Journal of the Geological Society, London, Vol. 155, 1998, pp. 773–785. Printed in Great Britain
Mineralogy, sulphur isotope geochemistry and the development of sulphide structures at the Broken Spur hydrothermal vent site, 2910 N, Mid-Atlantic Ridge I. B. BUTLER 1 *, A. E. FALLICK 2 & R. W. NESBITT 1 1 Department of Geology, Southampton Oceanography Centre, University of Southampton, Empress Dock, European Way, Southampton SO14 3ZH, UK 2 Isotope Geosciences Unit, Scottish Universities Research and Reactor Centre, Rankine Avenue, Scottish Enterprise Technology Park, East Kilbride G75 0QF, UK *Present address: Department of Earth Sciences, Cardiff University, PO Box 914, Cardiff CF1 3YE, UK (e-mail:
[email protected]) Abstract: A large collection of hydrothermal sulphides from the Broken Spur hydrothermal vent site, including representative samples of mound sulphide materials, has been characterized using optical mineralogy and sulphur isotope analysis. Young mound sulphides from Broken Spur have a pyrrhotitedominated mineralogy unusual for bare ridge vent systems. However, pyrrhotite is metastable and is ultimately converted to iron disulphides. Mature sulphides are indurated, recrystallized and contain abundant quartz. Sulphide mound materials are developed by three major processes: (i) coalescing of chimney structures; (ii) accumulation of talus from mass wasting and (iii) precipitation and growth in response to hydrothermal flow. Progressive maturation of mound materials is by modification of primary textures, development of mineralogical zoning and replacement of metastable phases. Sulphur isotope analysis of 35 mineral separates returned ä34S values of 0.5 to +3.2‰. These values are similar to those previously measured for Broken Spur and Snakepit, but are distinctly 32S enriched compared to the TAG active mound and some Pacific sites. Seawater entrainment and sulphate reduction within the subsurface feeder zone below Broken Spur mounds do not appear to be important processes at Broken Spur, in contrast to the TAG active mound. Keywords: Mid-Atlantic Ridge, sulphides, sulphur, isotopes, hydrothermal vents.
The Broken Spur Vent Field was discovered in March 1993 by the RRS Charles Darwin Cruise CD76 (Murton et al. 1993) and is located at 2910 N on segment 17 of the Kane–Atlantis super-segment of the Mid-Atlantic Ridge (MAR) (Fig. 1). The area was revisited by RRS Charles Darwin cruise CD77 (Elderfield et al. 1993) and was later explored by ALVIN submersible dives (Murton & Van Dover 1993). Its geology and the taxonomy of vent biota is discussed by Murton et al. (1995a), the chemistry of vent fluids is discussed by James et al. (1995) and massive sulphides and hydrothermal sediments have been characterized by Duckworth et al. (1995) and Knott (1995). In 1994, the Broken Spur Vent Field was revisited by the British-Russian Atlantic Vents Expedition (BRAVEX) aboard RV Academik Mstislav Keldysh (BRAVEX Scientific Team 1994), and this forms the basis of this paper. Nine submersible dives were made using the two deep diving Mir submersibles. A large collection of vent biota, massive sulphides and fluid samples was recovered and an extensive library of dive video footage recorded. The Mir dives visited more active and inactive structures than had previously been explored by ALVIN and also recovered a more varied collection of massive sulphides, and in particular sulphide mound materials. Correlation of video observations with those made by the dive scientists has allowed the production of a new geological reconstruction of the known extent of the Broken Spur Vent Field (Fig. 2) (Nesbitt 1995). In this study, the evolution and alteration of sulphides at the Broken Spur Vent Field and inputs to the ore forming system are assessed by use of optical mineralogy and bulk sulphur isotope analysis. Future publications will address aspects of
high resolution, high sensitivity trace element analysis and in situ spatially resolved laser ä34S analysis applied to studies of hydrothermal sulphide genesis.
Geology The Broken Spur Vent Field is located at 3080–3110 m water depth within and around the axial summit graben of the neovolcanic ridge of the Mid-Atlantic Ridge. The region of hydrothermal activity so far explored extends over an area of 150 m E–W by >100 m N–S (Nesbitt 1995). The area occupied by the Broken Spur Vent Field is largely composed of pillow basalts with a light dusting of sediment and contains two intersecting fault sets (Murton et al. 1995a) trending 028 and 115 which represent possible fluid flow pathways. The Broken Spur Vent Field can be subdivided into two parts, the eastern valley, a graben structure, which trends approximately N–S and whose eastern wall marks the limit of the field, and the Western Plateau which covers the area west of the eastern valley (Fig. 2). Murton et al. (1995)b confirm that this setting continues along the entire length of the vent field. The eastern valley is 20–30 m across and forms a dominant structure running along the eastern margin of the field. This graben may represent the axial summit graben referred to by Murton et al. (1995a) or may itself be a feature contained within the axial summit graben; high resolution bathymetry is required for clarification. Both sides of the eastern valley consist of 30 m high near-vertical pillow basalt walls that strike N–S and are fault scarps. Pillow basalts 773
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Fig. 2. Cartoon reconstruction of Broken Spur based on observations made during the 9 MIR submersible dives. The explored extent of the vent site is 150 m N–S by >100 m E–W. After Nesbitt (1995).
Fig. 1. The location of the Broken Spur hydrothermal vent site and other hydrothermal fields on the Mid-Atlantic Ridge. In terms of basement composition, the Broken Spur site is similar to TAG and Snakepit (N type MORB), but different to the plume-influenced Lucky Strike and Menez Gwen sites (E-type MORB) and the serpentinite basement at 1445 N.
draped over the eastern valley walls suggest volcanism postdating the tectonic event that created the eastern valley (Murton et al. 1995b). The floor of the eastern valley is at 3110 m water depth and is the deepest part of the Broken Spur Vent Field; the top of the basalt walls level off at 3080 m. Major sulphide structures named the Bogdanov site (BX16 in Fig. 2) and Wasps Nest are contained within the eastern valley. Wasps Nest, situated at the southern extent of the Broken Spur Vent Field is a 23 m high steep-sided sulphide edifice and in 1994 consisted of a small venting structure adjacent to an anhydrite platform large enough to support a Mir submersible during vent fluid sampling. The Wasps Nest is oriented NE–SW and is cut by prominent N–S-trending fracture zones. The western wall of the eastern valley is marked by a series of active ledges cutting oxidized sulphide mounds, the most notable of which is the Bogdanov site. Eight metres below the Bogdanov site is another active ledge (BX05), and both cut through a tall, narrow oxidized sulphide mound which is effectively a veneer plastered on the western wall of the eastern valley. The southern end of the Western Plateau contains the Spire and Saracens Head edifices discovered during the original ALVIN dive programme (Murton & Van Dover 1993). Saracens Head is located on the lip of the eastern valley due
west of Wasps Nest. In the period between discovery in 1993 and the Mir dive program in 1994, Saracens Head developed two chimneys approximately 6 m and 3 m tall (Nesbitt & Murton 1995a, b). Other features on the Western Plateau are the Dogs Head, White Button, Triple Chimney and Parkers Chimney. Details of the sulphide structures visited by BRAVEX dives are shown in Table 1. The western margin of hydrothermal activity is not marked by any major changes of elevation or geological structure, but the sea-floor gradually drops away to the west. A single E–W transect 100 m away from the vent site revealed a series of small (3 m mound of relict sulphides
Table 2. Classification of sulphide types at Broken Spur Morphology
Activity
Composition
Notes
Type I: chimney (immature) Type Ia Bulbous beehive diffuser structures Type Ib Organ pipe chimneys
Focused and diffuse hydrothermal Focused, black smoker
Not sampled by BRAVEX
Type II: chimney (mature) Type IIa Diffuser structures, elongate or bulbous Type IIb Chimneys; thick sulphide walls
Constructional anhydrite and structural Fe oxides Constructional anhydrite, chalcopyrite conduit fill
Focused and diffuse. Often inactive Focused, black smoker
Distinctive concentric zoning and lamination May be multiple walled
Type III: mound sulphide Type IIIa Ledge sulphides; shelf structures or i mound crust
Constructional sulphides and some oxides Constructional chalcopyrite, some anhydrite
Diffuse hydrothermal, may be inactive
Constructional sulphides. Fe and Zn sulphides, anhydrite crusts Variable composition; Cu, Fe, Zn sulphides; result of mass wasting Fe and Cu sulphides, crystalline quartz
Texturally immature, often microporous
Type IIIb
Brecciated mound talus
Inactive, associated with focused hydrothermal flow
Type IIIc
Mature mound talus; indurated low porosity
Inactive, associated with focused hydrothermal flow
Fragile, thin walled and zoned
Product of reworking of wasted sulphide on mounds May represent exposed mound core sulphide
This scheme is based upon that used by Duckworth et al., (1995) for Broken Spur sulphides but has been modified and extended to encompass the variety of sulphide types recovered by BRAVEX. extracts from acid volatile sulphides were recovered from two samples from which it was troublesome to obtain a clean mineral separate. For this, the sample was heated with excess 50% HCl in a Quickfit round-bottomed flask for 2 hours. A Liebig condenser refluxed H2O, while evolved H2S gas was carried through the system by flowing N2 gas. The gas extract was passed through distilled-deionized H2O to remove HCl before being passed through a 20% Zn(CH3COO)2/ NH4OH solution. The ZnS precipitate was treated with 10% AgNO3 to form Ag2S and washed with concentrated ammonia to remove traces of Zn(CH3COO)2. The product was filtered, washed and freeze-dried. For analysis, samples were intimately mixed with Cu2O and reacted at 1076C (Robinson & Kusakabe 1975); the extracted SO2 was cryogenically purified and isotopically analysed on a SIRA II dual inlet mass spectrometer. Data are presented in conventional
fashion as ä34S‰ relative to the Can˜on Diablo Troilite (CDT) standard, and the precision of the data is 0.2‰ (1ó).
Mineralogy and textures Massive sulphides were recovered from six of the Broken Spur edifices. For this discussion it is convenient to use the classification devised by Duckworth et al. (1995) for sulphide materials of the ALVIN sample suite. In this classification, samples are divided into three types based on hydrothermal setting, morphology, and mineralogy. The three classes are: (i) type I or immature chimneys/diffusers; (ii) type II or mature chimneys/diffusers; (iii) type III or mound sulphides. Due to
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Wasps Nest
Triple Chimney
Spire
Bogdanov Site
Dogs Head
Type I: chimney (immature) Type Ia Observed Observed Observed Type Ib AMK3348-S-02 AMK3348-S-01 AMK3435-S-56 Observed AMK3426-S-45 Type II: chimney (mature) Type IIa AMK3434-S-54 Type IIb AMK3434-S-55 Observed Type III: mound sulphides Type IIIa AMK3435-S-58 AMK3426-S-43 AMK3434-S-53 AMK3425-S-37 AMK3434-S-50 AMK3434-S-47 AMK3425-S-38 AMK3434-S-51 AMK3425-S-39 AMK3434-S-52 AMK3425-S-40 AMK3425-S-41 AMK3425-S-42 AMK3434-S-48 AMK3434-S-49 Type IIIb AMK3435-S-57 AMK3426-S-44 AMK3426-S-46 Type IIIc AMK3348-S-03
Fig. 3. A summary of the mineralogical composition of sulphide types recovered from Broken Spur by BRAVEX, using the classification scheme for sulphides modified from Duckworth et al. (1995). All assessments of mineral abundance are based on visual estimates.
the sampling strategy employed by BRAVEX and the greater variety of mound sulphides recovered, the above classification requires some extension of the type II and type III categories. The classification scheme used here is summarized in Table 2 and a full listing of the BRAVEX sample suite is shown in Table 3. Overall, the mineralogy of the BRAVEX sample suite is major marcasite, pyrrhotite, and sphalerite, abundant chalcopyrite, isocubanite, pyrite, Fe-oxyhydroxides, quartz, and anhydrite, minor amorphous silica, wurtzite, bornite and atacamite, and trace jarosite, covellite, chalcocite/digenite and galena. The dominant mineralogy of each of the sulphide types is summarized in Fig. 3.
Type I sulphide-sulphate structures Type I structures are immature organ pipe chimney and beehive diffusers, and are characterized by major construc-
tional anhydrite. Type Ia beehive structures like those discussed by Duckworth et al. (1995) and Rickard et al. (1994) were observed during the BRAVEX dives but were not sampled. Type Ib structures were sampled from the top of the Saracens Head, Wasps Nest and Triple Chimney edifices. Fragile thin walled organ pipe chimneys occur as individual structures or coalesce into ‘pan-pipe’ arrays. Constructional anhydrite forms the walls of the organ pipes and the inner conduit lining is chalcopyrite. Contained within the anhydrite is minor euhedral pyrite, chalcopyrite and trace boxwork pyrrhotite. The outer walls of the structures are coated with mixed anhydrite and Fe-oxyhydroxides. One sample of type Ib sulphide from Saracens Head (Sample AMK3434-S-55) shows the development of more mature textural features, including a thicker constructional chalcopyrite wall modified to secondary bornite and chalcocite/digenite at the contact with the anhydrite wall. Trace covellite is associated with bornite and chalcocite/digenite within the anhydrite wall.
Type II sulphide structures Mature thick-walled chimneys of the type sampled from the Spire by ALVIN (Duckworth et al. 1995) were not recovered by BRAVEX; however, a single large inactive diffuser structure (Sample AMK3434-S-54) was recovered from the top of Saracens Head. The diffuser is quite different from the type Ia structures previously described by Duckworth et al. (1995) and Rickard et al. (1994) in that its mineralogy is dominated by structural sulphide, rather than anhydrite and Fe-oxyhydroxides. Indeed, the sample is similar in mineralogy and zoning to structures described from Snakepit by Fouquet et al. (1993). By analogy with the evolution of chimney structures from an anhydrite dominated to a sulphide dominated mineralogy, this sample is included in the type II category as a type IIa mature diffuser. The diffuser is a large (403020 cm), broadly cylindrical structure with a bulbous base. There is a concentric crust-tocore zoning (Fig. 4). Rickard et al. (1994) describe the type Ia diffuser with an open central conduit around which the diffuser
BROKEN SPUR MINERALOGY AND S ISOTOPES
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Fig. 4. Mineralogy, textures and zoning within the inactive diffuser structure from Saracens Head. Photomicrographs taken in plane polarized reflected light unless otherwise stated. (a) Collomorphic marcasite crust (zone 1). (b) Dendritic marcasite overgrown by sphalerite (zone 3, crossed polars). (c) Polycrystalline marcasite with anhedral sphalerite sheathed by isocubanite (zone 5). (d) Core conduit assemblage of porous mixed sulphides (zone 7). Below the micrographs is a reconstruction of the crust to core zoning of the diffuser. Structures X and Y are hydrothermal fluid conduits. Refer to text for a full description of the zones. Mc1, collomorphic marcasite; Mc2, dendritic marcasite; Mc3, euhedral marcasite; Mc4, polycrystalline marcasite; Sp1, anhedral/collomorphic sphalerite; Sp2, euhedral sphalerite; Wu, wurtzite; Po1, tabular pyrrhotite; Po2, boxwork pyrrhotite; Cp, chalcopyrite; Iss, isocubanite; Oxide, Fe-oxyhydroxides; SiO2, amorphous silica.
is constructed. In sample S-54 the core of the diffuser (Z7, Fig. 4) is clogged by a porous (c. 50% porosity) friable assemblage of major pyrrhotite with abundant sphalerite and minor chalcopyrite, hexagonal ZnS (wurtzite?) and marcasite. A single
narrow (
