of compressive stress in a rock. Two states of stress are depicted by Mohr circles (i) and (ii). In stress state (i) the differential stress is large and the effect of increasing fluid pressure will bring it into contact with the failure envelope in the shear domain. In stress state (ii) the differential stress is small and increasing fluid pressure will bring it into contact with the failure envelope in the tensile domain, resulting in the formation of extensional hydraulic fractures (from Halls et al., 2000).
In Cornish examples of greisen vein systems (e.g. Cligga Head, St Michael’s Mount) it is not possible to see the full 3D extent of the vein system. Halls et al. (2000) report on Chinese exposures where this is possible. The individual veins in the array pinch out in all directions and take on the form of disc-shaped tensional fractures (within a zone bounded by points where stress conditions no longer allowed the fractures to propagate – in an isotropic rock the vein system would lie in a spheroidal or ellipsoidal zone near the roof of the intrusion) and it is therefore likely that the Cornish examples have a similar morphology. Many Cornish greisen veins show textures indicative of single-pass infilling (each vein acting as a closed system in its own right), over time, with rate of mineral growth < rate of fracture opening, but others show repeated infilling due to ‘autogenous hydraulic pulsation’ (Halls, 1987, 1994). The original fracture forms in the manner outlined above, with fluid pressure in the fracture > pore fluid pressures in the wallrocks.
381
Figure 6.12. The formation of extensional fractures. The two stress states illustrated will give rise to parallel fractures, normal to σ3 (a) where the differential stress is relatively large, or randomly-oriented B
B
fractures (b) where the stress is hydrostatic (the Mohr circle is reduced to a point) and the differential stress is therefore zero, with no preferred direction of opening (from Halls et al., 2000).
Crystallisation crack-seals the fracture until internal fluid pressures in the volatile reservoir again reach the point at which failure occurs. The weak point occupied by the original vein is reopened and the fracture is able to propagate to a higher structural level, still bound by tensile conditions (see Figure 6.13). This process may be repeated a number of times until internal fluid pressures finally fall back or stress conditions change. This staged opening/reopening of veins can allow different parageneses to be present in adjacent veins if they formed at different times. Abrupt changes in the physical and chemical conditions prevailing in the fracture system are unlikely and this is reflected in the gradual changes in the mineralogy of individual veins, where zoning is present, which is in marked contrast to the situation seen in the main-stage lodes.
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Figure 6.13. The formation of extensional hydraulic fractures by autogenous pulsation (after Halls, 1994).
Where lodes and greisen veins occur together, the main-stage lodes typically post-date greisen veins. The main-stage lodes acted as channels for more evolved Sn-B fluids, from deeper within the granite, than those seen in the greisen veins (Halls, 1994). They typically occupy faults and the early stages of their structural evolution appear to have been dominated by rapid fracture dilation, related to seismic events. Another factor in their development was the large amount of boron in the residual fluids; with the crystallisation of tourmaline, the sudden reduction of water solubility within the magma/fluid residuum (London and Manning, 1995), saw a corresponding rise in vapour pressures that rapidly overcame confining pressures. The resultant fractures, in many cases, appear to have communicated up-dip with lower pressure regimes. In these instances the sudden catastrophic loss of pressure in the system lead to explosive decompression and hydraulic decrepitation (Halls and Allman-Ward, 1986; AllmanWard et al., 1982) as pore fluid pressures in the wallrocks suddenly greatly exceeded fracture fluid pressures, resulting in (if the tensile strength of the rock is exceeded) the spalling off of segments of the wallrocks along the fracture. The resulting brecciated texture, with near instantaneous crystallisation trapping the entrained clasts (see Figure 6.14) is typical of lodes in their early stages of development.
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The sudden pressure loss and decrepitation lead to the production of much fine, comminuted debris and irregular clasts of wallrock and pre-existing lode infill. Rather than develop into a fluidised system, with rapid transport of material up the conduit, textures (microcrystalline intergrowths of tourmaline, quartz and cassiterite, which form the matrix for the hydraulic and cataclastic breccias) suggest very rapid adiabatic crystallisation, with the majority of clasts ‘frozen’ close to their point of origin (Halls, 1994).
Figure 6.14. The formation of hydrothermal breccias by explosive decompression (after Halls, 1994).
At South Crofty Mine such textures were very common in the tourmaline-dominated ‘blue peach’-bearing lodes at depth (below 245 fm level). A number of lodes showed the development of hydraulic breccias, with a quartz-cassiterite matrix cementing earlier fine-grained tourmalinite clasts. Occasionally, inherited clasts of wolframite could be observed where a main-stage lode had reactivated the lode fracture previously occupied by an earlier quartz-feldspar-wolframite assemblage. Some lodes showed more than one episode of breccia formation, initiated by movements along the fault plane hosting the lode. Later, lower-energy, banded, dilational infillings were also common, especially on the hangingwall of the lodes, marking the shift to a much less energetic, less violent form of deposition over time.
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The formation of hydrothermal breccias reaches its most extreme in the case of breccia pipes (Goode and Taylor, 1980; Halls and Allman-Ward, 1986; Allman-Ward et al., 1982; Taylor and Pollard, 1993). These features originate when a deep-seated reservoir of boron-rich fluid undergoes catastrophic decompression. At Wheal Remfry, the sudden decompression (at depths of 2.6-3.8 km) lead to massive decrepitation of the wallrocks, fluidisation and transport of material from depth, implosion of material from above and near instantaneous crystallisation of the microcrystalline tourmaline matrix (Halls, 1994). Similar conditions probably gave rise to the breccia lodes of the Gwinear district and elsewhere (Goode and Taylor, 1980), with the sudden decompression of large, super-pressurised, deep-seated fluid reservoirs by fracture propagation or fault activation. With the decline in fluid pressures over time, and at higher structural levels, the textures formed during hydraulic decompression were replaced by banded, dilational infilling (with occasional mylonites produced during faulting) and later open-space infilling (characterised by open, vuggy textures).
6.4.3 Structural Controls on Mineralisation. The lodes of the orefield are not subject to a uniform set of controls. They have complex relationships with the granite contact, elvan dykes, caunter lodes, crosscourses and each other, which may be contradictory, even within very small areas (Phillips, 1814; Thomas, 1819; Carne, 1822; Henwood, 1843; Collins, 1912). The granite contact is important as a structural control in that it marks the boundary between the fracture/joint sets present in the granite and those present in the overlying killas, which were exploited by the lodes during mineralisation. In the CamborneRedruth District, the number of lodes that span this boundary are relatively few, and those that do are the ‘major’ long-strike/long-dip structures, such as Pryce’s Lode or Dolcoath Main Lode. Most known lodes are only developed in the granite (tin-bearing) or in the killas (copper-bearing), though they share similar strike trends and dips. This is probably due to the differing rheological properties of the rocks in response to the regional stress field and internal thermal and fluid pressure stresses within the cooling granite (Jackson, 1979).
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The granite contact was also seen as a thermal barrier by Davison (1921, 1927) and Dewey (1925), and while it is true that hypothermal W-Sn mineralisation is closely associated with the granite (most production came from within 1 km of the contact; Jackson et al., 1989), Dines (1934, 1956) was able to show, at Geevor Mine, that the tin zone dips at a shallower angle than the contact itself and payable extensions to lodes (Garnett, 1966b) in the granite pass out into the killas (Figure 6.15).
Figure 6.15. The relationship between the granite/killas contact and the economic tin zone at Geevor Mine, Pendeen. The lodes passed through the contact without disturbance, loss of grade or change in mineralogy, apart from cassiterite content (dependent on position relative to the boundaries of the tin zone). The seaward pitch of the tin zone saw several mines in the St Just District follow lodes for considerable distances beneath the sea (after Dines, 1956).
A similar relationship between the granite contact and the economic tin zone was described in the Camborne-Redruth district (Llewellyn, 1946) and at South Crofty Mine (LeBoutillier, 1995), with the tin zone pitching, at a shallow angle, to the north and west, suggesting that granite/killas temperatures, around the contact, had largely equilibrated by the onset of main-stage mineralisation. Within the killas, the deposition of ore minerals was affected by the lithology of the host rocks (Collins, 1912; Alderton, 1993). Lode fractures that passed through arenaceous units rarely saw economic mineralisation deposited within them, while the same lode passing through pelitic host rocks was often mineralised. The classic example of this type of behaviour is shown by the long section of Main Lode in the Chiverton Mines (see Figure 6.16), near Blackwater [SW734463].
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Figure 6.16. Lithological control on mineralisation at the Chiverton Mines. The section is drawn in the plane of Main Lode; the lack of stoping in the sandstone units reflects the barren nature of the lode within beds of this lithology (after Alderton, 1993).
The relationship between lodes and elvan dykes presents every possible permutation (Hosking, 1964, 1988; Henwood, 1843; Collins, 1912). Lodes may intersect (but not fault) dykes, may fault dykes, may be faulted by dykes, may be deflected by dykes, or share a common hangingwall or footwall with a dyke (e.g. Wheal Jane Mine, near Truro). Mineralisation may be diminished on intersecting a lode or may swell, become richer, or form ladder veins and stockworks (see Figure 6.17). The close spatial and temporal association of the lodes and elvans shows that mineralisation and elvan magmatism were broadly contemporaneous and diachronous. At South Crofty Mine, in the North Pool section, an elvan dyke was found to cut reactivated Sn-W replacement bodies and a main-stage Sn lode (No:6N Lode). In another example an elvan dyke, refracted around the granite contact (Taylor, 1963) on the 260 fm level, was host to a series of mineralised stringers (an up-dip continuation of the No:9 Lode) that petered out on passing from the dyke into the killas above. At Wheal Jane and Mount Wellington mines (Rayment et al., 1971; Kettaneh and Badham, 1978) near Truro, mineralisation is intimately associated with the footwall contacts of elvan dykes. Much of the Sn-Cu-Zn mineralisation has been ‘ponded’ or trapped under the footwall of the ‘B’ Elvan and occupies a shear zone along the margin of the dyke (see Figure 6.18). 387
Figure 6.17 Relationships between main-stage lodes and elvan dykes (after Hosking, 1988).
Both the mineralisation and dykes were viewed as infilling fractures generated by the intrusion of the granite batholith, but Cotton (1972) suggested that the host fractures
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may have been pre-granite in origin and formed preferential sites for both dyke emplacement and mineralisation.
Figure 6.18. A section through the B Lode of Wheal Jane Mine. Polymetallic, multi-phase mineralisation has been impounded under the footwalls of the A and B elvan dykes, via feeder lodes and footwallparallel shear zones (after Rayment et al., 1971).
Pre-existing fault systems have played a major role in the tectonic history of the region (Andrews et al., 1998), influencing the formation of sedimentary basins, sites of granitic emplacement and mineralisation. The NNW-SSE trending wrench faults known as fluccans, trawns, crosscourses or guides (depending on area and type of infill) occur throughout the Cornubian Orefield. Although the low temperature chalcedony infill typical of many crosscourses (LeBoutillier, 1996) is late in the sequence of mineralisation in the orefield, the fractures themselves appear to have been active (with little or no infill) from the earliest phases of mineralisation. Hosking (1974) considered that many of the Sn vein systems in Cornwall developed from tension fractures or second-order shears that developed between pairs of wrench faults, and that under shear conditions these fractures could be dilated to form a ladderlike vein system. The distance between the pair of wrench faults determined the strike
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length of the lode, whilst the duration and degree of movement on the faults affected the structural complexity, mineralogy and payability of the lode. The pairs of bounding faults can act as impounding structures, with metal grades significantly raised adjacent to the faults (Carne, 1822) and notable changes in mineralogy within the lode (Henwood, 1843, noted that Malkin Lode of Ding Dong Mine (near Morvah, Penwith) changed in mineralogy on approaching a crosscourse). When a number of wrench faults operate at the same time, the lode fractures develop independently within each pair; however, lithological controls may be such that the lodes developed between one pair is very similar to that between neighbouring pairs.
Figure 6.19. The wrench fault-bound lode system of Wheal Vor, near Breage (after Garnett, 1961).
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Figure 6.20 Diagram showing the influence of crosscourse fracture sets on the formation of the mainstage lodes at Geevor Mine (after Garnett, 1961).
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This has sometimes lead to the pattern being interpreted as a series of lodes dislocated by a later set of transverse faults, when in fact it is the lodes which are the later feature. There are numerous examples (Collins, 1912; Henwood, 1843) where a lode is greatly enriched against a crosscourse, and (when mined through and the assumed extension of the lode is located) on the other side of the fault is low-grade or barren. Perhaps the best example of this phenomenon is shown by Wheal Vor [SW625302], near Breage (Garnett, 1961; Hosking, 1974; Taylor, 1979), close to the Tregonning-Godolphin Granite (see Figure 6.19). Garnett (1961, 1962) considers the mechanism important in the formation of the lodes of Geevor Mine and cites examples where lode widths and grades are elevated adjacent to crosscourses and the lodes beyond are poor or barren (see Figure 6.20).
Figure 6.21. A long section through South Crofty Mine, showing the relative position of the Great Crosscourse and the reserve areas adjacent to it.
The lodes either side of the Great Crosscourse (a composite wrench fault ‘zone’, ~100 metres wide, marked on surface by the valley of the Red River [SW663404]) near Camborne (see Figure 6.21) were considered to be continuations of the same structures (Collins, 1912; Taylor, 1965; Farmer, 1991), but the number of lodes on either side is uneven (though some show similarities in width, texture and mineralogy) and a number of lodes show distinct ‘ponding’ of tin grades against the crosscourse ,e.g. Dolcoath
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Figure 6.22. A plan of the 400 fathom level, South Crofty Mine, showing the position of the Great Crosscourse and the lodes worked. Although the spacing of the lodes on each side and their mineralogy appears similar, grade trends suggest that the crosscourse pre-dated the lodes and influenced their formation. Original drawing by South Crofty Mine Survey Department.
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Main Lode and its subsidiary structures (the Little Crosscourse on the west and the Red River Crosscourse on the east side of the Great Crosscourse) and No:8 lode of South Crofty Mine. Within South Crofty Mine the No:4 Lode and Roskear D lode were equated with each other, on either side of the crosscourse (Farmer, 1991). They share some mineralogical and structural characteristics, but while the No:4 Lode (east of the crosscourse) is highly enriched (with extensive mineralised wallrock) against the crosscourse, the Roskear D lode (west of the crosscourse) carries much lower grades (South Crofty Mine, unpublished data). This suggests that the Great Crosscourse (see Figure 6.22) also predated the lodes and may be of pre-granite origin. Elsewhere within South Crofty Mine (Taylor, 1965), a crosscourse can be seen to intersect both the No:3 and No:4 lodes (in Robinson’s Section, east of Robinson’s Shaft), but only the No:3 Lode is displaced; showing that the fault was active between the deposition of the No:3 Lode (associated with early W-As-bearing replacement bodies and quartz floors) and the No:4 Lode, before being infilled with late-stage chalcedony.
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Chapter 7: The Tectonics and Paragenesis of Mineralisation in SW Cornwall. 7.1 Introduction The Cornubian Orefield stretches from western Dartmoor and North Devon, to Land’s End. It was, from the late 16th to the late 20th Century, one of the most productive P
P
P
P
orefields in the World and was one of the most intensively studied, at a time when theories of ore genesis and zonation were in their infancy. By the time that modern scientific methods and ideas were brought to bear on the on the orefield, almost all of the mines had closed, and the details we now have of particular mines relate largely to their later periods of deep working. As a consequence there are a number of studies that give excellent details of the deeper hypothermal, tourmaline-dominated parageneses within SW Cornwall (Walters, 1988; Holl, 1990; Garnett, 1962; Jackson, 1977; Alderton, 1976; Taylor 1965; Farmer, 1991) at Geevor Mine, Wheal Jane, Wheal Pendarves and South Crofty Mine; but details of the old, early-worked upper levels are lacking. Desk-based studies contributed to theories of zonation during the 20th Century P
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(Davison, 1921; Dewey, 1925; Dines 1934; Hosking, 1964; Willis-Richards and Jackson, 1989; Willis-Richards, 1990) using mine production records to map metallogenic zones and ‘emanative centres’ across the orefield. While this approach gave a ‘broad brush’ picture of the orefield, it was fundamentally flawed. The production details for many mines are scant or non-existent, and only show what the mines sold, not what the lodes actually contained. Ores of metals thought to be of no value or application (which was several until the mid-19th Century, including tungsten P
P
and zinc) were discarded, as were components of complex ores that were unable to be recovered (or if they were, had a detrimental effect on the recovery of other metals). Such components never appeared in the production records, even though they may have made up a significant proportion of the ore at the time of mining. This chapter seeks to redress the balance by detailing the mineralogy of lodes from several districts (including some from outside the main study area) in their near-surface extensions, and examines the tectonic and paragenetic associations within and between the major granites, in terms of a continuation of the tectonic processes that were operating during granite emplacement.
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7.2 The Land’s End Granite. 7.2.1 Introduction.
Figure 7.1. The mineralised districts of the Land’s End Peninsula.
In describing the mineralisation across the Land’s End Peninsula, Dines (1956) divided the area into five mineralised districts, based on local geography and his theories regarding emanative centres (Dines, 1934, 1956). In his ‘The Metalliferous Mining Region of South-West England’ he utilised old mine records and plans to build up a comprehensive picture of the economic geology across the peninsula and the rest of the orefield (extending as far as Somerset), which has formed the basis of much of the later mining-related research conducted across south-west England. This study has focussed on all aspects of mineralisation, not only that which was exploited economically (indeed, most mine workings are no longer accessible), and for this purpose a revised
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version of Dines’ map (see Figure 7.1, above) of the mining districts has been established to illustrate the disposition of mineralised structures across the area. A total of seven districts have been outlined in the current study. These are heavily influenced by Dines’ previous work and proximity to the coast (inland exposures are poor and most of the relatively few, scattered mines, particularly in the southern half of the peninsula, between Penzance and Sennen, were small affairs, now largely obscured); these are:- (1) The St Just-Pendeen District, occupying the NW flank of the peninsula; (2) The Morvah-Sancreed District, occupying most of the rest of the peninsula west of Zennor, except the south coast; (3) The Towednack District, stretching from the Hayle Estuary (and including St Ives) to Porthglaze cove and as far south as Towednack village; (4) The Trencrom District, lying adjacent to the south and extending as far south as the area around Ludgvan; (5) The Penzance-Ludgvan District, encompassing Mount’s Bay as far east as Perranuthnoe; (6) The Lamorna District, running along the south coast of the peninsula from Mousehole to Porthguarnon; and (7) The St Levan District, continuing west along the south coast to Land’s End.
7.2.2 The Towednack District. The Towednack District comprises a roughly rectangular area, bounded by the Hayle Estuary in the east and a line drawn southeast from Porthglaze Cove [SW442385] west of Zennor, in the west, the coastline in the north and an E-W line running south of Towednack village [SW487380] in the south. The area comprises the northeast section of the coarse-grained Zennor Lobe Granite and two areas of fine-grained granite (part of the Castle-an-Dinas Granite and the Knill’s Monument Granite near Carbis Bay [SW519385]) inland, and a belt of killas (slates, greenstones and metabasic volcanics) forming the coastline around the north coast, past St Ives [SW518405] and on into St Ives Bay. The major lode trend is ENE-WSW (normal to the granite/killas contact), with some lodes trending E-W and a later (cross-cutting) set, trending NW-SE. The main-stage lodes are cut by crosscourses (termed ‘trawns’ in this area; Noall, 1993) trending around N-S (see Figure 7.2). The area produced tin and copper and minor amounts of arsenic, pyrite, ochre, zinc and uranium (Dines, 1956). This last was produced from Wheal Trenwith (in St Ives) and some other mines (in the form of pitchblende) and was originally used as a glass colourant (green), prior to its radioactive properties being 397
known. Most of the mines in the west of the district were small and unimportant. The main producers straddle the granite/killas contact close to St Ives. There is a broad zoning pattern observed in the area, illustrated by Rosewall Hill and Ransom United [SW498394], St Ives Consols [SW501397] and Wheal Trenwith [SW509400]. The granite contact occurs in the eastern part of St Ives Consols and the western part of Wheal Trenwith. The mines produced (in order) 1,500 tons of black tin, >16,000 tons of back tin and 450 tons of copper ore, and 20 tons of black tin and >13,000 tons of copper ore (Dines, 1956). The majority of the tin recovered in the district was raised from the granite, and the majority of copper from the killas. Collins (1912) notes that the tin (and copper) zone dips NE at a shallower angle than the granite contact (as in the St Just area) and that the ore shoots pass out beneath the sea. The mines here did not extend seawards like those around Geevor, the fractured killas allowed too much water to percolate into the workings, which were soon inundated and abandoned.
Figure 7.2. A contoured pole plot and rose diagram for all lodes and minor veins in the Towednack District. Lode data collated from Dines (1956) and this study.
The nature of the lodes is poorly recorded. At Wheal Trenwith the lodes consisted of brecciated killas cemented by quartz (crowded with chlorite plates) and copper/iron/zinc sulphides (Dines, 1956). Lodes exposed in the adits around Carbis Bay (Wheal Margery) show a similar morphology. Lodes at Wheal Providence (situated in granite, just south of St Ives) consisted of quartz (crowded with tourmaline) with brecciated schorl rock with cassiterite (Dines, 1956), while the lode at Rosevale Mine [SW458379] near Zennor is composed of massive blue peach and tourmalinite, similar in appearance to the deep lodes of South Crofty Mine. 398
7.2.2.1 Paragenesis: Carnelloe Mine. Carnelloe Mine [SW442387], or Carnelloe Consols, lies on the coast 1.20 km WNW of Zennor (see Figure 7.3). The mine is typical of the small workings along this section of the coastline and was worked only intermittently before finally closing in 1876 (Dines, 1956). A total of 12 lodes are included in the sett, but only two or three are likely to have been worked. These predominantly trend NE-SW and are cut by later NW-SE trending lodes and three N-S crosscourses (Dines, 1956).
Figure 7.3. A sketch map of mine setts and lodes on the coast around Zennor.
Although never vigorously worked or fully explored, the property was the subject of a serious exploration proposal (the mine was thought of as another possible Levant Mine) by Consolidated Goldfields in the 1960’s that only failed when permission was refused to drill on the adjoining Dollar Mine (Noall, 1993). The steeply-dipping (80° SE) NE-SW trending (~042°) lodes (see Figure 7.4) are exposed in the cliffs around Porthglaze Cove [SW44183864] and typically consist of a
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banded quartz-haematite assemblage with chalcedony, tourmaline (schorl) and secondary limonite (see Plate 7.1), varying in width from 0.30-0.50 m.
Figure 7.4. A rose diagram of vein and lode orientations at Carnelloe Mine. Sn-bearing quartz-schorlhaematite ± cassiterite lodes and veins trend 035°-045° and are cut by later quartz-haematite veins trending ~340° and ~010°.
Plate 7.1. One of the lodes of Carnelloe Mine (dipping 042°/80°/SE) exposed in the cliffs. The lode is banded and vuggy at this position and is heavily stained by secondary iron hydroxides. Thin ribbons of dark blue tourmaline can be seen (centre), which become more important and prominent closer to sea level. Rule 0.50 m for scale.
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At adit level (just above the high water mark) the proportion of tourmaline in the lodes is much greater and it forms discreet bands within the lodes, often with dip-slip slickenlines on exposed surfaces, suggesting that these lodes originated (and continued to act) as extensional faults; the growth of quartz crystals in individual layers, normal to the banding, also supports the repeated orthogonal opening of the fault. The banded, or laminated, patterning is indicative of repeated extensional opening (Taylor, 1992; Robert and Poulson, 2000) resulting in punctuated fluid release (due to ‘saw-tooth’ pressure release; Sibson, 2001) and resultant open-space infill. Such textures are radically different from the hydrothermal breccias (Taylor and Pollard, 1993; Taylor, 2000) seen in the deeper levels of mines such as South Crofty, and indicate a less highenergy depositional environment with relatively lower fluid pressures. The lodes at beach level and broken ore on the old dressing floors consists of a banded quartz (+ haematite) gangue containing spots, and larger masses, of chalcocite (Cu2S), B
B
with minor secondary cuprite (Cu2O) and malachite (Cu2(CO3)(OH)2) as coatings (the B
B
B
B
B
B
B
B
extensions of these lodes were reputedly worked for copper at Gurnards Head Mine nearby). The ore also contains a more plentiful cassiterite/quartz/schorl assemblage and this seems to have been the main production ore of the mine (see Plates 7.2 and 7.3).
Plate 7.2. Carnelloe Mine. A large subhedral poikilitic cassiterite crystal (brown), enclosing zoned subhedral schorl crystals, in a quartz matrix. A tourmaline-rich band lies on the right of the photograph. Taken under plane polarised light (PPL) at × 4 magnification. Field of view ~2 mm.
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Plate 7.3. Carnelloe Mine. Large zoned cassiterite crystal. Taken under PPL × 4 magnification.
The ore consists of coarse (up to 5 mm) cassiterite in seams and individual crystals, embedded in quartz. Although schorl is present in the lode, the vast majority of the cassiterite appears to be concentrated in those bands where schorl is a minor component, or appears to be absent (this phenomenon was also seen at South Crofty Mine, where much cassiterite deposition took place in a cassiterite/quartz phase) in hand specimen. Under the microscope (see Plates 7.2 and 7.3, above), cassiterite appears as large, euhedral to subhedral, zoned crystals. The concentric zoning reflects changes in the iron concentration of the ore fluids (Farmer et al., 1991), tinting the crystals from a light to deep brown. Where present, the subhedral to euhedral schorl crystals are also zoned. The morphology, zoning and size of the crystals is indicative of crystallisation within an open space environment with free-flow of ore-bearing fluids through the system. The paragenesis of the ore is a simple one and consists of essentially Schorl ± quartz ± cassiterite, quartz ± cassiterite and quartz ± haematite in a series of depositional episodes. Much of the haematite appears to be a late infilling and appears to post-date the main-stage mineralisation (filling in late cracks within the quartz bands); it may be derived from the alteration of the host metabasic rocks. That haematite enclosed within the lode may possibly be derived, as elsewhere, from the breakdown of chlorite. In any event, the association of haematite with quartz, cassiterite and schorl is a paragenesis typical of the north and west of Penwith (though it most often occurs in the St Just 402
District). A tin assay of a sample of the ore from the dressing floors gave a tin value of 4.5%, which is 50% higher than the (still very rich) run of the mine (R.O.M) ore, which was ~3% Sn (Noall, 1993). On the cliff top, the lodes are accompanied by a swarm of sub-parallel veins (occupying joints in the greenstone), ranging from 0.10 m down to 1 mm in width. Several of the narrower vein (
