Reduction Of Seawater Sulfate Research Articles

Geology, mineralogy and stable isotope geochemistry of the Kabwe carbonate-hosted Pb–Zn deposit, Central Zambia

The carbonate-hosted Kabwe Pb–Zn deposit, Central Zambia, has produced at least 2.6 Mt of Zn and Pb metal as well as minor amounts of V, Cd, Ag and Cu. The deposit consists of four main epigenetic, pipe-like orebodies, structurally controlled along NE–SW faults. Sphalerite, galena, pyrite, minor chalcopyrite, and accessory Ge-sulphides of briartite and renierite constitute the primary ore mineral assemblage. Cores of massive sulphide orebodies are surrounded by oxide zones of silicate ore (willemite) and mineralized jasperoid that consists largely of quartz, willemite, cerussite, smithsonite, goethite and hematite, as well as numerous other secondary minerals, including vanadates, phosphates and carbonates of Zn, Pb, V and Cu. Galena, sphalerite and pyrite from the Pb–Zn rich massive orebodies have homogeneous, negative sulphur isotope ratios with mean δ 34S CDT permil (‰) values of − 17.75 ± 0.28 (1 σ), − 16.54 ± 0.0.27 and − 15.82 ± 0.25, respectively. The Zn-rich and Pb-poor No. 2 orebody shows slightly heavier ratios of − 11.70 ± 0.5‰ δ 34S for sphalerite and of − 11.91 ± 0.71‰ δ 34S for pyrite. The negative sulphur isotope ratios are considered to be typical of sedimentary sulphides produced through bacterial reduction of seawater sulphate and suggest a sedimentary source for the sulphur. Carbon and oxygen isotope ratios of the host dolomite have mean δ 13C PDB and δ 18O SMOW values of 2.89‰ and 27.68‰, respectively, which are typical of marine carbonates. The oxygen isotope ratios of dolomite correlate negatively to the SiO 2 content introduced during silicification of the host dolomite. The depletion in 18O in dolomite indicates high temperature fluid/rock interaction, involving a silica- and 18O-rich hydrothermal solution. Two types of secondary fluid inclusions in dolomite, both of which are thought to be related to ore deposition, indicate temperatures of ore deposition in the range of 257 to 385 and 98 to 178 °C, respectively. The high temperature fluid inclusions contain liquid + vapour + solid phases and have salinities of 15 to 31 eq. wt.% NaCl, whereas the low temperature inclusions consist of liquid + vapour with a salinity of 11.5 eq. wt.% NaCl. Fluid transport may have been caused by tectonic movements associated with the early stages of the Pan-African Lufilian orogeny, whereas ore deposition within favourable structures occurred due to changes in pressure, temperature and pH in the ore solution during metasomatic replacement of the host dolomite. The termination of the Kabwe orebodies at the Mine Club fault zone and observed deformation textures of the ore sulphides as well as analysis of joint structures in the host dolomite, indicate that ore emplacement occurred prior to the latest deformation phase of the Neoproterozoic Lufilian orogeny.

Mineralogical and geochemical characterization of hydrocarbon seep sediments from the Gulf of Mexico

A common characteristic of the Gulf of Mexico (GOM) seafloor, hydrocarbon seepage produces asphalt volcanism, brine pools, gas hydrates and authigenic carbonates. We studied authigenic carbonates from two areas: Green Canyon (Louisiana slope, 1000 mbsl) and the Chapopote diapir (abyssal plain, southwestern GOM, 2900 mbsl). Carbonates consist of oily, matrix-supported limestones with variable porosity and a fine-grained matrix of calcite. They occur in Green Canyon as both concretions formed below the seafloor and as slabs of seabed pavements. In each case, the carbonates are mudstones with a mud matrix consisting of high-Mg calcite, disseminated pyrite and detrital quartz. The carbonate slabs have vug microporosity and contain barite that occurs as vug lining. The precipitation of barite results from a reaction between barium-rich and sulfate-free seep fluids and downwards-diffusing seawater. The carbonates in Chapopote occur in relation to tar flows and form crusts of wackestones with bivalve shells. The matrix is composed by low-Mg calcite with disseminated pyrite and detrital quartz. The carbonate mud shows depletion in 13C, with δ 13C between −36.9‰ and −23.0‰. Carbonates precipitate as a consequence of anaerobic methane oxidation coupled with seawater sulfate reduction. δ 18O values (3.4–4.4‰) suggest precipitation at temperatures between 5 and 9 °C.

Stable Isotope Geochemistry of the Carboniferous Zn-Pb-Cu Sediment-Hosted Sulfide Deposits, Northeastern Spain

Carboniferous sedimentary rocks in the southwestern Catalonian Coastal Ranges contain stratiform (Zn, Pb, Cu) sulfide deposits. Stable isotope compositions of sulfur, oxygen, and hydrogen were studied in five localities in order to establish the source of hydrothermal fluids and the influence of metamorphism on these deposits. Sulfur isotopes were analyzed in pyrrhotite, pyrite, sphalerite, chalcopyrite, and galena. The δ34S(CDT) values lie mostly between -1.0 and +7.0‰ (average +4.4‰). Except for pyrrhotite-pyrite pairs, all the sulfide minerals show isotopic disequilibrium. δ34S values suggest that sulfur was derived by thermochemical reduction of seawater sulfate in a deep-circulating convective system. Fluid inclusions were studied in quartz crystals from the sediment-hosted stratiform occurrences. They are two-phase (L-V), with salinity between 1.1 and 18.0 wt% NaCl eq., and homogenization temperatures mainly in the range of 220-260°C. Oxygen and hydrogen isotopes were analyzed in chlorite crystals from four ore-bearing beds. δD(V-SMOW) values in chlorite range from -63 to -33‰, whereas δ18O values lie between +2.7 and +6.8‰. δ18O(V-SMOW) values in quartz range between +8.4 and +10.7‰. Most of the calculated oxygen and hydrogen isotope compositions of ore-forming fluids plot close to the isotopic composition of seawater. Therefore, the ore-forming fluid must have been mainly seawater that interacted with sedimentary rocks during convective circulation within a basin. This process took place in a subsiding Carboniferous basin divided into sedimentary highs and depressions by synsedimentary normal faults, which acted as permeable areas that favored fluid circulation.

Ore-Forming Processes in Irish-Type Carbonate-Hosted Zn-Pb Deposits: Evidencefrom Mineralogy, Chemistry, and Isotopic Composition of Sulfides at the LisheenMine

Lisheen is a strata-bound zinc-lead deposit formed during the Mississippian by replacement of hydrothermally dolomitized, grossly stratiform breccia bodies located near the base of the Carboniferous Waulsortian Limestone. It represents one of a number of carbonate-hosted massive sulfide ore deposits in the Irish ore field that, due to several unique features, have been classified as Irish type. Disseminated pyrite occurs in preore dolomite and around the margins of preore dolomite clasts within dolomite breccias. Early fine-grained sphalerite-pyrite mineralization occurs as infill of intergranular dolomite porosity. Locally, massive to semimassive iron sulfide is observed, mainly comprising pyrite with lesser marcasite. A complex polymetallic sulfide assemblage typifies the main ore stage, dominated by fine-grained disseminated, massive or colloform sphalerite and galena, with minor pyrite, chalcopyrite, arsenopyrite, tennantite, nickel- and cobalt-bearing minerals. Silver occurs in solid solution in tennantite, galena, and sphalerite. Dolomite and barite dominate the gangue, with lesser calcite. Main-stage mineralization involved the progressive replacement of preexisting iron sulfides and the dolomite breccias, initially by replacement of the breccia matrix and ultimately by replacement of clasts. Coarse crystalline sphalerite and euhedral galena crystals are generally restricted to fracture-fill mineralization or vugs within main ore-stage assemblages where they occur with euhedral dolomite and calcite. Barite intergrown with main ore-stage sulfides has δ 34S values of 14.3 to 18.1 per mil, consistent with the derivation of sulfate from coeval Carboniferous seawater. The δ 34S values for sulfides range from –44.1 to +11.8 per mil, with a mean value of –13.7 per mil, typical of the Irish ore deposits. The dominant low δ 34S signature is considered to be the result of bacterial reduction of coeval seawater sulfate. Extremely low δ 34S values, in the range of –38 to –44 per mil, are only observed in preore disseminated pyrite; such extreme fractionations are thought to be due to low bacterial sulfate reduction rates coupled with oxidative cycles in near-sea floor pore waters. Main ore-stage sulfides have δ 34S values in the range of –4 to –18 per mil, with a mode of –10 per mil, consistent with a typical bacterial fractionation from coeval seawater sulfate. Isotopic equilibrium between cogenetic sulfides is not observed. The bacteriogenic sulfur component was probably transported from bacterial colonies fringing the ore system by low-temperature brines. The δ 34S values of late ore-stage sulfides mainly range from –20.2 to +12.0 per mil, with the majority having relatively high values (mean = –3.0 ± 8.5‰, 1 σ ) interpreted as being due to the presence of a hydrothermal sulfur component, leached from the lower Paleozoic basement. For galena and sphalerite there is a general increase in δ 34S values with depth in the system, with time, and with proximity to east-west– and northwest-trending faults. These relationships suggest that input of hydrothermal sulfur from depth via fractures became increasingly important. Hydrothermal sulfur appears to be more important at Lisheen than the other major Irish deposits. Galena lead isotope analyses gave average 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb values of 18.183, 15.594, and 38.080, respectively. These data do not correlate with ore-stage, galena texture or δ 34S. The results are comparable to previous data from Lisheen and from Silvermines, 35 km to the west, implying a common lead source in the lower Paleozoic basement. The textural, mineral, chemical, and isotopic evidence suggests that main-stage ore was precipitated as a consequence of rapid supersaturation, caused by fluid mixing within the permeable dolomite breccias. This process involved relatively high temperature (ca. 240°C), metal-bearing solutions derived from a basement-equilibrated fluid reservoir (carrying Zn, Pb, Fe, Cd) and shallow, saline (ca. 25 wt % NaCl equiv) formation waters rich in bacteriogenic H2S. Minor metals (Cu, As, Ni, Co) are thought to have been stripped from the footwall Old Red Sandstone during hydrothermal alteration around fault conduits. The availability of abundant seawater sulfate, operation of open-system bacterial sulfate reduction, and episodic availability of free oxygen imply that ore formation cannot have occurred at significant depth below the paleosea floor. Cessation of mineralization was due to a cut-off of the sulfur-rich brine supply, possibly by deposition of impermeable hanging-wall sediments. This process of ore formation is consistent with evidence from the other economic Irish-type deposits in the ore field.

Intracratonic crustal seawater circulation and the genesis of subseafloor zinc-lead mineralization in the Irish orefield

We have determined the chemical composition of ∼350-m.y.-old solutions extracted from fluid inclusions, and strontium isotopic compositions of hydrothermal minerals from the Irish zinc-lead orefield. These data show that ore-forming fluids were derived from evaporated seawater and acquired metals by deep circulation within fractures in continental crust. Mineralization occurred in the near-seafloor environment when these solutions returned to the surface via thermohaline convection and mixed with brines rich in H 2 S produced by bacterial reduction of seawater sulfate. The results indicate that deep penetration of seawater or evaporated seawater into the continental crust can occur in rift zones or extending passive margins and that this process can generate large volumes of base metal ore-forming solutions. Our results are inconsistent with topographic flow models for mineralization in the district, and support deep convection models for ore formation. The widespread development of evaporitic brines on the Laurussian continental margin under late Paleozoic greenhouse conditions is likely to have been critical for generating numerous accumulations of base metals in sedimentary basins at this time.

Sulfur Isotope Evidence for Magmatic Contributions to Submarine and Subaerial Gold Mineralization: Conical Seamount and the Ladolam Gold Deposit, Papua New Guinea

Conical seamount is a submarine alkali basalt volcano situated 10 km south of Lihir Island in the Tabar-Feni island chain, Papua New Guinea. Polymetallic (Zn-Pb-Ag-Au-As-Sb) veins, pyritic stockwork, and hydrothermally altered rocks discovered on the summit of Conical seamount represent a unique example of submarine gold mineralization with mineralogical, chemical, and textural characteristics common to some subaerial epithermal systems. The giant Ladolam epithermal gold deposit (42 Moz Au) is situated in the crater of an alkali stratovolcano on Lihir Island. Portions of the epithermal mineralization and alteration assemblages at Ladolam are similar to those observed on Conical seamount. Conical seamount has the lightest, hydrothermal (i.e., nondiagenetic) sulfide d34S values (-17.5 to +6.1%) measured to date from modern sea-floor hydrothermal systems. A previous model for the Conical seamount hydrothermal system suggested a contribution of magmatic volatiles in the earliest stages of mineralization (stage 1), followed by a combination of sulfur leached from the underlying volcanic rocks and reduced seawater sulfate during the main base and precious metal precipitating event (stage 2). New sulfur isotope data allow a reinterpretation of this model that suggests a greater input of magmatic volatiles, plus boiling, throughout both stage 1 and 2 mineralization. Sulfides from the Ladolam gold deposit have a range of sulfur isotope values (-12.9 to +3.6%) similar to that of Conical seamount. These sulfur isotope data have previously been interpreted as the result of magmatic volatiles escaping from a crystallizing magma and mixing with dilute meteoric ground water. Anhydrite associated with the gold mineralization has two populations of d34S values (8-14 and 20-22%), which indicate the interaction of magmatic volatiles with seawater that has previously been discounted. The range of sulfide and sulfate d34S values at Ladolam is reinterpreted to be due to a complex interaction of magmatic hydrothermal fluids, meteoric ground water, and seawater. Native sulfur in thermal areas in Luise caldera and pyrite in offshore hydrothermal vents in Luise harbor have similar d34S values to the Ladolam epithermal mineralization, indicating that fluids similar to the young Ladolam ore-forming event are still reaching the floor of Luise caldera. Sulfur isotope data from Conical seamount and the Ladolam deposit suggest that magmatic volatiles have contributed a significant amount of sulfur to both these gold-rich hydrothermal systems.

Geochemistry of the Ganesh Himal zinc-lead deposits, central Nepal Himalaya

A dolomite hosted strata-bound high-grade (19-25%) zinc-lead sulphide deposit occurs, between 4,000 m and 5,100 m over an area of 5 km2 in the ~ 7 km wide MCT zone of the Ganesh Himal area of central Nepal. The host crystalline milky white sugary dolomite occurs in a repeated sequence of garnet-mica schist, quartzite, calc-schist and concordant amphibolite of older Lesser Himalayan sequence (Upper Nawakot Group), all showing ductile deformation. The rocks along with the ore have undergone at least three phases of deformation. A series of overturned, steep northerly dipping and NE to ENE plunging anticlines and synclines form the dominating structure, characterised by disharmonic shape of the dolomite host rock because of apparent squeezing out from limbs into the axial regions. Compared to their strike lengths, the ore bodies and the host rock bodies have long extension along the plunge direction. The ore has a very simple composition of sphalerite-galena-pyrite with a little pyrrhotite, magnetite and chalcopyrite. Chemically, it consists mainly of zinc along with iron, lead, silver and very low silica, silicates and alumina. Concentrations of trace and rare elements are very low. Ore body types vary from dissemination and bands to massive sulphide lenses, arranged en echelon parallel to the schistosity/ bedding. The rocks of the area were subjected to almandine-amphibolite facies of metamorphism, to 750±150 MPa pressure and 500 °C to 750 °C temperature conditions. The latest thermal event was as young as ~12 Ma. The lead isotope data are interpreted to establish an age of 875 to 785 Ma for the Ganesh Himal deposits, while sulphur isotope data imply an age greater than 650 Ma. The Ganesh Himal deposit appears to be Vindhyan equivalent in the Himalaya, showing highest metal values as on date. Only two out of six occurrences in the area have been explored so far. The ore reserve estimates stand at 2.4 million tonnes with 14.66% zinc, 3.01 % lead, and 23.5 g/t silver. Taking into account all the occurrences the Ganesh Himal basin might have had 861,000 tonnes zinc and 182,000 tonnes lead at the minimum. The lithologic sequence represents a shallow marine facies of deposition. The δ34 S values for Ganesh Himal sulphides indicate that the sulphur was probably produced by biogenic reduction of contemporaneous seawater sulphate. The lead isotope ratios of Ganesh Himal deposit fall on a single-stage growth curve, on which also fall many big deposits of the world, indicating that the Ganesh Himal zinc-lead deposit has high potential. The primary control of the mineralisation is stratigraphic, the present ore body configuration being controlled by south vergent folding related with the metamorphism and thrusting along the MCT. Pyrite framboids and geochemical characters of ore indicate that the mineralisation is syngenetic sedimentary, and it may have been deposited in association with a bioherm or reef in an anoxic environment. The source of metal ions is not clear yet, but the contemporaneous basic rock bodies providing the metal ions could be a possibility.

POSSIBLE SUBMARINE ADVANCED ARGILLIC ALTERATION AT THE BASIN LAKE PROSPECT, WESTERN TASMANIA, AUSTRALIA

The Basin Lake copper-gold prospect lies in western Tasmania’s Mount Read Volcanics and is hosted in a series of calc-alkaline andesites, quartz-feldspar porphyries, mudstones, carbonates, and sandstones between the Tyndall Group and the Central Volcanic Complex. Alteration at the Basin Lake prospect occurs over a strike length of 1.4 km and includes thin, strata-bound pyrophyllite-quartz-paragonite-kaolinite-pyrite-alunite alteration zones, up to 12 m wide and containing up to 50 wt percent pyrophyllite, with local fluorite veining. These zones grade out to paragonite-muscovite-kaolinite-quartz-pyrite and muscovite-carbonate-chlorite alteration zones. Extensive propylitic alteration (chlorite-carbonate-epidote) affects most other rocks outside these zones. Mineralization consists of thin strata-bound zones of massive and vein pyrite, tennantite, and chalcopyrite, with trace covellite and galena, hosted mainly within an intensely silicified core of the pyrophyllite-quartzsericite alteration zone. Pyrite has δ34S values of –1.4 to +6.9 per mil, although marginal vein pyrite in the propylitic zone has δ34S values around 12.4 per mil. Large silicified glacial erratic boulders at surface contain massive and vein pyrite, enargite, and tennantite, with minor barite, and trace covellite, stannoidite, and mawsonite. Pyrite and enargite have δ34S values of 1.7 to 6.8 per mil; barite has δ34S values around 35.2 per mil with 87Sr/86Sr around 0.7108. The alteration and mineralization at the Basin Lake prospect is similar to that associated with high-sulfidation copper-gold systems formed by acidic, relatively oxidized fluids. A new geochemical vector, here termed the “advanced argillic alteration index” [AAAI = 100 (SiO2)/(SiO2 + 10MgO + 10CaO + 10Na2O)], has been devised to help quantify the intensity of alteration. The values of the AAAI at Basin Lake are similar to those of several high-sulfidation epithermal deposits. The low sulfide δ34S values are similar to those at other sulfide occurrences in the Mount Read Volcanics that have previously been considered to be barren, are lower than those of nearby volcanic-hosted massive sulfide deposits, and may indicate a magmatic fluid component. However, the δ34S and 87Sr/86Sr values of Basin Lake barite at the assumed highest exposed level of the system and higher δ34S values in pyrite from marginal veins are similar to those of Cambrian volcanic-hosted massive sulfide systems, indicating the involvement of reduced seawater sulfate at these locations. Calcite carbon and oxygen isotope values, silicate oxygen isotope values, and the unusual abundance of carbonate close to advanced argillic alteration indicate fluid mixing and suggest that acidic, magmatic fluids were likely neutralized by seawater. This occurrence strengthens the case for prospecting the Mount Read Volcanics and other similar submarine volcanic belts for copper-gold and gold-only deposits that formed by the actions of hyperacid oxidized fluids.

A study of sulfur isotopes in determining the genesis of Goynuk and Celaldagi Desandre Pb–Zn deposits, eastern Yahyali, Kayseri, Central Turkey

Pb–Zn deposits are widespread and common in various part of the Taurus Belt. Most of the deposits are related to faults in limestones and dolomitic limestones. Goynuk and Celaldagi Desandre deposits are typical of those found in the Yahyali district of the Aladag region, located northwest of the eastern Taurus. The concordant nature of these deposits with wall rocks differentiates them from other deposits in the same region. The following typical important features characterize the concordant and fault-related deposits: (1) the absence of any rocks indicating magmatic activity, (2) no evidence of thermochemical changes along the ore body-wall rocks contact, (3) the presence of a simple mineral assemblage in the paragenesis, (4) the presence of marcasite, reflecting the low temperature occurrence, (5) considerable carbonatization and sulfatization, and (6) the higher abundance of Pb, Zn, Cu, Ni and Co in wall/host rocks compared with those in the unaltered limestones. The δ 34S values for galena are within the range of −0.5 to 0.9 ‰ ( n=10, Mean=0.17±0.49) for Goynuk and 8.4 to 10.9 ‰ ( n=10, Mean=9.28±0.75) for Celaldagi Desandre. The origin of sulfur in Celaldagi Desandre is probably seawater, whereas Goynuk reflects the complete reduction of seawater sulfate within a closed basen. The concordant Pb–Zn deposits in the Aladag region are carbonate-hosted deposits. These were probably associated with rifting processes in the Eastern Mediterranean prior to the Early–Middle Triassic. The fault related deposits formed from leaching, transportation and re-deposition of the concordant deposits by seawater within fault zones.

Subsurface processes at the lucky strike hydrothermal field, Mid-Atlantic ridge: evidence from sulfur, selenium, and iron isotopes

At Lucky Strike near the Azores Triple Junction, the seafloor setting of the hydrothermal field in a caldera system with abundant low-permeability layers of cemented breccia, provides a unique opportunity to study the influence of subsurface geological conditions on the hydrothermal fluid evolution. Coupled analyses of S isotopes performed in conjunction with Se and Fe isotopes have been applied for the first time to the study of seafloor hydrothermal systems. These data provide a tool for resolving the different abiotic and potential biotic near-surface hydrothermal reactions. The δ 34S (between 1.5‰ and 4.6‰) and Se values (between 213 and 1640 ppm) of chalcopyrite suggest a high temperature end-member hydrothermal fluid with a dual source of sulfur: sulfur that was leached from basaltic rocks, and sulfur derived from the reduction of seawater sulfate. In contrast, pyrite and marcasite generally have lower δ 34S within the range of magmatic values (0 ± 1‰) and are characterized by low concentrations of Se (<50 ppm). For 82Se/ 76Se ratios, the δ 82Se values range from basaltic values of near −1.5‰ to −7‰. The large range and highly negative values of hydrothermal deposits observed cannot be explained by simple mixing between Se leached from igneous rock and Se derived from seawater. We interpret the Se isotope signature to be a result of leaching and mixing of a fractionated Se source located beneath hydrothermal chimneys in the hydrothermal fluid. At Lucky Strike we consider two sources for S and Se: (1) the “end-member” hydrothermal fluid with basaltic Se isotopic values (−1.5‰) and typical S isotope hydrothermal values of 1.5‰; (2) a fractionated source hosted in subsurface environment with negative δ 34S values, probably from bacterial reduction of seawater sulfate and negative δ 82Se values possibly derived from inorganic reduction of Se oxyanions. Fluid trapped in the subsurface environment is conductively cooled and has restricted mixing and provide favorable conditions for subsurface microbial activity which is potentially recorded by S isotopes. Fe isotope systematic reveals that Se-rich high temperature samples have δ 57Fe values close to basaltic values (∼0‰) whereas Se-depleted samples precipitated at medium to low temperature are systematically lighter (δ 57Fe values between −1 to −3‰). An important implication of our finding is that light Fe isotope composition down to −3.2‰ may be explained entirely by abiotic fractionation, in which a reservoir effect during sulfide precipitation was able to produce highly fractionated compositions.

Sulfur isotope distribution in Late Silurian volcanic‐hosted massive sulfide deposits of the Hill End Trough, eastern Lachlan Fold Belt, New South Wales

Volcanic‐hosted massive sulfide (VHMS) deposits of the eastern Lachlan Fold Belt of New South Wales represent a VHMS district of major importance. Despite the metallogenic importance of this terrane, few data have been published for sulfur isotope distribution in the deposits, with the exception of previously published studies on Captains Flat and Woodlawn (Captains Flat–Goulburn Trough) and Sunny Corner (Hill End Trough). Here is presented 105 new sulfur isotope analyses and collation of a further 92 analyses from unpublished sources on an additional 12 of the VHMS systems in the Hill End Trough. Measured δ34S values range from −7.4‰ to 38.3‰, mainly for massive and stockwork mineralisation. Sulfur isotope signatures for polymetallic sulfide mineralisation from the Lewis Ponds, Mt Bulga, Belara and Accost deposits (group 1) are all very similar and vary from −1.7‰ to 5.9‰. Ore‐forming fluids for these deposits were likely to have been reducing, with sulfur derived largely from a magmatic source, either as a direct magmatic contribution accompanying felsic volcanism or indirectly through dissolution and recycling of rock sulfide in host volcanic sequences. Sulfur isotope signatures for sulfide mineralisation from the Calula, Commonwealth, Cordillera and Kempfield deposits, Peelwood mine and Sunny Corner (group 2) are similar and have average δ34S values ranging from 5.4‰ to 8.1‰. These deposits appear to have formed from ore fluids that were more oxidising than group 1 deposits, representing a mixed contribution of sulfur derived from partial reduction of seawater sulfate, in addition to sulfur from other sources. The δ34S values for massive sulfides from the John Fardy deposit are the highest in the present study and have a range of 11.9–14.5‰, suggesting a greater component of sulfur of seawater origin compared to other VHMS deposits in the Hill End Trough. For barite the sulfur isotope composition for samples from the Commonwealth, Stringers and Kempfield deposits ranges from 12.6‰ to 38.3‰. More than 75% of barite samples have a sulfur isotope composition between 23.4 and 30.6‰, close to the previously published estimates of the composition of seawater sulfate during Late Silurian to earliest Devonian times, providing supporting evidence that these deposits formed concurrently with the Late Silurian volcanic event. Sulfur isotope distribution appears to be independent of the host rock unit, although there appears to be a relation linking the sulfur isotope composition of different deposits to defined centres of felsic volcanism. The Mt Bulga, Lewis Ponds and Accost systems are close to coherent felsic volcanic rocks and/or intrusions and have sulfur isotope signatures with a stronger magmatic affinity than group 2 deposits. By contrast, group 2 deposits (including John Fardy) are characterised by 34S‐enrichment and a lesser magmatic signature, are generally confined to clastic units and reworked volcanogenic sediments with lesser coherent volcanics in the local stratigraphy, and are interpreted to have formed distal from the magmatic source. An exception is the Belara deposit, which is hosted by reworked felsic volcanic rocks and has a more pronounced magmatic sulfur isotope signature.

Sulfur isotope distribution in Late Silurian volcanic‐hosted massive sulfide deposits of the Hill End Trough, eastern Lachlan Fold Belt, New South Wales

Volcanic‐hosted massive sulfide (VHMS) deposits of the eastern Lachlan Fold Belt of New South Wales represent a VHMS district of major importance. Despite the metallogenic importance of this terrane, few data have been published for sulfur isotope distribution in the deposits, with the exception of previously published studies on Captains Flat and Woodlawn (Captains Flat‐Goulburn Trough) and Sunny Corner (Hill End Trough). Here is presented 105 new sulfur isotope analyses and collation of a further 92 analyses from unpublished sources on an additional 12 of the VHMS systems in the Hill End Trough. Measured δ34S values range from ‐7.4% to 38.3%, mainly for massive and stockwork mineralisation. Sulfur isotope signatures for polymetallic sulfide mineralisation from the Lewis Ponds, Mt Bulga, Belara and Accost deposits (group 1) are all very similar and vary from ‐1.7% to 5.9%. Ore‐forming fluids for these deposits were likely to have been reducing, with sulfur derived largely from a magmatic source, either as a direct magmatic contribution accompanying felsic volcanism or indirectly through dissolution and recycling of rock sulfide in host volcanic sequences. Sulfur isotope signatures for sulfide mineralisation from the Calula, Commonwealth, Cordillera and Kempfield deposits, Peelwood mine and Sunny Corner (group 2) are similar and have average δ34S values ranging from 5.4% to 8.1%. These deposits appear to have formed from ore fluids that were more oxidising than group 1 deposits, representing a mixed contribution of sulfur derived from partial reduction of seawater sulfate, in addition to sulfur from other sources. The δ34S values for massive sulfides from the John Fardy deposit are the highest in the present study and have a range of 11.9–14.5%, suggesting a greater component of sulfur of seawater origin compared to other VHMS deposits in the Hill End Trough. For barite the sulfur isotope composition for samples from the Commonwealth, Stringers and Kempfield deposits ranges from 12.6% to 38.3%. More than 75% of barite samples have a sulfur isotope composition between 23.4 and 30.6%, close to the previously published estimates of the composition of seawater sulfate during Late Silurian to earliest Devonian times, providing supporting evidence that these deposits formed concurrently with the Late Silurian volcanic event. Sulfur isotope distribution appears to be independent of the host rock unit, although there appears to be a relation linking the sulfur isotope composition of different deposits to defined centres of felsic volcanism. The Mt Bulga, Lewis Ponds and Accost systems are close to coherent felsic volcanic rocks and/or intrusions and have sulfur isotope signatures with a stronger magmatic affinity than group 2 deposits. By contrast, group 2 deposits (including John Fardy) are characterised by 34S‐enrichment and a lesser magmatic signature, are generally confined to clastic units and reworked volcanogenic sediments with lesser coherent volcanics in the local stratigraphy, and are interpreted to have formed distal from the magmatic source. An exception is the Belara deposit, which is hosted by reworked felsic volcanic rocks and has a more pronounced magmatic sulfur isotope signature.

Isotopic composition of gypsum in the Macquarie Island ophiolite: Implications for the sulfur cycle and the subsurface biosphere in oceanic crust

The O, S, and Sr isotope compositions were determined for 17 samples of gypsum that replaced anhydrite in the sheeted-dike complex of the Macquarie Island ophiolite. Elevated d34S (26.2%-29.0%) and d18O values (12.5%-14.4%) of gypsum compared to those of seawater sulfate are the result of microbial sulfate reduction. Low organic carbon contents and little sulfate reduction in sediments, plus a large basaltic Sr component in the gypsum (87Sr/86Sr = 0.70446-0.70524), indicate that the sulfate source was not pore waters in the overlying sediment. Low d34S values of sulfide in basalt lavas are consistent with microbial reduction of seawater sulfate within the volcanic rocks. Tectonic activity at the slowspreading ridge allowed evolved formation waters to enter hot sheeted-dike complex basement, resulting in heating and precipitation of anhydrite. Results show that microbes can leave geochemical tracers of their activity in oceanic basement and that anhydrite can be preserved in oceanic crust and may be of significance for the oceanic sulfur budget.

Genesis of the Dogankuzu and Mortas Bauxite Deposits, Taurides, Turkey: Separation of Al, Fe, and Mn and Implications for Passive Margin Metallogeny

The Taurides region of Turkey is host to a number of important bauxite, Al-rich laterite, and Mn deposits. The most important bauxite deposits, Dog ˘ankuzu and Mortas¸, are karst-related, unconformity-type deposits in Upper Cretaceous limestone. The bottom contact of the bauxite ore is undulatory, and bauxite fills depressions and sinkholes in the footwall limestone, whereas its top surface is concordant with the hanging-wall limestone. The thickness of the bauxite varies from 1 to 40 m and consists of bohmite, hematite, pyrite, marcasite, anatase, diaspore, gypsum, kaolinite, and smectite. The strata-bound, sulfide- and sulfate-bearing, low-grade lower part of the bauxite ore bed contains pyrite pseudomorphs after hematite and is deep red in outcrop owing to supergene oxidation. The lower part of the bauxite body contains local intercalations of calcareous conglomerate that formed in fault-controlled depressions and sinkholes. Bauxite ore is overlain by fine-grained Fe sulfidebearing and calcareous claystone and argillaceous limestone, which are in turn overlain by massive, compact limestone of Santonian age. That 50-m-thick limestone is in turn overlain by well-bedded bioclastic limestone of Campanian or Maastrichtian age, rich with rudist fossils. Fracture fillings in the bauxite orebody are up to 1 m thick and consist of bluish-gray-green pyrite and marcasite (20%) with bohmite, diaspore, and anatase. These sulfide veins crosscut and offset the strata-bound sulfide zones. Sulfur for the sulfides was derived from the bacterial reduction of seawater sulfate, and Fe was derived from alteration of oxides in the bauxite. Iron sulfides do not occur within either the immediately underlying or overlying limestone. The platform limestone and shale that host the bauxite deposits formed at a passive margin of the Tethys Ocean. Extensive vegetation developed on land as the result of a humid climate, thereby creating thick and acidic soils and enhancing the transport of large amounts of organic matter to the ocean. Alteration of the organic matter provided CO2 that contributed to formation of a relatively 12 C-rich marine footwall limestone. Relative sea-level fall resulted from strike-slip faulting associated with closure of the ocean and local uplift of the passive margin. That uplift resulted in karstification and bauxite formation in topographic lows, as repre

Sulphur isotope composition of Silurian shale-hosted PGE-Ag-Au-Zn-Cu mineralisations of the Prades Mountains (Catalonia, Spain)

Several Silurian metamorphosed shale-hosted sulphide occurrences have been studied in the Prades Mountains, southern part of the Coastal Catalonian Ranges. Most of the sulphides are found as stratiform or shale-disseminated occurrences. Pyrrhotite is the most abundant sulphide mineral. Chalcopyrite and arsenopyrite are less common. Gold, Pd-bearing lollingite (partly replaced by arsenopyrite), sperrylite, hessite, clausthalite, altaite, galena, sphalerite, molybdenite, scheelite, and V-Cr oxides and silicates are minor components. Sulphur isotopic analyses were made on three outcrops (Roca de Ponent, Coma Fosca and Sant Miquel) and in the Silurian black shales of the Sant Bernat series, in order to determine the origin of the sulphur which formed these deposits. The Coma Fosca and Sant Miquel outcrops yield a narrow range of δ34S values (–11.3 to –4.6‰), whereas the Roca de Ponent and Sant Bernat series have a wider range (–19.9 to –7.6‰ in pyrrhotite, and up to +36‰ in pyrite from the top of the Sant Bernat series). In the Roca de Ponent outcrop the δ34S values decrease with increasing stratigraphic height. All the deposits have a mean δ34S composition close to –9‰. Annealing of pyrrhotite took place during the Hercynian metamorphism. Later, during the retrograde metamorphism, pyrite replaced pyrrhotite, and arsenopyrite replaced lollingite. Large-scale compositional inhomogeneities survived metamorphism (from pyroxene to amphibolite facies). A hydrothermal source of S has been postulated for the Coma Fosca and Sant Miquel outcrops. Sulphur in these deposits is from leaching of sulphides from the underlying rocks. In the Roca de Ponent outcrop and in the Sant Bernat series, sulphur was derived from a hydrothermal source and from biogenic reduction of seawater sulphate. The contribution of the latter source increased with time. Biogenic reduction of seawater sulphide in a closed or semi-closed system for sulphate is considered responsible of the high δ34S values of pyrite from the uppermost part of the Sant Bernat series. Based on textural and isotopic evidence, we propose an exhalative origin for the precious metals, cogenetic with the stratiform sulphides.

Variations of sulfur isotopes in metamorphic rocks from Bamble Sector, southern Norway: a laser probe study

Variations of sulfur isotopes in metamorphic rocks from Bamble Sector, southern Norway, are investigated using a Q-switched, pulsed mode, laser combustion technique. The results indicate that laser-related fractionations are reproducible (1 σ=0.2‰), provided uniform conditions are maintained during analysis. Bamble is a Precambrian high-grade metamorphic terrain. A transition from amphibolite- to granulite-facies occurs across the 25-km width of the Sector. Supracrustal rocks of mid-Proterozoic ages were intruded by felsic and mafic magmas and metamorphosed during the Sveconorwegian orogenic episode. Earlier studies indicated that Bamble rocks equilibrated under a relatively high f O 2 , 2–3 log units above QFM buffer, during peak metamorphism [J. Geochemical Exploration 31 (1989) 149; J. Geology 100 (1992) 447; Contrib. Mineral Petrol. 1139 (2000) 180], which led to phase changes in sulfides. Pyrrhotite, or Po mss, has variably been replaced by pyrite±magnetite. Mantling of pyrite by magnetite is common. Metasupracrustal rocks display a large variation in δ 34S CDT (−1‰ to +12‰), consistent with data from sedimentary rocks of similar age. No distinction can be made between amphibolite, transition, and granulite zones. The isotopic signature of sulfides was determined by bacteriogenic reduction of seawater sulfate with an initial δ 34S value of 20–30‰. Tonalitic–trondhjemitic gneisses from granulite zone show a large scatter in δ 34S CDT (0‰ to +8‰), implying that the parent magmas were contaminated by sedimentary materials. This is supported by S/Se ratios. Mafic rocks from various metamorphic zones display a clustering of δ 34S CDT values around 0‰, and S/Se ratios in the range 3×10 3 to 6×10 3. This is consistent with a mantle source for S. Ni–Cu sulfide ores locally associated with the mafic rocks, however, are enriched in 34S, indicating the involvement of sedimentary S in the formation of the ores. Mass and isotopic balance calculations indicate that oxidation of pyrrhotite, or Po mss, proceeded without significant addition or removal of S. Sulfur liberated during mantling of Fe–sulfides by magnetite was in a reduced form and reprecipitated in short distances as secondary overgrowth and fracture filling pyrite. The large variation of δ 34S values for the supracrustal rocks, and the preservation of the original isotopic features for various rock units suggest that no isotopic homogenization occurred in spite of the high-grade metamorphism.

BACTERIA WERE RESPONSIBLE FOR THE MAGNITUDE OF THE WORLD-CLASSHYDROTHERMAL BASE METAL SULFIDE OREBODY AT NAVAN, IRELAND

We report sulfur and lead isotope analyses of mine concentrates from Navan, the largest zinc deposit in Ireland. These samples, each representing up to a million tonnes of ore, show very limited isotopic variation, with mean δ 34 S = –13.6 ± 2 per mil (n = 20), and galena concentrate mean 206 Pb/ 204 Pb = 18.19 ± 0.03 (n = 7). Calculations suggest =90 percent of the Navan sulfides were derived through bacteriogenic reduction of Mississippian seawater sulfate, whereas metals were acquired from a local, orogenic crustal source beneath the orebody. Enhanced bacterial activity was fundamental to ore deposition at Navan: no bacteria, no giant ore deposit.

Fluid inclusion and sulfur isotope studies in probable modern analogue Kuroko-type ores from the JADE hydrothermal field (Central Okinawa Trough, Japan)

Fluid inclusions and sulfur isotopic compositions were studied in sulfides and barites from the JADE active hydrothermal field in the Central Okinawa Trough. The mineral assemblages in the JADE field strongly resemble ancient Kuroko-type deposits. The formation of massive sulfide mineralization is related to alteration of felsic volcanic rocks by deep penetrating heated seawater. Fluid inclusions in sphalerite from stockwork mineralization show homogenization temperatures that range between 270°C and 360°C and strong variations in salinity (2–15 equiv. wt.% NaCl). Varying salinities are attributed to two-phase separation that partially occurred at deeper levels. The resulting brine favored the transportation of other metals besides Pb, Zn, Fe, Cu, and led to a complex tetrahedrite/tennantite and enargite-bearing sulfide mineralization in the stockwork region. Fluid inclusions in enargite, therefore, show salinities more than three times higher than seawater. Even though the δ 34S values of sulfide separates from stockwork mineralization suggest a common sulfur source, ore deposition occurred under non-equilibrium isotopic conditions. Barite in stockwork mineralization, smokers, and mounds on the seafloor precipitated via mixing of hydrothermal solution with seawater. Microthermometric data of barite-hosted fluid inclusions point to maximum formation temperatures of between 150°C and 200°C. Partial reduction of seawater sulfate is indicated by locally observed δ 34S values in barite heavier than the ambient seawater.

Metal leaching and inorganic sulfate reduction in volcanic-hosted massive sulfide mineral systems: Evidence from the paleo-Archean Panorama district, Western Australia

Comprehensive studies of the well-preserved, paleo-Arehean (3.6-3.2 Ga) Panorama volcanic-hosted massive sulfide district of Western Australla provide compelling evidence that metals were leached from the hase of the volcanic pile and redeposited at its top in volcanic-hosted massive sulfide deposits. This leaching provided more than enough metal to form known deposits, implying that direct magmatic input of metal is not required. Sulfur depletion from the base of the volcanic pile was associated with an increase in Fe2O3/FeO and hematitic alteration. These data, combined with sulfur isotope data, indicate that seawater sulfate reduction was facilitated by the olddation of rock FeO to hematite at high temperature in the H2S stability field. This is the first time that seawater sulfate reduction has been demonstrated regionally in an ancient volcanic-hosted massive sulfide mineral system. The data presented here require pah-Archean seawater to be sufate bearing.

Constraints on the energy and chemical balances of the modern TAG and ancient Cyprus seafloor sulfide deposits

The size, chemical composition, energy flux, and fluid composition of the TAG hydrothermal sulfide deposit at the Mid‐Atlantic Ridge and the size, chemical composition and reaction zone characteristics of the Skouriotissa volcanogenic massive sulfide deposit of the Troodos ophiolite in Cyprus are used to examine the energy requirements and chemical balances associated with the generation of a large volcanogenic massive sulfide deposit. We conclude that formation of large sulfide deposits from oceanic hydrothermal systems is a geologically rapid process and occurs on timescales of hundreds of years, with episodes of activity as short as a few tens of years separated by thousands of years of inactivity. About 2×1019 J of energy supplied at high temperature is required to form a deposit the size of the TAG mound. Metals (up to 4 times the mass of any element present in the sulfide deposit) are leached out of relatively small reaction zones most likely at the base of the sheeted dikes. Chemical balance can be struck for all elements except sulfur with a reaction zone 1–2 km3 in volume from which a small proportion of iron and large proportions of copper, zinc, and manganese are removed. A sulfur balance requires that a significant fraction of sulfur be derived from reduction of seawater sulfate, as suggested by stable isotope analyses. We argue that the principal source of energy that drives hydrothermal circulation is latent heat of crystallization of magma close to the top of the plutonic section. Furthermore, we speculate that activity of the TAG hydrothermal system is related to periods of more rapid magma supply from the mantle at magma supply rates similar to those observed in volcanoes in Hawaii and Iceland.