The Yellow Mountain Granite has been dated at 420.6±2.8 Ma by Black (2007). Pogson (1991b) concluded that the Yellow Mountain Granite is comagmatic with ...
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GEOLOGICAL SURVEY OF NEW SOUTH WALES DEPARTMENT OF INDUSTRY
A geological and geophysical interpretation of the bedrock units in the Nymagee 1:250 000 map sheet area by GARY R. BURTON (Senior Geologist)
Geological Survey Report No: GS2015/0142 Dated: June 2016 Departmental File No: Map Reference: Nymagee Bobadah Gindoono Kilparney Lachlan Downs Mount Allen Nymagee
1:250 000 1:100 000 1:100 000 1:100 000 1:100 000 1:100 000 1:100 000
SI/55-2 8233 8232 8132 8033 8032 8133
Accompanying Plans: Nymagee 1:250 000 geophysical and geological interpretation map ©
State of New South Wales through NSW Department of Industry, Skills and Regional Development 2016. You must obtain permission from the Department to copy, distribute, display or store in electronic form, any part of this publication, except as permitted under the Copyright Act 1968 (Cwlth).
Disclaimer The information contained in this publication is based on knowledge and understanding at time of writing (2016). However, because of advances in knowledge, users are reminded of the need to ensure that information upon which they rely is up to date. No warranty about the accuracy, currency or completeness of any information contained in this document is inferred (including, without limitation, any information in the document provided by third parties). While all reasonable care has been taken in the compilation, to the extent permitted by law, NSW Department of Industry, Skills and Regional Development and the State of New South Wales exclude all liability for the accuracy or completeness of the information, or for any injury, loss, or damage whatsoever (including without limitation liability for negligence and consequential losses) suffered by any person acting, or purporting to act, in reliance upon anything contained herein. Users should rely upon their own advice, skills, interpretation and experience in applying information contained in this publication. The product trade names in this publication are supplied on the understanding that no preference between equivalent products is intended and that the inclusion of a product name does not imply endorsement by the Department over any equivalent product.
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ABSTRACT The Nymagee 1:250 000 map sheet area (Nymagee sheet) is located in central New South Wales. Geologically, the area consists of Ordovician metasedimentary rocks, with minor mafic units, intruded by Silurian granites which are all variably overlain by Late Silurian to Early (to possible Middle) Devonian clastic sediments, volcanics and volcaniclastic rocks of the Cobar Supergroup. The area was geologically mapped in detail by the Geological Survey of New South Wales in the 1970s and 1980s but detailed geophysical data only became available in the 1990s. This work is an attempt to produce a bedrock, or solid geology, map of the Nymagee sheet by integrating the existing geological mapping with the geophysical data. Aeromagnetic data was particularly useful for delineating near surface features, especially faults, while gravity data was particularly useful for delineating deeper features, especially some granitic bodies. Water bore logs and exploration drillhole data were useful for supporting the interpretation in some areas. The interpretation map produced herein groups various rock packages together by similar age and formation environment. The main results of this study are – •
• • • • • • • •
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Based upon magnetic data and a substantial amount of company exploration drilling, a belt of metasaediments, mafic volcanics and quartz-magnetite rocks, interpreted as part of the Ordovician Girilambone Group, has been identified in the Melrose area. The mafic volcanics are chemically similar to those hosting copper mineralisation at Tritton and Tottenham. It follows that the Melrose area may be prospective for a similar style of copper mineralisation. Also within the Melrose area, based upon company exploration work including aeromagnetics and drilling, the presence of the Fountaindale granodiorite at depth. The presence of the Tollingo granodiorite south of the Erimeran Granite, based upon magnetic data and company exploration drilling. It is here speculated that the Tollingo granodiorite may have been an earlier intrusion stopped out by the Erimeran Granite. The presence of (probable) Silurian granites at depth within the Girilambone Group in the northeastern part of the area as well as small granite bodies at depth adjacent to the Erimeran and Thule granites, and a speculated granite at depth east of Mount Hope. A more detailed distribution of intrusive rocks associated with the Tarran Volcanics and a more detailed distribution of Devonian dolerite dykes. The Scotts Craig Fault is interpreted to run the full north-south length of the Nymagee sheet and may have been active during deposition of the Cobar Supergroup. The Boothumble Formation is interpreted to rest upon and encircle basement highs – the (previously defined) Boothumble Anticline and the (newly defined) Pine Ridge Dome. The interpretation of a high strain zone southeast of the Gilgunnia Granite which may be prospective for Cobar style gold, silver and base metal deposits. The interpretation of the Bogolo Zone – a north-northeasterly trending zone containing dykes of dolerite and associated rocks between the converging Rookery and Hathaway faults and the Nyora Fault south of the Nymagee sheet area. It appears that the Bogolo Zone has acted as a tensile zone between these major faults. Geochemical data supports the interpretation that the Parkvale Gabbro is comagmatic with dolerite dykes which occur throughout the area and that they are intraplate tholeiites. The delineation of Cainozoic magnetic palaeochannels.
The interpretation map may be useful as a framework for mineral deposit studies. Future work to help better understand the geological framework of the Nymagee sheet area should involve the collection and modelling of detailed gravity data and seismic imaging of the top 10 kilometres of the crust.
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CONTENTS INTRODUCTION GEOLOGICAL SYNOPSIS OF THE NYMAGEE SHEET AREA Ordovician (meta)sedimentary rocks Benambran Orogeny Silurian granites Cobar Supergroup Shallow marine clastic and volcanic/volcaniclastic sequences Deep marine clastic and volcanic/volcaniclastic sequences Siluro-Devonian Granites Tarran Volcanics and associated intrusive rocks Early Devonian shallow marine to terrestrial clastic sediments and carbonates Cobar deformation Mulga Downs Group and Kanimblan Orogeny BEDROCK INTERPRETATION METHOD Sources of data Magnetic data Gravity data Radioelement data Drillhole data Process of interpreting the data and constructing the bedrock interpretation map BASEMENT INTERPRETATION Discussion of the stratigraphic units in the bedrock interpretation Ordovician metasediments Erimeran and Derrida granites, Tarran Volcanics and dolerite dykes The Melrose and Tollingo magnetic features Interpreted granites at depth Cobar Supergroup rocks Structural features Relationship between Cobar Supergroup rocks and faulting The Scotts Craig Fault The Rookery and Hathaway faults Structures in the Gilgunnia area Dolerite dykes and the ‘Bogolo Zone’ The Tarran Volcanics Magnetic palaeochannels IMPLICATIONS FOR THE GEOLOGICAL/STRUCTURAL DEVELOPMENT OF THE NYMAGEE SHEET AREA IMPLICATIONS FOR MINERAL SYSTEMS CONCLUSIONS / RECOMMENDATIONS ACKNOWLEDGEMENTS REFERENCES APPENDIX 1 Sample data APPENDIX 2 Geochemical character of an altered basaltic rock from the Rosedale area APPENDIX 3 Geochemical character of the Parkvale Gabbro and a (probable) Devonian dolerite dyke
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List of Figures Figure 1. Location of the Nymagee 1:250 000 map sheet area Figure 2. Bedrock interpretation of the Nymagee 1:250 000 sheet
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page Figure 3. Aeromagnetic data coverage for the Nymagee 1:250 000 map sheet area 16 Figure 4. Total Magnetic Intensity image for the Nymagee 1:250 000 map sheet area 17 Figure 5. First Vertical Derivative image of the TMI data for the Nymagee sheet area 17 Figure 6. Gravity coverage for the Nymagee 1:250 000 map sheet area 18 Figure 7. Isostatically corrected gravity image for the Nymagee map sheet area 19 Figure 8. Radioelement image for the Nymagee 1:250 000 map sheet area 19 Figure 9. Nymagee metallogenic map with water and mineral exploration bores 20 Figure 10. Interpretation of the pre-Cobar Supergroup sequence on the Nymagee sheet 22 Figure 11. Interpretation of the Cobar Supergroup on the Nymagee map sheet area 22 Figure 12. Geological interpretation for the Black Range Syncline area 23 Figure 13. Geological interpretation of the Erimeran Granite 25 Figure 14. Geological interpretation of the Tollingo and Fountaindale areas 27 Figure 15. Geological interpretation for the Erimeran Granite and area to its northeast 28 Figure 16. Geological interpretation for the area around Mount Hope 30 Figure 17. Geological interpretation for the area containing the Thule Granite 31 Figure 18. Geological interpretation of the Boothumble Anticline and the Pine Ridge dome 31 Figure 19. Interpreted extent of the Scotts Craig Fault on the Nymagee map sheet area 34 Figure 20. Geological interpretation for the Gilgunnia Granite and adjacent areas 35 Figure 21. Map showing interpreted extent of the Bogolo Zone 37 Figure 22. Interpreted magnetic palaeochannels on the Nymagee sheet area 38 Figure 23. Cross-section ABC 40 Figure 24. Cross-section DEFG 41 Figure 25. Location and migrated profile of seismic line 09GA-RS2 42 Figure 26. Geological map of the Cobar-Nymagee area 44 Figure 27. Zr/Ti vs Nb/Y diagram for Rosedale sample 64 Figure 28. Spider diagram for the Rosedale altered basalt sample 64 Figure 29. Chondrite-normalised REE plot for the Rosedale altered basalt sample 65 Figure 30. Rosedale altered basalt sample plotted on Th-Hf/3-Ta diagram 65 Figure 31. Rosedale altered basalt sample plotted on Zr/4-2Nb-Y diagram 66 Figure 32. Zr/Ti vs Nb/Y diagram Parkvale Gabbro and dolerite dyke plotted 69 Figure 33. Spider diagram for the Parkvale Gabbro and dolerite dyke samples 70 Figure 34. Chondrite-normalised REE plot for Parkvale Gabbro and dolerite dyke samples 70 Figure 35. Parkvale Gabbro and dolerite dyke samples plotted on Zr/4-2Nb-Y diagram 71 Figure 36. Parkvale Gabbro and dolerite dyke samples plotted on Th-Hf/3-Ta diagram 72 List of Tables Table 1. Rock groupings used to construct the basement interpretation in this study Table 2. Sample data for samples collected during this study Table 3. Thin section descriptions for samples collected during this study Table 4. Results of attempted condont indentifications from cherts Table 5. Whole rock geochemical data Map in back pocket of report Nymagee 1:250 000 map sheet geophysical and geological interpretation
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INTRODUCTION The Nymagee 1:250 000 map sheet area (henceforth referred to as the Nymagee sheet) is located in central New South Wales (Figure 1). The first published geological map of the area was by Brunker (1968), based on aerial photograph interpretation with field traverses. During the 1980s the area was mapped in detail by the Geological Survey of New South Wales and resulted in the production of six 1:100 000 scale geological maps covering the area – Mount Allen (Scheibner 1985), Gindoono (MacRae and Pogson 1985), Nymagee (MacRae 1988), Kilparney (Trigg 1988), Lachlan Downs (MacRae 1989) and Bobadah (Pogson 1991a). This mapping was compiled to 1:250 000 scale for the Nymagee 1:250 000 metallogenic map (Suppel and Pogson 1993) and remains the best available geological mapping for the area. A basement interpretation for the map sheet was given by Suppel and Pogson (1993) based upon the mapped geology, available aeromagnetic data and Landsat data. Subsequent to that work geophysical data coverage of the area has been vastly improved. The geological mapping of the 1980s however, although done without the benefit of detailed geophysical data (particularly magnetics), is considered here to be of high reliability. With the initiation of the Nymagee Mineral Systems project in 2011 it was considered pertinent to have an updated basement interpretation for the Nymagee sheet. This is necessary in order to better understand the geological framework (geological setting and history) for mineral systems studies and to tie in with new radiometric dating. In order to construct this, the existing geological mapping would be combined with an interpretation of the best available geophysical data. This report presents the resulting bedrock interpretation for the Nymagee 1:250 000 map sheet area, with a discussion of the data used, the method employed to undertake the interpretation and a discussion of the pertinent features of the interpretation. The first part of this report provides a synopsis of the geology of the Nymagee sheet. This is then followed by a description of the methods employed in interpreting the geophysical data and integrating it with the existing geological mapping. The third part is a discussion of particular stratigraphic and structural aspects arising from this interpretation. This is then followed by a discussion of the implications for the geological history and mineral systems of the area derived from the geophysical interpretation. In this report all grid references relate to the Metric Grid of Australia (MGA) Zone 55 using the Geodetic Datum of Australia (GDA) 1994 and all directions are relative to true north.
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Figure 1. Location of the Nymagee 1:250 000 map sheet area. Grid is MGA 1994. GEOLOGICAL SYNOPSIS OF THE NYMAGEE SHEET AREA Figure 2 shows the bedrock interpretation for the Nymagee sheet arrived at in this study. The reasoning used to construct the interpretation is given below. A simplified approach has been taken in that individual formations, generally, are not shown. Rather, the map, generally, shows stratigraphic units at either group level or grouped together by their interpreted depositional environment. Table 1 shows the groupings used. In this report the term ‘shallow marine’ refers to water depths where tidal currents and wave action affects the deposited material and incorporates shelf and coastal areas. “Deep marine” refers to water depths were there is no influence of wave action and little tidal current influence. Ordovician (meta)sedimentary rocks The oldest rocks on the Nymagee sheet are Ordovician and consist primarily of deep water turbidites. Suppel and Pogson (1993) assigned the Ordovician units within the Nymagee sheet area to the Girilambone and Tallebung groups. Although no age diagnostic fossils have thus far been identified from the Girilambone Group within the Nymagee sheet area 1, age diagnostic conodonts have been identified in areas adjacent to the sheet and indicate that the Girilambone Group ranges in age from at least Early Ordovician to (at least) late Middle Ordovician (Burton et al. 2012; Colquhoun, Meakin and Cameron. 2005). Graptolites from the Tallebung Group indicate an age range of Darriwilian (late Middle Ordovician) to Eastonian (Late Ordovician) (Trigg 1987 and references therein). Suppel and Pogson (1993) subdivided the Tallebung Group on the Nymagee sheet into the Eulendool Formation, consisting of quartz sandstone, quartzite and interbedded mudstone, siltstone and minor conglomerate, and the overlying Currawalla Formation, consisting of carbonaceous mudstone and siltstone with minor sandstone. 1
Several samples of chert from the Allendoon Chert (GR 468140 6447950) and a sample of chert from an unnamed chert unit from the Girilambone Group (GR 470260 6439780, in the vicinity of Delby homestead) collected by the author during this study failed to provide conodonts for age determinations. Refer to Tables 2 and 4, Appendix 1.
7 General stratigraphic position ?Post Cobar Supergroup rocks Cobar Supergroup rocks
Age Late Early to Middle or Late Devonian Early Devonian
Siluro-Devonian
Pre-Cobar Supergroup units
Silurian Ordovician
GS2015/0142 Rock grouping Mulga Downs Group Shallow marine to terrestrial clastic sediments and carbonates (Winduck, Walters Range and Yarra Yarra Creek groups) Tarran Volcanics and associated intrusive rocks granites Deep water volcanic/ volcaniclastic rocks Deep water (turbiditic) clastic sediments (with very minor volcanics/ volcaniclastics) Shallow marine volcanics/volcaniclastics Shallow marine (locally terrestrial) clastic sediments (minor limestone) (with minor volcanic/ volcaniclastic units) granites (mostly) (meta) sedimentary rocks
Table 1. Rock groupings used to construct the basement interpretation in this study. Note that recent work by Sherwin (2013) suggests that the Mulga Downs Group may be part of the Cobar Supergroup. Colquhoun, Hendrickx and Meakin (2005) suggested that the name “Tallebung Group” should be suppressed with the (Early to Middle Ordovician) Eulendool Formation to be placed in the Wagga Group and the Currawalla Shale (as they referred to it) to be placed in the (Late Ordovician) Bendoc Group. The Wagga Group can be correlated with the Girilambone Group (Burton et al. 2012), therefore the Eulendool Formation may be part of the Girilambone Group. That unit is interpreted to exist beneath Cobar Supergroup rocks throughout most of the area of the Nymagee 1:250 000 map sheet area, west of the Erimeran Granite. The westernmost occurrence of the unit is at GR 372730 mE 6447830 mN (MacRae 1989). Mafic rocks occur in association with Girilambone Group metasediments in the Break O’Day area (Break O’Day Amphibolite) (Pogson 1991 a and b; Burton 2015) and the Melrose area (unit Θgv) within the Gindoono 1:100 000 map sheet area (MacRae and Pogson 1985). Benambran Orogeny The Ordovician rocks have been subjected to the Benambran Orogeny, which has been dated at around 440 Ma (Fergusson et al. 2005). As a result of that deformation event uplift and erosion occurred prior to deposition of the Siluro-Devonian rocks.
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Silurian granites The Nymagee Igneous Complex was emplaced either prior to or during the Benambran Orogeny as parts of it contain multiple cleavages equated with Benambran fabrics within the Girilambone Group (Brown 1975). Attempts to date this intrusion using the SHRIMP zircon U–Pb technique have so far failed to identify a magmatic age. MacRae (1987) indicated that a 440 Ma age is likely, based on the work of Pogson and Hilyard (1981) and S.E. Shaw (pers.comm. to MacRae 1987). The next oldest rocks within the Nymagee sheet are the Silurian Erimeran, Derrida, and Thule granites and the Urambie Granodiorite, all of which are S-type (Suppel and Gilligan 1993, Blevin 1999). Recent SHRIMP U–Pb zircon dating for those bodies indicates an intrusion age for the Urambie Granodiorite of 428 Ma (Bodorkos et al. 2015), 426.3 ± 3.3 Ma for the Derrida Granite (Downes et al. 2016), 425.7 ± 2.4 Ma and 424.1 ± 2.9 Ma for the Thule Granite (Downes et al. 2016) and 424.5 ± 2.6 Ma for the Erimeran Granite (Downes et al. 2016). G. MacRae (pers.comm. 2014) suggested that the Erimeran Granite may be composed of multiple phases of different ages which have not yet been identified in the field.
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Figure 2. Bedrock interpretation of the Nymagee 1:250 000 sheet showing distribution of the main units based upon age and depositional environment. For legend and more detailed information refer to the full map in back pocket. Also shown are cross-section lines ABC and DEFG (Figures 23 and 24).
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Cobar Supergroup Shallow marine clastic and volcanic/volcaniclastic sequences The Cobar Supergroup overlies the Ordovician rocks and Silurian granites. The basal part is characterised by sandstone with intercalated conglomerate lenses of the Mouramba Group and conglomerates and sandstones of the Boothumble Formation (MacRae 1988; Trigg 1988). The Burthong Formation (Mouramba Group) also contains some rhyolitic porphyry lavas and tuffs (MacRae 1988), including the Guapa Tank Rhyodacite Member 2 (Trigg 1988). Those units were deposited within a terrestrial to shallow marine environment (MacRae 1987; Trigg 1987). The Baledmund Formation of the Kopyje Group also contains basal conglomerates and volcanic lenses, similar to the Mouramba Group, though it is dominated by siltstone, and contains minor limestone (the Beloura Tank Limestone Member). The remainder of the Kopyje Group, within the Nymagee sheet area, consists of volcanic units (Babinda Volcanics and Majuba Volcanics) (MacRae 1987, Pogson 1991b). Fossils from the limestone have been assigned to the Encrinurus-Molongia faunal zone (MacRae 1987 and references therein; Trigg 1987 and references therein), which is Late Ludlow to Pridoli in age (Sherwin 1992). The volcanic units present within the Nymagee sheet area have not been dated, however, the Florida Volcanics, which has been assigned to the Kopyje Group on the Canbelego 1:100 000 map sheet area (Felton 1981), has been dated (using SHRIMP U–Pb zircon) at 421.7 ± 2.7 Ma (Black 2005) (Late Silurian) 3. Pickett (1982) determined that the colour alteration of conodonts from the Beloura Tank Limestone Member of the Baledmund Formation suggested that the unit had been heated to 300° to 400° C. One explanation for this could be that the unit was deeply buried and hence there has been substantial erosion of overlying material. However, the possibility that the Tarran Volcanics (see below) may have thermally affected the unit cannot be discounted. The Mineral Hill Volcanics have not been directly dated, though a lava from the Mineral Hill area, informally referred to as the “Freytag Dome” by Blevin (pers. comm. 2012), has a SHRIMP zircon U–Pb age of 417.6 ± 3.2 Ma (Downes et al. 2016). Based on the relationship between the Mineral Hill Volcanics and other igneous bodies which have been dated using the SHRIMP zircon U–Pb method, a mid to late Silurian age of deposition is indicated (Morrison et al. 2004). The Yellow Mountain Granite has been dated at 420.6±2.8 Ma by Black (2007). Pogson (1991b) concluded that the Yellow Mountain Granite is comagmatic with the Majuba Volcanics, based upon their similar chemistries. No fossils have been found in the Majuba Volcanics (Pogson 1991b) but the unit interfingers with and is locally overlain by the Baledmund Formation (Kopyje Group) (Pogson 1991b), which contains, near Bobadah (Pogson 1991b), elements of the Late Silurian Encrinurus-Molongia fauna (after Sherwin 1980 and 1992). Sherwin (1996) placed the Mineral Hill Volcanics in 2
A sample of the Guapa Tank Rhyodacite has been collected for SHRIMP zircon U-Pb dating. A sample of tuff (GR 435230 6431860) from an unnamed unit within the Burthong Formation has also been sampled for SHRIMP zircon U-Pb dating. Refer to Tables 2 and 3, Appendix 1. 3 A sample of the Hennings Tank Tuff Member and another of the Hartwood Tuff Member (both of Baledmund Formation) have been collected for SHRIMP zircon U-Pb dating.Refer to Tables 2 and 3, Appendix 1.
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the Late Silurian, based upon fossils found within sediments from within the Mineral Hill Volcanics which have similarities with the Encrinurus-Molongia fauna. Sherwin (1996) described the Talingaboolba Formation as overlying, but interbedded with in its lowest parts, the Mineral Hill Volcanics. Fossils in the Talingaboolba Formation may indicate a Late Silurian depositional age (Sherwin 1996). The conclusion here is that there is consistent evidence for the start of deposition of the Cobar Supergroup in the Late Silurian with associated volcanism. Blevin and Jones (2004) classified the Mineral Hill, Majuba, Babinda and Florida volcanics as felsic I-types and included them in their “Majuba supersuite”. Deep marine clastic and volcanic/volcaniclastic sequences Deposition in the western part of the Cobar Basin in the Nymagee sheet area was deep water and was dominated by fine-grained turbidites of the Amphitheatre Group. The Lower Amphitheatre Group contains fossils assigned to the Encrinurus-Molongia faunal zone (Sherwin 1977 from MacRae 1987), including a questionable trilobite characteristic of the lowest part of the Cobar Supergroup (MacRae 1987). If correct, this means that deposition of the Lower Amphitheatre Group extends back to the Late Silurian. The Broken Range Group, consisting of turbidites with minor tuffs 4 and lavas, has an interfingering and gradational boundary with the Nombiginni Volcanics (see below) (Scheibner 1987). The Mount Knobby Formation, consisting of turbidites with local conglomerate lenses and rare tuffs, is considered to be similar in age to Kopyje Group rocks (Suppel and Gilligan 1993) 5. It contains the basal Kooranjie Conglomerate Member (MacRae and Pogson 1990). In the southern and western parts of the sheet area voluminous amounts of felsic volcanics were erupted into a deep water environment (the Mount Hope6 group and the Ural Volcanics of the Rast Group). Note that the name Shepherds Hill Volcanics, for the package of rocks mapped and described by Trigg (1987 and 1988), has been suppressed and supplanted by the Ural Volcanics (Meakin, Colquhoun and Cameron 2005). Meakin, Colquhoun and Cameron (2005) stated that the depositional age for the Ural Volcanics probably spanned a considerable period of time, ranging from the Lochkovian (and possibly Late Silurian based upon the zircon date mentioned above) to Early Emsian based upon fossil information and zircon U–Pb dates. The Ural Volcanics are aluminous A-types (Blevin and Jones 2004). The underlying Crossleys Tank Formation is also probably turbiditic (Trigg 1987) and represents part of a fining upward (and deepening) sequence which started with deposition of the Boothumble Formation. The Mount Halfway Volcanics (within the Mount Hope Group) have recently been dated at 422.5 ± 3.6 Ma by SHRIMP zircon U–Pb (Downes et al. 2016), placing them in the Late Silurian, consistent with the fossil age noted above. The Nombiginni Volcanics of A-Type affinity (Blevin and Jones 2004), at the top of the Mount Hope
4
A sample of tuffaceous rock from within the Broken Range Group (GR 398970 6369340) has been collected for SHRIMP zircon U-Pb dating. Refer to Tables 2 and 3, Appendix 1. 5 It is recommended that this unit be field checked as superficially, at least, it appears similar to Girilambone Group rocks. 6 Carol Simpson is undertaking a detailed petrographic study of these rocks.
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Group, has recently been dated at 419.1 ± 3 Ma (Downes et al. 2016), also late Silurian. 7,8 The Shume Formation, which separates the Upper and Lower Amphitheatre Groups, contains fossils ascribed to the Encrinurus-Molongia faunal zone of Sherwin (1980, 1992) (MacRae 1987) and indicates deposition in the Late Silurian (Late Ludlow to Pridoli (Sherwin 1992)). The base of the Upper Amphitheatre Group interfingers with the Mount Halfway Volcanics (MacRae 1987 and Burton 2012) and therefore it must extend into the Late Silurian (based on recent zircon dates). A possible Gravicalymene angustior trilobite has been found within the unit (Brunker 1970 in MacRae 1987) indicating an Early Devonian (Pragian) age 9,10. The Shume Formation has been interpreted to have been deposited as a proximal turbidite derived from the Erimeran Granite and Ordovician metasediments to the southeast (MacRae 1987). This may have tectonic implications as it sits within otherwise distal turbidites derived generally from sources to the north to southwest (MacRae 1987). Siluro-Devonian Granites The Gilgunnia Granite, of S-type affinity (Blevin and Jones 2004), has been dated at 422.5 ± 3.6 Ma (SHRIMP zircon U–Pb, Downes et al. 2016) and is coeval and comagmatic with the Mount Halfway Volcanics (Blevin 1999). Similarly, the Mount Allen Granite has recently been dated by SHRIMP zircon U–Pb at 422.8 ± 2.7 Ma (Downes et al. 2016). Scheibner (1987) described the Mount Allen Granite as intruding and having a gradational contact with the Double Peak Volcanics (Mount Hope Group), further supporting a Late Silurian age for the Mount Hope Group. One problem is with the recently dated A-type Boolahbone Granite (415.8 ± 3.1 Ma, SHRIMP zircon U–Pb, Downes et al. 2016), which Scheibner (1987) described as comagmatic with the Mount Kennan Volcanics (supported by Blevin (1999) and Blevin and Jones (2004) who described both units as A-type) which he placed at the base of the Mount Hope Group. Either the SHRIMP age is not indicative of the magmatic event, or the granite is not comagmatic with the volcanics, or there is a problem with the stratigraphic interpretation, perhaps due to structural reworking. The SHRIMP date is similar to the Tarran Volcanics (see below) which are I-type (Blevin and Jones 2004), though the lower unit of the Tarran Volcanics has A-type characteristics (Blevin 1999), suggesting that there may be a relationship between them. Tarran Volcanics and associated intrusive rocks The Tarran Volcanics has recently been dated by zircon U–Pb SHRIMP at 415 ± 3.9 Ma (Downes et al. 2016), which places them in the Early Devonian, after 7
Fossil-bearing sediments from the Nombiginni Volcanics were sampled by the author at GR397900 6388740, near from where the radiometric date was obtained. L.Sherwin (pers.comm. 2013) identified crinoid ossicles, confirming the marine depositional environment. 8 Samples have been collected from the Ambone, Regina, Double Peak, Coan, Mount Kennan and Goona volcanics for SHRIMP U-Pb dating. Refer to Tables 2 and 3, Appendix 1. 9 A sample of the Shuttleton Rhyolite Member, from the Shume Formation, has been collected for SHRIMP U-Pb zircon dating. Refer to Tables 2 and 3, Appendix 1. 10 Drilling by Peel Mining Ltd has intersected tuff at the base of the Upper Amphitheatre Group in the Mallee Bull area and this is amenable to sampling for SHRIMP zircon U-Pb dating.
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deposition of the shallow water sequences in the east and the major pulse of volcanism of the Mount Hope Group, but probably during deposition of the Upper Amphitheatre Group and before deposition of the Winduck, Walters Range and Yarra Yarra Creek Groups. As mentioned above, their I-type to A-type chemistry (Blevin 1999, Blevin and Jones 2004) and age indicates a possible relationship with the Boolahbone Granite, although they are situated some distance away from each other. As the Tarran Volcanics lie directly upon the Erimeran Granite and Ordovician rocks, east of the Rookery Fault, it is concluded that that area had either experienced nil sediment deposition or it had been an erosional area, with or without earlier deposition of Cobar Supergroup rocks, prior to 415 Ma. Dolerite dykes up to 10 m wide occur mainly within the Erimeran Granite (MacRae 1987, Trigg 1987, Pogson 1991a) though some also occur within the Cobar Supergroup (Trigg 1988). Their age is uncertain but they cross-cut porphyry dykes associated with the Tarran Volcanics and have been regionally metamorphosed (Pogson 1991b), suggesting that they are Early Devonian in age (Pogson 1991b). The Parkvale Gabbro, which clearly postdates the Majuba Volcanics (Pogson 1991b and field observations by the author) may be related to these dykes. Geochemical data collected during this study (Appendix 3) supports this possibility. Early Devonian shallow marine to terrestrial clastic sediments and carbonates The last phase of Cobar Supergroup deposition was shallow marine to terrestrial and comprises the (now) spatially separate units of the Winduck and Walters Range 11 groups. TheYarra Yarra Creek Group12, which is age equivalent, occurs in the eastern part of the sheet. All three groups contain fossils of the Howellella jaqueti faunal zone, indicating a late Silurian to Early Devonian (Late Pridoli to Pragian, Sherwin 1992) depositional age (Scheibner 1987; MacRae 1989; Pogson 1991b). However, Sherwin (1992) noted that fauna of the “Podolella” assemblage (Lochkovian) is present in the mid Winduck Group and that the Yarra Yarra Creek Group contains the Reeftonia (Pragian) and Spinella Pittmani (late Pragian to early Emsian) assemblages. These ages suggest that deposition of these groups had begun while other parts of the Cobar Supergroup were being deposited. The Winduck Group is shallow marine. Although its contact with the underlying Amphitheatre Group has not been observed it is inferred to be conformable, based on the age similarity of the two groups (MacRae 1989). The Walters Range Group sediments have been interpreted to have been deposited within a shallow marine environment with possible fluvial input (Scheibner 1987). Its contact with the underlying Amphitheatre Group is not exposed. Pogson (1991b) described the Yarra Yarra Creek Group as varying upward from basal outwash fan to offshore fine sands and silts followed by fluvial deposition. Pogson (1991b) described its contact with the underlying Majuba Volcanics as disconformable and probably unconformable and its contact with the Mineral Hill Volcanics and Tallingaboolba Formation as probably 11
A sample from the Walters Range Group has been collected for SHRIMP zircon U-Pb dating of detrital zircons for provenance dating. Refer to Tables 2 and 3, Appendix 1. 12 A sample from the Yarra Yarra Creek Group has been collected for SHRIMP zircon U-Pb dating of detrital zircons for provenance dating. Refer to Tables 2 and 4, Appendix 1.
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unconformable. Sherwin (1996) described its contact with the underlying Tallingaboolba Formation as paraconformable to low angle unconformable. The Yarra Yarra Creek Group also rests unconformably upon rocks of the Girilambone Group (Pogson 1991b). These relationships suggest that the earlier deposited shallow water sequences in the east may have been exposed prior to deposition of the Yarra Yarra Creek Group, but further west, the Winduck Group (and possibly Walters Range Group) represent a shallow water continuum of deposition from the Amphitheatre Group. At the Wonowinta silver mine the Winduck Group lies directly upon the Thule Granite and contains a basal arkose derived from the granite (Skirka 2002). This indicates that the granite was exposed and eroded prior to deposition of the Winduck Group in that area. The Mountain Dam Limestone (GR 447600 mE 6395260 mN) also rests directly upon basement rocks (Trigg 1988). Its age is early to mid Early Devonian (Trigg 1987 and references therein). Pickett (1983) gave an Early Pragian age for the limestone based on the conodont assemblage. Interestingly, Pickett (1982) determined from the conodont colour alteration that the unit had been heated to 300° to 400° C suggesting that it had been deeply buried, lending weight to the interpretation that the thickness of the Cobar Supergroup and any overlying units was far more substantial than is seen today. Cobar deformation Rocks of the Cobar Supergroup and older were subjected to the Cobar deformation (Scheibner and Basden 1998). The timing of this event is somewhat uncertain. Glen, Dallmeyer and Black (1992), using K-Ar and 40Ar/39Ar dating concluded that deformation occurred between 395 and 400 Ma (Late Early Devonian). Sun et al. (2000) questioned this conclusion and preferred an age range of 385 to 389.2 Ma for the deformation, based on their 40Ar/39Ar dating of samples from Endeavor (formerly Elura) mine and a review of Glen, Dallmeyer and Black’s (1992) data. Sun et al. (2000) rejected the 395-400 Ma deformation age range of Glen, Dallmeyer and Black (1992) on the basis that it was older than the depositional age of the Florida Volcanics, which they took as being 391±6.8 Ma based on their K-Ar age of biotite from that unit. However, that date is inconsistent with recent SHRIMP U–Pb zircon dating of the Florida Volcanics (421.7 ± 2.7 Ma), as discussed above. It is possible that Sun et al’s (2000) K-Ar date is a deformation age in line with that of Glen, Dallmeyer and Black (1992). A Rb-Sr age of 362±25.2 Ma obtained by Sun et al. (2000) for the Peak rhyolite, at the Peak mine south of Cobar, is imprecise and inconsistent with a recent SHRIMP U–Pb zircon date of 423.0±3.5 Ma (Black 2007). Mulga Downs Group and Kanimblan Orogeny The Mulga Downs Group is present on the western edge of the Nymagee sheet. Powell, Khaiami and Scheibner (1987) considered the Mulga Downs Group to range in age from late Early to Late Devonian, possibly even to Early Carboniferous. However, recent work by Sherwin (2013) favours an age range from Early Devonian (Pragian-Emsian boundary) to Middle Devonian (no younger than the Eifelian).
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Its contact with the underlying Cobar Supergroup rocks has been described as conformable through disconformable to locally unconformable by Powell, Khaiami and Scheibner (1987). Field observations by the present author and L. Sherwin made in October 2013 support a conformable contact between the Cobar Supergroup and Mulga Downs Group at GRs 381330 mE 6465220 mN and 380490 mE 6434350 mN. The Mulga Downs Group is interpreted to have been affected by the Kanimblan Orogeny during the Carboniferous (Scheibner and Basden 1998), but if the upper age limit ascribed by Sherwin (2013) is correct and the dating of Sun et al. (2000) (as discussed above) has merit, then it is possible that the major structures present within it are correlatable with the Cobar deformation.
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BEDROCK INTERPRETATION METHOD Sources of data The Nymagee 1:250 000 bedrock interpretation map is based primarily upon existing geological mapping. The six 1:100 000 geological maps which comprise the Nymagee sheet are considered to be reliable and therefore the resulting interpretation should be consistent with them. However, as there is much Cainozoic cover in the Nymagee sheet area geological boundaries needed to be inferred from geophysical data i.e. aeromagnetic imagery and gravity data. Magnetic data The best available aeromagnetic data for the Nymagee sheet area exists in two parts (Figure 3). The westernmost part of the sheet was flown at 400 m line spacing and 80 m flight height and released in 1996. The majority of the sheet, however, was flown at 250 m spacing and 60 m flight height and released in 1999. The data has been processed and is displayed as a Total Magnetic Intensity (TMI) image in Figure 4. Magnetic features have been reduced to the pole (RTP) so that peak magnetic responses are generally over the centre of the source body and the maximum gradients are situated above the edges of the source body (Isles and Rankin 2008). The first vertical derivative (1VD) of the TMI data is presented in Figure 5. The 1VD acts as a high pass filter (Isles and Rankin 2008) which helps to delineate near surface features.
Figure 3. Aeromagnetic data coverage for the Nymagee 1:250 000 map sheet area
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Figure 4. Total Magnetic Intensity (TMI) image reduced to the pole (RTP) for the Nymagee 1:250 000 map sheet area. Blue colours reflect relative magnetic lows while red colours reflect relative magnetic highs.
Figure 5. First Vertical Derivative (1VD) of the TMI RTP data for the Nymagee 1:250 000 map sheet area.
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Gravity data Gravity data coverage for the Nymagee sheet is variable, ranging from 5 km X 5 km station spacing to 2 km X 4 km station spacing with some areas covered by 1 km spaced stations along tracks (Figure 6). Figure 7 shows the isostatically corrected Bouger gravity image (Spencer and Musgrave 2006) for the Nymagee sheet area.
Figure 6. Gravity coverage for the Nymagee 1:250 000 map sheet area
Radioelement data Figure 8 is an image of the Nymagee sheet area showing the distribution of potassium (red), thorium (green) and uranium (blue). The data reflects surface features and its resolution is quite poor, hence it is not particularly useful for interpreting the geology beneath unconsolidated cover.
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Figure 7. Isostatically corrected gravity image for the Nymagee 1:250 000 map sheet area
Figure 8. Radioelement image for the Nymagee 1:250 000 map sheet area. Red areas are relatively rich in potassium; blue areas are relatively rich in uranium; green areas are relatively rich in thorium; white areas have equal amounts of all three elements
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Drillhole data Geological logs from water bores were compiled from information available via the New South Wales Natural Resource Atlas website (http://nratlas.nsw.gov.au). The locations of the bores are presented in Figure 9. Many bores have no logs. Those that do are of limited use as 1) the quality of the information can be difficult to assess; 2) the logged information sheds no further light on the geology than would the existing geological mapping; and 3) the representativeness of a particular hole can be difficult to assess. However, some drillholes which penetrated granite were found to be useful for interpreting granite at depth. Figure 9 also gives the locations of mineral exploration drillholes. The geological logs for a selection of these holes were examined, but in general the lithologies intersected are as expected from the available geological mapping. In a few cases (e.g. Fountaindale and Tollingo areas) mineral exploration drilling has been useful for the interpretation.
Figure 9. Nymagee 1:250 000 metallogenic map (Suppel and Pogson 1993) showing the locations of water bores and mineral exploration bores.
Process of interpreting the data and constructing the bedrock interpretation map As described above, the Nymagee sheet can be separated into three geological components: 1) The Cobar Supergroup rocks of Late Silurian to Early Devonian age; 2) Pre-Cobar Supergroup rocks, which consist of the Ordovician Girilambone and Tallebung Groups and several Silurian granitic intrusions; and 3) the Mulga Downs Group, which has previously been interpreted to post-date the Cobar Supergroup (e.g. Powell, Khaiami and Scheibner 1987) but may in fact be part of it (Sherwin 2013). It
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was decided that the interpretation should attempt to separate out these three components so that they could be examined in isolation. Because the Mulga Downs Group only occurs at the western extremity of the map sheet area, where aeromagnetic data coverage is generally of low resolution, it was considered that nothing was to be gained by placing the Mulga Downs Group into a separate layer. Therefore, the interpretation is divided into two separate GIS layers - one for the preCobar Supergroup basement (Figure 10) and one for the Cobar Supergroup and Mulga Downs Group (Figure 11), which, when combined, show the complete surface geology for the area (Figure 2). The interpretation was carried out in Arcview 10 with onscreen digitisation of interpreted lines. Each 1:100 000 geological map was examined and geological boundaries, using the groupings given in Table 1, were digitised. In places where geological boundaries and faults were reasonably well constrained by geological mapping, it was labelled as approximate. Where boundaries and faults were not well constrained by geological mapping, due to lack of exposure, they were interpreted and it was at this stage that the geophysics was examined for clues as to where unexposed boundaries could be. The magnetic data (both TMI RTP and 1VD) was used to interpret geological boundaries at or near the surface. This was done with varying degrees of confidence and the lines are generally coded as inferred. Obscured faults and granite-sediment contacts are generally more confidently interpreted due to the general contrast between rock types while obscured sediment-sediment and sedimentvolcanic boundaries, in which the magnetic contrasts are weak, are less confidently interpreted. The Cobar Supergroup layer was interpreted from existing mapping and aeromagnetic data only. The Pre-Cobar Supergroup layer was similarly interpreted, however, gravity data was used to delineate the boundaries of granitic bodies at depth. Granites, such as the Erimeran and the Derrida, produce distinct relative gravity lows. Once the interpretation process was completed the digitised lines and label points were combined to produce polygons of geological units.
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Figure 10. Interpretation of the pre-Cobar Supergroup rock sequence on the Nymagee 1:250 000 map sheet area.
Figure 11. Interpretation of the Cobar Supergroup and associated rock sequence on the Nymagee 1:250 000 map sheet area.
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BASEMENT INTERPRETATION A map of the final interpretation of the basement geology of the Nymagee sheet is provided in the back pocket of this report and partly reproduced in Figure 2. Discussion of the stratigraphic units in the bedrock interpretation Ordovician metasediments The extent of the Girilambone Group has been interpreted in this study much as has already been mapped and dominates the eastern part of the map sheet area. The Eulendool Formation has been retained as a separate unit, though it is most likely part of the Girilambone Group, and has been interpreted as being present beneath cover throughout the central and western parts of the Nymagee map sheet. The occurrence of Bendoc Group rocks surrounded by older Eulendool Formation (Girilambone Group) rocks in the southeastern part of the sheet, suggests that there is a north-northwesterly trending (faulted) syncline (the Black Range Syncline, Figure 12) which has been overprinted by the Derrida Granite and part of the Erimeran Granite and has been disrupted by later faulting. It must close at its northern end implying that it is southerly plunging. This structure most likely formed during the Benambran Orogeny.
Figure 12. Geological interpretation for the Black Range Syncline area Erimeran and Derrida granites, Tarran Volcanics and dolerite dykes The extents of the Erimeran and Derrida granites are clearly revealed by their relatively low gravity characteristics (Figure 7) but they are fairly neutral in their
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magnetic features (Figures 4 and 5). The strong relatively magnetic high features which dominate the northeastern and southeastern parts of the Erimeran Granite are here attributed to the Tarran Volcanics, their associated intrusive rocks and dolerite dykes (Figure 13). The 1VD imagery helps to clarify this relationship. By comparing the magnetic response with the mapped distribution of those volcanics and intrusive rocks it can be easily ascertained that similar magnetic features, particularly in areas of alluvial cover, are most probably also due to those rocks. The broadness of the TMI magnetic highs may be due to the erosional spreading of magnetic material from those bodies. The Tarran Volcanics and associated intrusions are responsible for eastnortheasterly and northwesterly trending linear magnetic highs while the dolerite dykes are responsible for northeasterly and northerly trending (and rare northwesterly trending) linear magnetic highs. This is discussed further in the Structure section below. Porphyry intrusions southeast of Nymagee (between GRs440450 6450000 and 443230 6443130) have been mapped and described by MacRae (1987, 1988). They intrude the Burthong Formation and it is here speculated that they may be related to the Tarran Volcanics as, like them, they have associated relative magnetic highs. The Melrose and Tollingo magnetic features The Melrose magnetic anomaly is a strong northwest-trending magnetic high on the eastern side of the Erimeran Granite (e.g. Mackenzie, Hill and Randell 2004) (Figure 14). MacRae and Pogson (1985) indicated that the limited outcrop within this area consists of Girilambone Group rocks with volcanics indicated by them to be andesitic. Field work by the present author identified quartz-magnetite rock at GR 481480 6399220, in association with a mafic volcanic rock. Petrology and whole rock geochemistry (Appendix 2) indicates that the mafic rock is an altered basalt which probably originated at a mid-ocean ridge contaminated by a plume component, similar to those associated with copper mineralisation in the Tritton and Tottenham areas (Burton 2011, Burton 2014). Aircore drilling by Triako Resources Limited indicates that the non-outcropping rocks in the Melrose magnetic anomaly consist mainly of siltstone/phyllite with andesite and subordinate carbonaceous shale (Mackenzie and Pienmunne 2008). Aircore drilling in the area by Paradigm Metals also intersected mafic volcanics (Logan 2011, Ashley 2011). It is therefore here interpreted that the Melrose magnetic anomaly is caused by Girilambone Group metasedimentary rocks with associated mafic volcanics and quartz magnetite rocks. The area has similarities to the Break O’Day area (Pogson 1991b) with the occurrence at Rosedale (Triako Resources Limited reverse circulation drillholes TYM046 to 056, 060 and 061) of garnet skarn-altered ‘andesitic’ volcanic and quartz fluorite veining adjacent to the Erimeran Granite (Mackenzie, Miller and Randell 2004) and andalusite spotting in the country rocks attributed to contact metamorphism by the Erimeran Granite (Worden and Carman 2010). However, Burton (2015) has shown that the Break O’Day Amphibolite probably originated as an oceanic island. It is noted that Blevin (2005b) described volcaniclastic rocks from the Fountaindale and Rosedale areas as being petrographically and chemically similar to those of the Macquarie Arc.
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Figure 13. a) TMI RTP image over the Erimeran Granite; b) 1VD image of same area; c) geological interpretation of the same area with selected rock types shown in legend The Fountaindale granodiorite is situated at depth and has an associated circular relative magnetic low (Figure 14). Blevin (2004) described rock samples obtained from drilling from this area as consisting of biotite and hornblende-bearing I-type granodiorite. A Re-Os date of 424.7±1.5 Ma was returned from a molybdenite sample from the Fountaindale granodiorite (Blevin 2003). Norman (2004) determined a 420±2 Ma zircon U–Pb date (using the laser ablation ICPMS method) for the Fountaindale granodiorite, but stated that the crystallisation age could be as old as the
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Re-Os date. The circa. 425 Ma date is the same as the recent SHRIMP age for the Erimeran Granite (as mentioned above). Mackenzie and Pienmunne (2008) noted that three other satellite intrusions had been intersected in drilling 1.2 km west, 2.8 km southeast and 4.4 km south-southeast of the main Fountaindale body. A magnetic high running east-west along the southern edge of the Erimeran Granite has been named the Tollingo aeromagnetic anomaly (e.g. Mackenzie and Pienmunne 2006) (Figure 14). Superficially it appears to be very similar in character to the Melrose magnetic anomaly. Gunn (2004) considered that the best explanation for the source of the Tollingo magnetic anomaly was a magnetic phase of the Erimeran Granite draped over less magnetic granite, and that it is thick in the south (where the magnetic feature is strongest) and thins towards the north (with a more subdued but still relatively high magnetic character). However, aircore drilling by Triako Resources Limited intersected granodiorite directly beneath unconsolidated cover material in this area, which Blevin (2005) described as an I-type similar to the Fountaindale granodiorite. Mackenzie and Pienmunne (2006) considered that this finding invalidated Gunn’s (2004) interpretation. Water bore GW016235 (GR 471180 mE 6385300 mN; Figure 14), situated in the more subdued magnetic zone to the north of the more intense magnetic high, penetrated 85 m of sandstone (interpreted here as Ordovician) above granite, hence that zone is here interpreted as consisting of the Tollingo granodiorite underlying a veneer of Ordovician metasediments. If the Tollingo and Fountaindale granodiorites are part of the same system it may be the case that they are both part of an early I-type granodioritic intrusion which was subsequently stoped out by the Erimeran Granite to leave only the eastern and southern edges intact. Interpreted granites at depth Relative gravity lows in the eastern part of the Nymagee map sheet area (Figure 15) are here interpreted as being caused by granitic bodies, possibly related to the Erimeran Granite, at depth i.e. within the Girilambone Group rocks such that they would have no surface exposure if the unconsolidated and Siluro-Devonian cover materials were removed. Apart from the gravity lows (similar to that for the Erimeran and Derrida granites) evidence supporting this interpretation is the presence of granite (probably dykes) within water bores GW002763 (GR 469950mE 6438540mN (at 46m depth)) and GW002716 (GR 475156mE 6440370mN (at 55m depth)). Other water bores have been drilled above these interpreted granite bodies but are not recorded as having intersected granite. Bore GW014156 (GR 479988mE 6420674mN) has been collared over the lowest of these gravity features where an obscured granite body, here termed the Merrimbah granite, is considered most likely to be. The hole went to a depth of 74 m without penetrating granite, suggesting that if a granite body is present its top is at least that deep. Other drillholes extend between 64 and 85 m depth without intersecting granite. It is here suggested that these interpreted granite bodies could be further investigated by looking for any evidence of contact metamorphism in the Girilambone Group rocks in their vicinity.
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Figure 14 a) TMI RTP image over Tollingo and Fountaindale magnetic anomalies; b) 1VD image for the same area; c) geological interpretation with reference showing selected units.
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A small, strong gravity low adjacent to the Erimeran Granite (GR 426750 mE 6405240 mN) is likely caused by a small granitic intrusion, here termed the White Tank granite (Figure 16). Whether this is part of the Erimeran or is a separate (younger?) body is unknown. It has no corresponding magnetic high. G. MacRae (pers.comm. 2014) has speculated that it may be associated with the overlying Guapa Tank Volcanics. Based upon mapped outcrops of Thule Granite centred on GRs 380230 6438020 and 381810 6433800 (MacRae 1989) and gravity data, it is interpreted that the Thule Granite extends westward, at depth, from its main area of exposure, beneath Cobar Supergroup rocks (Figure 17). Gravity data (the presence of relative gravity lows) also suggests that granite extends well to the southwest and south of the exposed area of the Thule Granite. A prominent circular gravity low centred at GR 375310 6390260 is most likely a stock-like granite body, here termed the Red Tank granite (Figure 16). The gravity feature is very similar to that for the White Tank granite and like that one, this feature has no corresponding magnetic feature. The inference is that both granites are of the same type. Another granite body is interpreted to exist beneath Cobar Supergroup rocks approximately 10 km east of Mount Hope, along the Scotts Craig Fault, based on the presence of a relative gravity low (Figure 16). However, this interpretation is speculative.
Figure 15 a) Isostatically corrected Bouger gravity image for area including the Erimeran Granite and area to its northeast with the locations of water bores indicated; b) geological interpretation for the same area with reference showing selected units.
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Subtle magnetic features (specifically, a smoother TMI character for the granite as opposed to a noisier TMI character for the surrounding rocks) suggest that the Gilgunnia Granite may extend laterally beneath sedimentary and volcanic rocks of the Cobar Supergroup for up to approximately 2.5 km from its exposed area. The lack of a corresponding gravity feature (at the resolution of the data) associated with this and other Siluro-Devonian granites could be due to the lack of density contrast between the granites and their country rocks and/or that they are not deeply rooted. Cobar Supergroup rocks The geophysical data has been of very limited value for interpreting the spatial distributions of Cobar Supergroup rocks. This is probably because, apart from the Tarran Volcanics and their associated intrusive phases, and dolerite dykes, there is very little magnetic contrast between the units and they are probably not of any substantial thickness to produce detectable gravity features at the resolution of the data. Magnetic data is mainly useful for delineating some structures, but in the main the distribution of Cobar Supergroup rocks as shown in this interpretation follows the existing mapped distribution. An exception to this general statement occurs in the southern part of the Nymagee sheet area where shallow water sedimentary rocks of the Boothumble Formation (as mapped by Trigg 1988) appear to have an associated relative magnetic high (Figure 18). This is particularly clear on the 1VD imagery and the magnetic data suggests that the Boothumble Formation in that area continues beneath cover and rests upon a north-northwest trending basement high. Trigg (1988) named this feature the Boothumble Anticline. A similar feature (here referred to as the Pine Ridge Dome) is interpreted from existing mapping in the vicinity of GR 426000 6383000. The Mount Knobby Formation is suspicious in that one outcrop (GR 491260 6358020) of it examined by the author looked very much like the Girilambone Group in that it consists of graded beds containing at least two cleavages. Exposures of this unit should be investigated to help clarify this apparent anomaly.
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Figure 16 a) isostatically corrected Bouger gravity image for the area around Mount Hope, showing roads; b) geological interpretation, showing only basement features, for the same area.
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Figure 17. a) isostatically corrected gravity image for the area containing the Thule Granite on the Nymagee sheet; b) geological interpretation for the same area.
Figure 18. a) 1VD image of the area of the Boothumble Anticline and the Pine Ridge dome; b) geological interpretation for the same area.
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Structural features The magnetic data has been particularly useful for determining the distribution of faults as these are revealed either as linear zones of relatively low magnetic intensity or as distinct discontinuities across magnetic trends attributable to lithological features. The aeromagnetic data accurately reveals the positions of faults and also indicates that what have been previously interpreted as individual faults are, in many cases, actually complex networks of interconnected faults which are better described as fault zones. Also, the data shows that these fault zones bifurcate and anastomoze so that defining the end points of any particular fault is difficult and naming particular faults or fault zones is quite subjective. Relationship between Cobar Supergroup rocks and faulting MacRae (1987, p 57) stated that “In the northeastern quarter of the [Nymagee 1:100 000] sheet [from GR 440950 6453700 to GR 446590 6438120], the Burthong Formation is conformably underlain by the Baledmund Formation. The Baledmund Formation consists of massive to finely laminated siltstones, and the boundary with the sandstones of the Burthong Formation is placed where sandstone becomes the dominant lithology. This boundary can be confidently recognised even when scree is being mapped”. In this study the contact between the Burthong and Baledmund Formations in that area is interpreted to be a fault, presumably an east-dipping thrust. Similarly, the boundary between the Roset Sandstone and the Burthong Formation in the area between GRs 423730 6411860 and 424840 6405390 is here interpreted to be a fault. The belt of Kopyje Group rocks running northwesterly in the northeastern part of the Nymagee area, as well as their immediate basement rocks, have been substantially affected by the northwesterly trending Cumbine, Walton, Walkers Hill and Yellow Mountain fault zones. The Melrose Fault Zone strongly affects the southern continuation of the Kopyje Group rocks in the eastern part of the Nymagee sheet area. The Jackermaroo Fault Zone, in the northwestern part of the Nymagee area, has produced apparent structural complexity in rocks of the Winduck and Mulga Downs groups. Numerous northerly trending faults affect the Mount Hope and Amphitheatre groups in the western part of the study area. The Scotts Craig Fault The Scotts Craig Fault (Figure 19) to the east of Mount Hope is apparent in the magnetic data as a subtle north-south oriented linear trend. It is also apparent in the gravity data as a boundary between a relative gravity high to its west and a low to its east. In the vicinity of Gilgunnia the Scotts Craig Fault is lost among a swarm of northeast-trending faults but it is notable that existing mapping indicates that the Mount Hope Group volcanic rocks in the southwestern part of the sheet area terminate, but with an interfingering relationship with the Amphitheatre Group (MacRae 1987 and 1989, Burton 2012), along the extrapolated position of the fault. North of Gilgunnia the fault is difficult to trace but there is a north-south trending relative gravity high, the eastern edge of which follows the extrapolation of the Scotts Craig Fault onto the Cobar 1:250 000 map sheet to link with the Rookery Fault in the vicinity of the Queen Bee mine.
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The implication is that the Scotts Craig Fault is a relatively old structure. It may have been initiated in the Benambran Orogeny and/or it may have been active during deposition of the Cobar Supergroup and controlled the distribution of the Mount Hope Group to its west. In the southwestern part of the Nymagee sheet area magnetic data indicates that Tertiary palaeochannels to the west of the Scotts Craig Fault drain easterly and appear to terminate against the fault (Figure 22). This suggests that there has been some movement along the fault either during or since deposition of those channels. It is highly probable that other structures like the Scotts Craig Fault were initiated during the Benambran Orogeny and post-Benambran deformation has reactivated those structures at various subsequent times. However, in the main, it is not a simple matter to deduce which structures were already present when the Cobar Supergroup rocks were deposited. The Rookery and Hathaway faults In this study the Rookery Fault as defined by MacRae (1988) has here been labelled the Hathaway Fault and the structure labelled the Ironbark Fault by MacRae (1988) has here been labelled the Rookery Fault as this appears to be more closely aligned with the Rookery Fault as mapped by Felton, Brown and Fail (1983). However, as mentioned above, the naming of individual faults in areas of such structural complexity is subjective. The Rookery and Hathaway faults converge and appear to terminate within the Erimeran Granite and form the northeastern boundary to the Bogolo Zone (see below). The Tarran Volcanics rest ontop of the Erimeran Granite east of the Hathaway Fault with only intrusive equivalents of the Tarran Volcanics occurring to the west of it. It may therefore be the case that there was a component of west-block-up movement along the Hathaway Fault subsequent to the deposition of the Tarran Volcanics. Structures in the Gilgunnia area Interpreted northeast-trending faults in the Gilgunnia area, one of which continues further north and arcs around the eastern end of the Gilgunnia Granite, may reflect strain partitioning adjacent to the granite body (Figure 20). A series of vein style mineral occurrences, the most significant of which is at the May Day mine, occurs in that area over a strike length of about seven kilometres. Mapping at the May Day mine (Burton 2012) suggests that mineralisation there is associated with a northeasterly trending shear in a zone of high strain. It is quite likely that the May Day mineralisation was emplaced into a transpressive zone, similar to that proposed by Glen (1990) and Smith (1992) for the Cobar mineral field. It may therefore be the case that the Gilgunnia area is prospective for the further discovery of Cobar type mineral occurrences. The current interpretation suggests that the Boolahbone and Coan granites have been sheared along northerly trending faults either during or after their emplacement. This is consistent with observations made by Scheibner (1987).
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Figure 19. Interpreted extent of the Scotts Craig Fault on the Nymagee 1:250 000 map sheet area
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Figure 20. geological interpretation for the Gilgunnia Granite and adjacent areas Dolerite dykes and the ‘Bogolo Zone’ Based upon existing geological mapping and the present geophysical interpretation, it is clear that numerous dolerite dykes are present in the eastern part of the Nymagee sheet. They mainly occur within the Erimeran Granite and apparently occupy two main regions – i) the eastern lobe of the Erimeran Granite, where north-northeasterly
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trending dolerite dykes are conspicuous in the TMI and 1VD data; and ii) within a north-northeasterly trending linear zone, up to about 14 km wide, referred to here as the Bogolo Zone (Figure 21). Within this zone dolerite dykes trend northerly to northnortheasterly with a few trending north-northwesterly. The northern end of the zone terminates at the Rookery Fault while to the south, it continues through the area of Fifteen Mile Tank (GR 434030 6352150), where dykes have been mapped trending north-northeasterly, through the Boothumble Formation (Trigg 1988). Trigg (1987) also noted that geophysical studies and drilling carried out by Electrolytic Zinc Co. of Australasia Ltd had identified other, north-northwesterly trending dykes in the Ural Volcanics in the vicinity of GR 424610 6362490. The southern end of the Bogolo Zone terminates at the Nyora Fault on the Cargelligo 1:250 000 sheet, where similar, but east-northeast trending, dykes have been mapped within the Ural Volcanics in the area of GR 425000 6305000 (Meakin et al. 2006). Trigg (1987) noted that the dykes have vertical dips. He also noted that a subvertical foliation in the Urambie Granite trends between 000º and 020ºM, which is parallel to the trend of the Bogolo Zone. The foliated phase of the Erimeran Granite also occurs within this structural zone but the foliation of that unit trends between 030º and 090ºM (Trigg 1987), and is probably older. Both Trigg (1987, and references therein) and Meakin (2005) noted that the dykes actually span a range of compositions including dolerite, monzonite, quartz monzodiorite, monzodiorite, diorite, gabbro, syenite and microdiorite. It is clear that they post-date deposition of the Boothumble Formation and Meakin (2005) considered that they probably post-dated deposition of the Ural Volcanics. Pogson (1991b) interpreted the presence of chlorite, carbonate and ?magnetite alteration as being due to greenschist facies metamorphism and therefore considered that they predated the Cobar deformation and were Early Devonian in age. However, it is possible that they are younger. During this study whole rock geochemical samples were taken from the Parkvale Gabbro and a dolerite dyke which cross-cuts both the Erimeran Granite and a felsic porphyry associated with the Tarran Volcanics. Analysis of the data (Appenix 3) indicates that the gabbro and the dolerite are within-plate tholeiites and may be comagmatic. It is here proposed that the Bogolo Zone may represent a northeasterly trending zone of tensile failure which accommodated components of dextral strike slip movement along the Rookery-Hathaway fault system and the Nyora Fault. Apart from enclosing the distribution of known dolerite dykes, the Bogolo Zone is not associated with any obvious large scale gravity or magnetic feature. A north-northeasterly trending dolerite dyke occurs in the Norma Vale area, centred approximately at GR 405630 6433110, adjacent and parallel to the northwestern edge of the Gilgunnia Granite (Peak Gold Mines Pty Ltd unpublished data).
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Figure 21. the Bogolo Zone (red shaded area). An interpreted zone containing northeasterly trending dolerite dykes within the Nymagee and Cargelligo 1:250 000 map sheet areas. The Tarran Volcanics The mapped and interpreted distribution of the Tarran Volcanics and associated intrusions indicates that they are associated with linear structures which occupy trends
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ranging from northeast through to southeast. MacRae (1987) indicated that the Tarran Volcanics pre-date the dolerite dykes, as they have been cross-cut by them. Magnetic palaeochannels Magnetic palaeochannels have been interpreted from the TMI and 1VD data (Figure 22). These are dendritic magnetic features which reflect Tertiary palaeochannels containing maghemite pisoliths. Figure 22 shows the result of that interpretation.
Figure 22. Magnetic palaeochannels interpreted from TMI and 1VD data for the Nymagee sheet area superimposed upon interpreted geological units
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IMPLICATIONS FOR THE GEOLOGICAL/STRUCTURAL DEVELOPMENT OF THE NYMAGEE SHEET AREA Figures 23 and 24 are geological cross-sections of the Nymagee sheet area based upon existing mapping and the geophysical interpretation carried out in this study. The cross-sections are “best guesses” as there is very little quantitative regionally significant depth information for the area and modelling of gravity features was unavailable. The eastern half of seismic line 09GA-RS2, acquired in 2009 by Geoscience Australia in collaboration with the (then) New South Wales Department of Primary Industries, overlaps approximately with the western half of cross-section ABC (Figure 25). The Blue Mountain Fault is distinctive in the seismic profile and there is a suggestion in the seismic imagery that the Woorara Fault and faults to its west may dip more shallowly to the east than interpreted in cross-section AB. However, the Scotts Craig Fault is not apparent in the seismic imagery, nor is there any distinctive outline of the Gilgunnia Granite. The distinction between basin and basement rocks is also not clear in the seismic section. It is difficult to unravel the details of the present structural architecture of the Nymagee sheet area, and developing a detailed understanding of its geological history is probably impossible. The subsurface orientations of faults are unknown and it is difficult to be sure about when particular faults which affect the basement rocks were initiated and reactivated let alone the senses of movement and displacements along them. It is also difficult to know if faults within the basin are extensions of deeper basement structures or not. Del Ventisette et al. (2006) conducted analogue modelling of rift basin inversion and demonstrated that a range of complex deformation patterns can be produced within the basin fill and that the types of pattern produced and the reactivation mode of existing faults is dependant upon the compression angle with respect to the initial rift basin orientation. Smith (1992) used analogue modelling to estimate that the compression which produced inversion of the Cobar Basin was directed at about 60º to the basin margin. Del Ventisette et al. (2006) showed that reactivation of rift basins at such high angles (50º, 70º and 90º to the basin margin) caused basement faults to be reactivated as thrusts with a component of strike-slip movement. Where there is a ductile basal layer in the basin, new thrust structures are produced, including pop-ups, which do not join up with pre-existing basement faults. Where there is no ductile basal layer the structures produced within the basin fill are extensions to those in the basement. According to the terminology of Glen (1990) the bulk of the Cobar Supergroup rocks of the Nymagee sheet area occur within the Mount Hope and Rast troughs and the Canbelego-Mineral Hill Belt, with Winduck Group and stratigraphically equivalent rocks situated on the Winduck, Walters Range and Kopyje shelves (Figure 26). The Mount Hope and Rast troughs are here regarded as lateral (southern) extensions of the Cobar Basin and it is quite likely that all three are part of the one single (Cobar) basin. The present author does not recognise the need to define the Walters Range Shelf as an intervening basement high as the Walters Range Group can be interpreted as a faulted remnant of a once extensive sheet (including the Winduck and Yarra Yarra Creek groups) across the region.
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Figure 23. Cross-section ABC (broken into three segments, all with the same scale). Refer to Figure 2 for location.
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Figure 24. Cross-section DEFG (broken into two segments, each with the same scale). Refer to Figure 2 for location.
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Figure 25. a) Location of the eastern part of Geoscience Australia’s seismic line 09GA-RS2 showing common depth points (CDPs) as bold numbers; b) migrated seismic profile for the first 3 seconds (note V/H ≈ 2); c) geological cross-section AB, from Figure 23.
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Glen (1990) regarded the initiation of the Cobar Basin as a sinistral transtensional event with extension oriented northeast-southwest. Seismic reflection profiling across the Cobar Basin (as defined by Glen 1990) has been interpreted by Drummond et al. (1992) to indicate that the Cobar Basin is a ramp basin situated above an easterly dipping detachment fault and that its western margin is steeper than its eastern margin. Extension along this detachment led to deepening of the basin in its western part. Glen et al. (1994), based upon the seismic reflection data, estimated a maximum basin thickness of six kilometres. Given that the Mount Halfway Volcanics, dated at 422.5 Ma, interfinger with turbiditic sediments in the west and the Florida Volcanics, dated at 421.7 Ma, interfinger with more shallow water sequences in the east, it can be assumed that in the Late Silurian the basin was deeper to the west than in the east, consistent with Glen et al. (1994). The presence of granitic clasts within the basal Cobar Supergroup (e.g. Trigg 1987) indicates that the Erimeran Granite was exposed at the time of its deposition and the presence of the Tarran Volcanics on top of the Erimeran Granite indicates that the granite was exposed at 415 Ma. As discussed above, the Yarra Yarra Creek Group rests unconformably upon older units while the stratigraphically equivalent Winduck and Walters Range groups appear to be conformable with the underlying marine sediments. This suggests that there was uplift in the eastern part of the Nymagee sheet prior to deposition of the Yarra Yarra Creek Group. The occurrence of the Mountain Dam Limestone in the Marobee area indicates that marine conditions prevailed there in the Early Pragian. Dolerite dykes intruded parts of the Nymagee sheet after emplacement of the Tarran Volcanics and possibly before inversion of the Cobar Basin but their exact timing is unknown. The Parkvale Gabbro may have been emplaced at the same time as the dykes during a phase of within-plate tholeiitic volcanism. The Cobar Supergroup rocks were deformed in the Cobar deformation (Scheibner and Basden 1998) when the Cobar Basin underwent inversion. Glen (1990) and Smith (1992) interpreted that deformation to be dextral transpressional with principal compression oriented northeast-southwest. Burton et al. (2012) suggested that isolated packages of Cobar Supergroup rocks in fault-bounded synclinal keels in the Sussex and Byrock areas were likely to be remnants of a once continuous sedimentary sequence. It is here suggested that a similar interpretation is applicable to the entire Cobar Supergroup sequence in the Nymagee sheet area, such that it may have actually once been continuous and what now form separate belts of rock were once all interconnected, but with an overall shallow water depositional environment in the east deepening to the west with various degrees of volcanic input. Later deformation followed by erosion produced the compartmentalised geometry now present. The colour alteration of conodonts from the Mountain Dam Limestone and Beloura Tank Limestone Member, as noted by Pickett (1982), as mentioned above, is consistent with the sequence once having been much thicker, probably due both to burial and tectonic processes, with subsequent substantial erosion.
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Figure 26. Geological map of the Cobar-Nymagee area after Glen (1990). IMPLICATIONS FOR MINERAL SYSTEMS With regards to particular areas which may have mineral potential, it is possible that gold-bearing quartz veins could be associated with areas of high strain within fault zones adjacent to granites, similar to those adjacent to the Weethalle and Yalgogrin granites on the Cargelligo map sheet (Burton and Downes 2005). Such areas include
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the Eulendool Fault Zone adjacent to the Derrida Granite and the Walton Fault Zone adjacent to the Erimeran Granite. Altered basalt in the Melrose area is chemically similar to mafic rocks which host copper mineralisation in the Tritton and Tottenham areas. Hence it is speculated that the Melrose area may have potential to host similar copper-rich volcanic associated massive sulphide deposits. Cobar type deposits are associated with zones of high strain (e.g. Glen (1987), Lawrie and Hinman (1998), Stegman (2001)). The Gilgunnia Fault Zone, southeast of the Gilgunnia Granite, may be such a zone which may have formed by strain partitioning around the granite. Features at the May Day mine (developed upon a Cobar type deposit) within that area (Burton 2012) are consistent with transpression within a high strain zone. It is therefore possible that the Gilgunnia Fault Zone has potential to host other Cobar type deposits. CONCLUSIONS / RECOMMENDATIONS The intention of this study of the Nymagee sheet area was to add to the existing mapped geology by incorporating interpretations of the geophysical data which only became available after the geological mapping was completed. It is clear that the interpretation conducted here does not greatly change the overall understanding of the geological setting and geological history of the area as developed by previous workers. The main outcome of this study has been to highlight particular areas and give alternative geological interpretations for them. Examples include: the interpretation of granites within the basement rocks; a more detailed distribution of the Tarran Volcanics and associated intrusions; a more detailed distribution of dolerite dykes and the interpretation of the Bogolo Zone as a possible tensile zone linking the Rookery/Hathaway faults with the Nyora Fault; the recognition of more faults and fault zones than previously mapped; and, based on company drilling, the recognition of the Tollingo granodiorite and the interpretation of the Melrose magnetic anomaly as being due to mafic volcanics and associated quartz-magnetite rocks within the Girilambone Group. Constructing meaningful geological cross-sections through the Nymagee sheet area has been limited by the lack of geophysical modelling, particularly of the gravity data, and the lack of detailed near-surface seismic data. It is therefore recommended that any further work to better understand the solid geology of the area should include seismic lines to image the top ten kilometres of the crust and modelling of gravity data, perhaps with more detailed gravity surveys in particular areas. ACKNOWLEDGEMENTS I would like to thank Steven Trigg (Geological Survey of New South Wales) and Greg MacRae (Variscan Mines Limited) for their constructive comments and advice on an earlier draft of this report. Michael Bruce(Geological Survey of New South Wales) is thanked for providing helpful advice regarding interpretation of whole rock geochemical data.
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SHERWIN L. 1992. Siluro-Devonian biostratigraphy of central New South Wales. Geological Survey of New South Wales, Quarterly Notes 86, 1–12. SHERWIN L. 1996. Narromine 1:250 000 Geological Sheet SI/55-3: Explanatory Notes. Geological Survey of New South Wales, Sydney. SHERWIN L. 2013. Revised Nymagee 1:250 000 Siluro-Devonian time space plot. Geological Survey of New South Wales, Report GS2013/1885. SKIRKA M. 2002. Pasminco Exploration, Exploration Licence 5774: Wonawinta. Annual report for the period 6 September 2001 to 5 September 2002. Geological Survey of New South Wales, File GS2002/875. SMITH J.V. 1992. Experimental kinematic analysis of en echelon structures in relation to the Cobar Basin, Lachlan Fold Belt. Tectonophysics, 214, 269–276. SPENCER R. & MUSGRAVE R.J. 2006. Isostatic and decompensative correction of gravity data from New South Wales. Exploration Geophysics 37 (3), pp 210–214. STEGMAN C.L. 2001. Cobar deposits: still defying classification! Society of Economic Geologists Newsletter, 44, 14–26. SUN S. S. 1980. Lead isotopic study of young volcanic rocks from mid-ocean ridges, ocean islands and island arcs. Philosophical Transactions of the Royal Society, A297, 409–445. SUN Y., JIANG Z., SECCOMBE P.K. & FENG Y. 2000. New dating and a review of previous data for the development, inversion and mineralisation of the Cobar Basin. In McQueen K.G. & Stegman C.L. eds. Central West Symposium Cobar 2000, Extended Abstracts, pp. 113–116. SUPPEL D.W & GILLIGAN L.B. 1993. Nymagee 1:250 000 Metallogenic Map SI/55-2: Metallogenic Study and Mineral Deposit Data Sheets. Geological Survey of New South Wales, Sydney. SUPPEL D.W & POGSON D.J. 1993. Nymagee 1:250 000 Metallogenic Map SI/55-2. Geological Survey of New South Wales, Sydney. THOMPSON R. N. 1982. British Tertiary volcanic province. Scottish Journal of Geology, 18, 49–107. TRIGG S.J. 1987. Geology of the Kilparney 1:100,000 Sheet 8132. Geological Survey of New South Wales, Sydney. TRIGG S.J. 1988. Kilparney 1:100 000 Geological Sheet 8132. Geological Survey of New South Wales, Sydney.
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APPENDIX 1
SAMPLE DATA
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sample number BOBGB1
site 8233GRB0010.01
easting 468200
northing 6447970
stratigraphic unit Allandoon Chert
GRB Delby 01
8233GRB001101
470270
6439750
GRB Allendoon 1
8233GRB0012.01
468170
6447920
Unnamed chert in Girilambone Group Allandoon Chert
GRBMU02
8132GRB0001.01
445780
6382030
GRBMU03
8132GRB0002.01
445840
6382030
GRBMU04
8132GRB0003.01
445520
6382110
GRBMO002
-
445780
6394050
GRBNY01
8134GRB0001.01
436960
6463240
GRBNYMAUG001
NYMG-GRB0001
443140
6455730
GRBNYMAUG002
NYMG-GRB0002
414180
6445420
GRBNYMAUG003
NYMG-GRB0003
446910
6438700
GRBNYMAUG004 GRBNYMAUG005
NYMG-GRB0004 NYMG-GRB0005
443310 435240
GRBNYM13SEP001 GRBNYM13SEP002
NYMG-GRB0006 NYMG-GRB0007
470360 470330
Urambie Granodiorite Urambie Granodiorite Urambie Granodiorite Marobee Conglomerate Nymagee Igneous Complex Hartwood Tuff Member Shuttleton Rhyolite Member
purpose Conodont age determination Conodont age determination Conodont age determination geochronology
notes Submitted
TS number -
Submitted
-
Submitted
-
submitted
T88797
petrology
submitted
T88798
petrology
submitted
T88799
geochronology
Not submitted
-
geochronology
Not submitted, TS only Not submitted, TS only Not submitted, TS only
T88802
Not submitted, TS only submitted Not submitted, TS only submitted submitted
T88911
geochronology geochronology geochronology
6443050 6431850
Hennings Tank Tuff Member Unnamed unit Burthong Formation
6422700 6422670
Parkvale Gabbro Parkvale Gabbro
geochemistry petrology
petrology geochronology
T88909 T88910
T88912 T88913 T88914 T88915
Table 2. Samples collected during this study. Site refers to site number in the Geological Survey of New South Wales FieldObs database. Geochemical data was not available at the time of writing. All samples collected for geochronology, except Urambie Granodiorite, were held in storage for later processing and had not been submitted at time of writing.
55
Sample number GRBNYM13SEP003
GS2015/0142
GRBNYM13SEP004
site NYMG-GRB0008 NYMG-GRB0009
easting 470310 390410
northing 6422710 6394300
stratigraphic unit Parkvale Gabbro Mount Kennan Volcanics
purpose petrology geochronology
notes submitted Not submitted, TS only
TS number T88916 T88917
GRBNYM13SEP005
NYMG-GRB00010
393340
6392630
Goona Volcanics
geochronology
Not submitted, TS only
T88918
GRBNYM13SEP006 GRBNYM13SEP007
NYMG-GRB00011 NYMG-GRB00012
416510 427770
6413460 6400080
petrology geochronology
NYMG-GRB00013
394550
6381320
GRBNYM13SEP013
NYMG-GRB00014
386590
6378510
submitted Not submitted, TS only Not submitted, TS only Not submitted, TS only
T88919 T88920
GRBNYM13SEP012
Amphitheatre Group Guapa Tank Rhyodacite Member Double Peak Volcanics Coando Volcanics
GRBNYM13SEP016 GRBNYMOCT13-002 GRBNYMOCT13-003 GRBNYMOCT13-006
NYMG-GRB00015 NYMG-GRB00016 NYMG-GRB00016 NYMG-GRB00017
440730 481480 481480 398970
6400730 6399220 6399220 6369340
geochemistry geochemistry petrology geochronology
NYMG-GRB00018
393570
6361890
GRBNYMOCT13-009
NYMG-GRB00019
387650
6375950
Ambone Volcanics
geochronology
GRBNYMSEP13-008
NYMG-GRB00020
420660
6352890
geochronology
GRBNYMSEP13-017
NYMG-GRB00021
489810
6410180
Whoey Tank Formation Ewolong Formation
submitted submitted submitted Not submitted, TS only Not submitted, TS only Not submitted, TS only Not submitted, TS only Not submitted, TS only
T88923 T88924 T88925 T88926
GRBNYMOCT13-008
Dolerite dyke Girilambone Group Girilambone Group Broken Range Group Regina Volcanics
Table 2. continued.
geochronology geochronology
geochronology
geochronology
T88921 T88922
T88927 T88928 T88929 T88930
56
GS2015/0142
TS number
Description
T88797
Tonalite. Consists mainly of plagioclase, quartz and biotite/chlorite with one grain of alkali feldspar. Plagioclase forms elongate to needle-like grains about 1 mm long with some up to 2 mm long. They are overprinted by very fine-grained sericite. There is no obvious preferred orientation to the grains. Quartz grains, which appear rounded to irregular in shape range from 3 mm to 6 mm across and envelop the plagioclase grains such that they sit in a “sea” of quartz. Biotite grains occur as ragged tabs with alteration to chlorite and are around 0.25 mm long. These are associated with extremely fine opaque grains, which are probably magnetite. There is no preferred orientation and they also occur within the quartz grains. One subhedral phenocrysts of ?alkali feldspar is present and is about 2 mm long. It contains biotite flakes and has extremely fine sericite alteration through it. Monzonite. Consists of quartz, alkali feldspar, plagioclase and biotite/chlorite. Alkali feldspar is probably microcline with some having cross-hatch twinning and a lot of grains having a laminated (Perthite) appearance. Grains are subhedral to irregular and contain both biotite and plagioclase grains (i.e. the alkali feldspar is interstitial to those minerals). Quartz grains are irregular, between 0.5 and 1 mm across and are also interstitial to plagioclase and biotite. There is one elongated quartz phenocryst about 5 mm in length. Plagioclase grains are commonly small, columnar to euhedra/subhedral and range in length from 0.1 mm to 0.5 mm in length. There is a vague preferred orientation to the grains. There are also subhedral phenocrystal grains between about 2 mm and 6 mm in length, some of which show zoning. Biotite forms ragged tabular grains between about 0.1 and 0.3 mm long. They show partial alteration to chlorite. There is no obvious preferred orientation. One large biotite grain is 2 mm long. The rock consists of phenocrysts of plagioclase, with lesser quartz and biotite, in a groundmass of quartz, alkali feldspar, plagioclase and biotite. Granite. Consists of plagioclase, alkali feldspar, quartz and biotite/chlorite. Plagioclase forms euhedral to subhedral lath-like grains between 0.5 and 4 mm in length. Some show oscillatory zoning. Alkali feldspar grains are irregular and are between about 1 mm and 4 mm in length. They have lamellar exsolution features and may indicate that the mineral is microcline but simple twinning is common. Quartz forms irregular grains between 1 and 4 mm across. They are undulose with very weak recrystallisation. Biotite grains are ragged tabs with partial alteration to chlorite and show a lot of radioactive zircon damage. There are some grains, probably alkali feldspar, which have been badly corroded by sericite. A fairly homogeneous granite. Granite. Contains quartz, alkali feldspar, plagioclase, biotite and minor muscovite and ?tourmaline. Quartz grains are irregular, undulose and partly recrystallised. Average grain size is about 3.5 mm. Some are up to 7mm across but are made up of smaller recrystallised zones. Plagioclase grains have corroded (very fine sericite altered) cores. Average grain size is about 2 mm with some up to 4 mm long. Grains are subhedral. Alkali feldspar is microcline and generally has a distinct cross-hatch twinning pattern. Forms irregular to subhedral grains between about 2 and 5 mm across. Biotite forms small platy grains
