Early Sulfide Research Articles

Gold-Bearing Mesothermal Veins from the Gubong Mine, Cheongyang Gold District, Republic of Korea: Fluid Inclusion and Stable Isotope Studies

The Gubong deposit consists of five massive, gold-bearing mesothermal quartz veins that fill fractures oriented northeast and northwest along fault shear zones over an area of 14 km2 in Precambrian metasedimentary rocks of the Gyeonggi massif. The veins are divided into three groups, based on their orientation and location. They have a ribbon texture that is interpreted to result from repeated hydraulic fracturing events. Mineral deposition was associated with hydrothermal fluid overpressuring within and below the active fault shear zones. The vein mineralogy and paragenesis of the veins allow two separate discrete mineralization episodes to be recognized, separated by a major faulting event. The ore minerals are contained within quartz and calcite associated with fracturing and healing of veins that occurred during both mineralization episodes. Wall-rock alteration minerals during stage I, the main ore stage, include sericite, chlorite, and minor pyrite and carbonates. Sulfide minerals deposited along with electrum during this stage include arsenopyrite, pyrite, pyrrhotite, sphalerite, marcasite, chalcopyrite, galena, and argentite. Electrum also was deposited during stage II mineralization in the one group of veins, along with pyrite, sphalerite, chalcopyrite, and galena, but the second stage of deposition in other veins was barren. Petrographic examination of textural relationships among sulfides, fluid inclusions, microfracturing, and quartz shows chronological and genetic relationships between gold deposition and fluid entrapment. Systematic studies of fluid inclusions in stage I vein indicate two contrasting events: a relatively high temperature (203°–432°C) and pressure (943–2,098 bars) event related to early sulfide deposition and associated with H2O-CO2-CH4-NaCl ± N2 fluids (less than about 13.4 wt % NaCl), and a lower temperature (202°–399°C) and pressure (670–850 bars) late sulfide depositional event involving H2O-NaCl fluids (3.9–17.3 wt % NaCl). The H2O-NaCl fluid involved in ore-related stage II mineralization had a salinity of 0.4 to 4.2 wt percent NaCl and a homogenization temperature of 201° to 378°C. The calculated sulfur isotope compositions of hydrothermal fluids from the stage I veins (δ34SH2S = 3.5–10.5‰) indicate that ore sulfur was derived mainly from a magmatic source but also in part from sulfur contained in the host rocks. The calculated and measured oxygen and hydrogen isotope compositions of the ore-forming fluids (stage I: δ18OH2O = 1.1–9.0‰, δD = −92 to −21‰; stage II: δ18OH2O = −0.1 to +0.3‰, δD = −95 to −93‰) indicate that the fluids were derived from magmatic and/or deep-seated metasedimentary rocks (stage I) and evolved by mixing with local meteoric water (stage II), by limited water-rock exchange and by degassing during mineralization in uplift zones. The H2O-NaCl fluids involved in the stage I development of these veins represent fluids that evolved either through unmixing of H2O-CO2-CH4-NaCl ± N2 fluids following a decrease in fluid pressure or through mixing with deeply circulating meteoric waters, possibly as a result of uplift and/or unloading during mineralization. The H2O-NaCl fluid involved in stage II was derived from meteoric water. Early deposition of gold in stage I was caused by a decrease in sulfur fugacity (H2S loss) that accompanied the immiscible separation of carbonic vapor from H2O-CO2-CH4-NaCl ± N2 fluids. Gold in late stage I and stage II veins was precipitated from H2O-NaCl fluids by cooling and by dilution caused by mixing with meteoric water.

Sulphur isotope constraints on formation conditions of the Luiswishi ore deposit, Democratic Republic of Congo (DRC)

Luiswishi is a Congo-type Neoproterozoic sediment-hosted stratiform Cu–Co ore deposit of the Central Africa Copperbelt, located northwest of Lubumbashi (DRC). The ores form two main Cu–Co orebodies hosted by the Mines Subgroup, one in the lower part of the Kamoto Formation and the other at the base of the Dolomitic Shales Formation. Sulphides occur essentially as early parallel layers of chalcopyrite and carrolite, and secondarily as late stockwork sulphides cross-cutting the bedding and the early sulphide generation. Both types of stratiform and stockwork chalcopyrite and carrolite were systematically analyzed for sulphur isotopes, along the lithostratigraphic succession of the Mine Series. The quite similar δ 34S values of stratiform sulphides and late stockwork sulphides suggest an in situ recrystallization or a slight remobilization of stockwork sulphides without attainment of isotopic equilibrium between different sulphide phases (chalcopyrite and carrolite). The distribution of δ 34S values (−14.4‰ to +17.5‰) combined with the lithology indicates a strong stratigraphic control of the sulphur isotope signature, supporting bacterial sulphate reduction during early diagenesis of the host sediments, in a shallow marine to lacustrine environment. Petrological features combined with sulphur isotopic data of sulphides at Luiswishi and previous results on nodules of anhydrite in the Mine Series indicate a dominant seawater/lacustrine origin for sulphates, precluding a possible hydrothermal participation. The high positive δ 34S values of sulphides in the lower orebody at Luiswishi, hosted in massive chloritic–dolomitic siltite (known as Grey R.A.T.), fine-grained stratified dolostone (D.Strat.) and silicified-stromatolitic dolomites alternating with chloritic–dolomitic silty beds (R.S.F.), suggest that they were probably deposited during a period of regression in a basin cut off from seawater. The variations of δ 34S values (i.e. the decrease of δ 34S values from the Kamoto Formation to the overlying Dolomitic Shales and then the slight increase from S.D.2d to S.D.3a and S.D.3b members) are in perfect agreement with the inferred lithological and transgressive–regressive evolution of the ore-hosting sedimentary rocks [Cailteux, J., 1994. Lithostratigraphy of the Neoproterozoic Shaba-type (Zaire) Roan Supergroup and metallogenesis of associated stratiform mineralization. In: Kampunzu A.B., Lubala, R.T. (Eds.), Neoproterozoic Belts of Zambia, Zaire and Namibia. Journal of African Earth Sciences 19, 279–301].

Platinum-group element distribution in the Main Zone and Upper Zone of the Bushveld Complex, South Africa

The platinum-group element (PGE) contents of the upper portions of the Bushveld Complex were investigated with three questions in mind: (a) In natural systems does magnetite concentrate Os, Ir, Ru and Rh (IPGE), as has been observed in experimental systems? (b) Is there a Au–Pd-enriched layer present such as observed in the upper parts of the Skaergaard intrusion? (c) Can changes in metal ratios be used for prospecting for PGE deposits? In the sulfide-poor Main Magnetite Layer of the eastern Bushveld, Ir and Rh are enriched relative to Pt, Pd and Au and this could be because magnetite preferentially concentrated Ir and Rh over Pt, Pd and Au. However, in most other magnetite layers no enrichment was observed. This could be because most magnetite layers contain approximately 1% sulfides and the PGE budget is dominated by the sulfides. These sulfides obscured the effects of magnetite collecting IPGE, because sulfides collect all the PGE and the partition coefficients for the PGE into a sulfide liquid are much greater than the partition coefficient for IPGE into magnetite. The weighted average of the platinum-group elements (PGE) and Au over the 2000 m of sampled stratigraphy is Au 2.1 ppb, Pd 1.7 ppb, Pt 1.7 ppb, Rh 0.18 ppb, Ru<0.5, Ir 0.16 ppb, Os<0.5 ppb. Compared to the marginal rocks (presumed initial liquids) of the Bushveld Complex the PGE and Au are severely depleted. Only one sample (a leuconorite in the first cyclic unit) contained Pt and Pd at economic grade (Pt 2 ppm, Pd 2 ppm). The overall depletion of the PGE in the Upper Zone (despite the presence of 1% to 3% sulfides) could be the result of the PGE having been stripped from the magma by early sulfide liquid which had already settled out of the magma to form the world famous platinum reefs lower in the magma chamber. In addition to the overall depletion of PGE, PGE/S ratios decrease up section indicating that the sulfide fraction is poorer in PGE up section. This is interpreted to be the result of continued depletion of the silicate liquid as sulfides constantly settle out of the silicate liquid. There appears to be little prospect of a Pd-reef type deposit in the Upper Zone. Comparison of the composition of the marginal rocks of the Bushveld with a weighted average for the complete 6 km of the Bushveld cumulates shows that the cumulate pile is much richer in compatible elements (Ir, Rh, Cr) and poorer in incompatible elements (Sm and Hf) than the marginal rocks. Two possible solutions to this are: (a) The magma emplaced into the chamber was a crystal mush and thus more mafic than marginal rocks to the intrusion or (b) fractionated magma has been removed from the cumulate sequence and either irrupted or intruded the country rocks as granites and granophyres.

Fluid inclusions and quantitative model for gold precipitation in the Tiouit deposit (Anti-Atlas, Morocco)

In the Tiouit gold deposit (Anti-Atlas, Morocco), a paragenetic study has revealed the existence mainly of two generations of gold, the earliest one was cogenetic with ferro-arseniferous stage I; the second generation (most important) is associated with the minerals of a cupro-zinciferous sulphide stage II. A microthermometric study of fluid inclusions demonstrates that the main part of the gold was introduced by an aqueous solution of relatively low salinity (1.7 m Nacl) and deposited at about 400 °C and 500 bars in microfractures crosscutting early sulphides. A calculated quantitative model for the fluid-mineral interaction demonstrates that a significant amount of gold (3 mg deposited from 1 kg of solution, i.e. 43% of total gold), transported as bisulphide complexes ( HAu ( HS ) 2 0 , AuHS 0 and Au ( HS ) 2 - ), can be deposited as a result of a decrease in both H 2S activity and oxygen fugacity produced by the alteration of pyrrhotite into pyrite.

Sulfur isotopic studies of continental flood basalts in the Noril’sk region: implications for the association between lavas and ore-bearing intrusions

Previous studies of both ore and non-ore-bearing intrusives in the Permo-Triassic flood basalts of the Siberian platform in the Noril’sk area have shown that high-grade Ni–Cu–platinum group elements (PGE) mineralization is associated with anomalously high δ 34S values of ∼8 to 12‰. In addition, several researchers have proposed that observed depletions in the Cu, Ni, and PGE content of basaltic lavas of the Nadezhdinsky ( Nd) Formation are related to diffusional exchange with, and upgrading in metal tenor of, sulfides in the volcanic conduit system. Sulfur isotopic studies of the lavas at Noril’sk were initiated to determine if interaction with crustally derived sulfur in the conduit system was evident, and if the Nd lavas in particular were characterized by an anomalous isotopic signature. δ 34S values of the lavas range from −4.5 to 8.7‰ Vienna Cañon Diablo Troilite (VCDT), with S concentrations from <40 to 1373 ppm. The majority of δ 34S values range from 0 to 4‰, and are similar to those from S-poor intrusions in the Noril’sk area. Although textural data are not supportive of early sulfide saturation and the presence of immiscible sulfide droplets in the lavas, recrystallization may have erased expected mineralogical and textural evidence. Mineralogical data indicate that hydrothermal alteration of the lavas has occurred, but S redistribution has been restricted to localized areas and δ 34S values have not been affected. The relatively low S concentrations of the lavas are thought to be due in large part to degassing of the lavas in the shallow conduit system and during eruption. Our calculations are consistent with the premise that degassing of basaltic magmas at temperatures in excess of ∼900°C at QFM leads to only minor 34S-depletion of sulfur remaining in the melt, and decreases in δ 34S values of less than 2‰ at 90% degassing. For this reason all lavas with δ 34S values in excess of ∼ 2‰ require a contribution of 34S-enriched country rock sulfur. Because of the high S content and δ 34S value (∼ 16–20‰) of evaporites in the country rocks at Noril’sk, contamination of less than 0.5% is required to explain the most 34S-enriched lavas. The Nd lavas have an average δ 34S of 2.9‰, but show no difference in S isotopic composition relative to the other lavas, suggesting that metal depletion involved only limited S transfer, or that exchange between mantle-derived S and S of crustal origin buffered δ 34S values to less than ∼5‰. Anomalously positive δ 34S values, similar to those of the ore-bearing intrusives in the Noril’sk region, are not consistently found in low-S rocks, either lavas or intrusives. Although the mechanism for the derivation of sulfide in the ore-bearing intrusions remain speculative, it is clear that the formation of sulfide ores characterized by high metal tenors proceeded only in the presence of sulfur of crustal origin.

Compositional Variations of Olivine and Sulfur Isotopes in the Noril'skand Talnakh Intrusions, Siberia: Implications for Ore-Forming Processes inDynamic Magma Conduits

Olivine from the Noril’sk I, Main and Northwestern Talnakh ore-bearing intrusions in the Noril’sk region occurs as inclusions, discrete grains, and poikilitic crystals. The original compositions of olivine inclusions have been well preserved by their host oikocrysts from reaction with interstitial silicate and sulfide liquids. In contrast, the compositions of discrete and poikilitic olivine crystals have been variably modified by reequilibration with trapped silicate and sulfide liquids. There are abrupt changes in the contents of nickel in olivine inclusions (500–1,500 ppm) between different rock units in all three intrusions, which indicates that the different rock units in these intrusions are not related to each other via in situ differentiation of a common parental magma but rather are consistent with separate pulses of magma with variable degrees of differentiation, possibly involving olivine crystallization and assimilation of siliceous crustal material in a staging chamber. The δ 34S values of the samples with high sulfur contents (>800 ppm sulfur) from all three ore-bearing intrusions vary between 8 and 14 per mil. Two sulfur-poor samples (<600 ppm sulfur) from the Northwestern Talnakh intrusion yield δ 34S values of 1.5 and 3.6 per mil. These values are similar to the values of the sulfur-poor rocks from the ore-barren intrusions in the region and illustrate the fact that high sulfur contents couple with increasing δ 34S values. Such a relationship is consistent with the addition of crustally derived 34S-enriched sulfur to the magma of the ore-bearing intrusions. In contrast to other models of sulfide segregation in closed systems, we propose that sulfide segregation occurred in an open system and the magma involved was undersaturated in sulfur and undepleted in chalcophile elements. We envision that the 34S-enriched sulfur was introduced via circulating fluids that transported sulfur from evaporite in the underlying sedimentary sequence to the magma in a dynamic magma conduit. The immiscible sulfide liquids that segregated during sulfur contamination settled down in the wider parts of the conduit as the magma continued to travel. Successive pulses of new magma entered the conduit, reacted with the early sulfide, and displaced much of the early magma in the conduit. The ore-bearing intrusions are thought to have crystallized within the wider parts of the conduits and their voluminous, sulfide-poor peripheral sills represent some (if not all) of the magma that flowed out of the conduits.

Amphibole zoning, a record of progressive oxidation during crystallization of mafic microgranular enclaves in the Kurobegawa Granitic Pluton

Sulfide, oxide, and amphibole in MME (mafic microgranular enclaves) in the young Kurobegawa Granitic Pluton (2 Ma) indicate that their textural and mineralogical features are characteristic of progressively oxidized magnetite-series granitoids. Hydrous mafic magma intruded into the chamber from the bottom and fragmented to form MME, subsequently vesiculated and floated through the chamber. The MME generally have sharp fine chilled margins and show no in situ mixing between the MME and the host granite. Amphibole, an early phase together with sulfides and plagioclase, began to crystallize just after the mafic magma intruded into the chamber bottom (10 km in depth) and then continued to crystallize as the MME floated. Amphibole shows strongly patchy zoning from brownish green pargasitic core to pale green magnesio-hornblende rim, with a systematic decreasing in Fe2+/(Fe2++Mg) with AlIV decreasing toward the rim. Early sulfide precipitation followed by late -stage magnetite formation and the simultaneous decreasing in Fe2+ during amphibole crystallization were resulted from progressive oxidation of iron in minerals. Transformation of sulfur species from SO2 to H2S probably oxidized iron in the minerals in this hydrous magma on cooling, as suggested in previous models for large sulfur supplies and magma oxidation in subduction related volcanisms.

Timing and genesis of base-metal mineralisation in black shales of the Upper Permian Ravnefjeld Formation, Wegener Halvø, East Greenland

Bituminous mud shales of the Upper Permian Ravnefjeld Formation (Zechstein 1 equivalent) are mineralised with zinc, lead and copper within a ca. 50 km2 area on Wegener Halvo in central East Greenland. The occurrence of base-metal sulphides in shale nodules cemented prior to compaction indicates an early commencement of base-metal mineralisation. In other cases, post-compactional sulphide textures are observed. Homogeneous lead isotope signatures of galena and sphalerite from the shales (206Pb/204Pb: 18.440–18.466; 207Pb/204Pb: 16.554–16.586; 208Pb/204Pb: 38.240–38.326) suggest that all base metals were introduced during a single hydrothermal event. Therefore, post-compactional textures are believed to result from recrystallisation of early diagenetic sulphides during deep burial in the Upper Cretaceous to Tertiary. Lead isotope signatures of galena hosted in Upper Permian carbonate build-ups are relatively heterogeneous compared to those of the shale-hosted sulphides. The observed relations indicate a shared lead source for the two types of mineralisation, but different degrees of homogenisation during mineralisation. This suggests that lead was introduced to the carbonate rocks and black shales during two separate events. δ34S of base-metal sulphides in the Ravnefjeld Formation lie between –12 and –4‰, whereas synsedimentary and early diagenetic pyrite in unmineralised shales in general have δ34S between –47 and –16.5‰. Early diagenetic pyrite in the Wegener Halvo area in general has δ34S 15 to 20‰ higher than the same pyrite morphotype in Triaselv in the western part of the basin. This relatively high δ34S can be explained by extensive microbial sulphate reduction within persistent euxinic (super-anoxic) bottom waters under which supply of isotopically light seawater sulphate (and disproportionation of intermediate sulphur compounds) was restricted. The sulphur in the base-metal sulphides is believed to represent sulphide-dominated pore water, enriched in 34S due to preferential removal of 32S by sulphate-reducing bacteria and precipitation of diagenetic pyrite in the near-seafloor environment. We suggest that the sulphide-dominated pore water was trapped in the shale formation prior to introduction of base-metal-bearing fluids through fractures in the underlying carbonates, and that sulphide precipitation took place when the two fluids met. δ34S values of carbonate-hosted base-metal sulphides fall within the same range as the shale-hosted ones. The relationship between barite and sulphides and evidence for pre-mineralisation entrapment of liquid hydrocarbons in the carbonates suggest that the sulphide in this case is derived by in-situ thermochemical sulphate reduction (TSR). Measured fractionation between sulphide and sulphate ranges from 18.5 to 24.4‰, suggesting temperatures of TSR around 70 to 100 °C. Vitrinite reflectance measurements in mineralised shale samples are all between 1.7 and 2.0%, except for samples taken close to a Tertiary dyke giving ca. 3.0%. Vitrinite reflectance data are comparable to previously published data from unmineralised shale samples in the area and could not be proven to correlate with the degree of mineralisation. This indicates that any early hydrothermal effect has been overprinted later, probably during deep burial in the Late Cretaceous to Early Tertiary as previously proposed.

The Role of Magma Mixing in the Genesis of PGE Mineralization in the Bushveld Complex: Thermodynamic Calculations and New Interpretations

The elegant magma-mixing model for the formation of the platinum-group elements (PGE) mineralization associated with chromite and sulfide in the Bushveld Complex proposed by Naldrett and von Gruenewaldt (1989) has recently been questioned by Cawthorn (1999). Based on his own calculation, Cawthorn concluded that the concave-upward sulfur solubility curve of a fractionated basaltic magma, which is central to the model, does not exist. Cawthorn (1999) pointed out that both the hanging-wall and footwall sequences of the Merensky reef formed from sulfide-undersaturated magmas instead of sulfide-saturated magmas as predicted by the model. Cawthorn argued that there is a mass-balance problem with the model if these rocks did not contribute PGE to the sulfides in the reef. Our thermodynamic calculations indicate that the concave-upward curve of S solubility, though different in detail from the original, is very much in evidence. Our calculations also indicate that the content of S dissolved in the residual magma may fall below the S solubility curve when the rate of S increase in the magma, due to the removal of silicate cumulates, is less than the rate of increase in its S solubility, due to the change in its temperature and composition during fractional crystallization. Mixing of such fractionated and yet sulfide-undersaturated magma with appropriate amounts of a primitive magma is still capable of achieving sulfide oversaturation in the hybrid. Once the immiscible sulfides settle out of the magma, the content of S dissolved in the hybrid may fall below its solubility curve again during further crystallization, thus no sulfide segregation is expected in the rocks crystallized subsequently. R-factor assessment needs to consider the consequence of sulfide suspension in magma. Small quantities of sulfide droplets formed during the normal course of fractional crystallization may remain suspended in magma until the next replenishment, and in this case they can be redissolved in the hybrid when the proportion of a new input is large enough. Because there is no early sulfide liquid segregation, both the residual and new magmas will not be depleted in PGE and are equally important for contributing PGE to the sulfide liquid segregated during a subsequent magma-mixing event.

Wetting Properties of Fe-Ni-Co-Cu-O-S Melts against Olivine: Implications for Sulfide Melt Mobility

The effects of transition metal content and the fugacities of oxygen ( f O2) and sulfur ( f S2) on the wetting behavior of molten sulfide against forsteritic olivine have been investigated in this study. Wetting behavior, or the mobility of a liquid phase within a coexisting solid matrix, is quantified by the dihedral angle, Θ, which is a function of the relative solid-solid and solid-liquid surface energies. In order to determine sulfide wetting behavior, experiments were performed at 1,300°C using a vertical tube furnace employing C-O-S gas mixtures to control f O2 and f S2. Samples consisted of mixtures of San Carlos olivine and Fe + (Ni, Cu, Co; 0–30 wt ‰) + S melts equilibrated for 24 to 168 hr. Experimental conditions ranged from f O2 = 10–8 to 10–10 and f S2 from 10–1.2 to 10–4 in accordance with values appropriate for basalt petrogenesis. Results of our experiments revealed that dihedral angles exhibited a marked increase with decreasing f O2, and variable dependence on melt metal composition. At f O2 = 10–8, all sulfide melt compositions were found to be wetting (i.e., Θ< 60°), whereas only those with <~15 wt percent added Cu, Co, or Ni were wetting at f O2 = 10–9, and no wetting compositions were encountered at f O2 = 10–10. In agreement with the results of other investigators, we found that values of Θ decreased as the mole fraction of oxygen in the melt increased, suggesting that metal oxide species in the melt are more likely to be surface-active with respect to olivine. In light of our experimental data, it is expected that the wetting behavior of natural sulfide liquids will depend on both the identity and quantity of nonferrous metal and the abundance of dissolved oxygen. Because of this latter effect, the prevailing conditions of both f O2 and f S2 are therefore likely to dictate sulfide melt mobility. In terms of the potential for sulfide melt metasomatism in the upper mantle, consideration of the range of f O2 reflected by natural sulfide liquids, mafic lavas, and upper mantle source regions reveals that conditions for sulfide melt mobility encompass much of this spectrum, even for many Ni- and Cu-rich natural liquid compositions. Thus, such liquids may be potent agents for redistributing siderophile and chalcophile elements in the upper mantle. Sulfide melt wetting behavior will also play a role in the final sulfide distribution on solidification of mafic-ultramafic magmas that achieve saturation in an immiscible sulfide liquid. Efficient sulfide segregation may occur in reduced magmas only by early sulfide settling through a largely liquid medium, inasmuch as late-formed sulfide liquid will become trapped in the solid silicate matrix. For the case of relatively oxidized magmas, the wettability of sulfide liquid at these conditions, combined with a low viscosity and high density, suggests that efficient compaction-driven sulfide segregation is possible, even over the relatively short cooling intervals likely for high level mafic intrusions.

Nd-Pb-Sr Isotope Systematics of Crustal Assimilation in theVoisey's Bay and Mushuau Intrusions, Labrador, Canada

Assimilation of crustal rocks by mafic magmas is considered important for the formation of sulfide ores. Radiogenic isotope systems may allow one to quantify the amount of contamination and to distinguish between possible contaminants. We have studied Sm-Nd, Rb-Sr, and U-Pb isotope systems in two mafic intrusions of the Nain Plutonic Suite: the 1333 Ma Voisey’s Bay intrusion that hosts the major Ni-Cu-Co sulfide deposit of the same name, and the 1317 to 1313 Ma Mushuau intrusion, which contains only minor sulfide mineralization. In addition, we have analyzed potential contaminants: Archean Nain gneisses of various composition, and Paleoproterozoic enderbitic gneisses and Tasiuyak paragneisses. The analyses were performed on carefully handpicked plagioclase and apatite fractions in order to obtain more reliable isotopic data and check for closed system behavior. The Voisey’s Bay intrusion has the most mantlelike, least-contaminated initial isotopic compositions among the basic intrusions of the Nain Plutonic Suite: ϵ Nd = –1 to –2, 87 Sr/ 86 Sr = 0.7034 to 0.7038, 206 Pb/ 204 Pb = 15.34 to 15.54, 207 Pb/ 204 Pb = 15.10 to 15.18, and 208 Pb/ 204 Pb = 35.24 to 35.56. These isotopic ratios are uniform in various troctolite and gabbro phases from all parts of the intrusion (Eastern Deeps, Discovery Hill zone, and Reid Brook zone). Isotopic signatures of the Mushuau intrusion are quite distinct from the Voisey’s Bay intrusion and typical for the rest of the Nain Plutonic Suite: ϵ Nd = –3 to –10, 87 Sr/ 86 Sr = 0.7034 to 0.7052, 206 Pb/ 204 Pb = 14.21 to 14.55, 207 Pb/ 204 Pb = 14.63 to 14.77, and 208 Pb/ 204 Pb = 34.36 to 34.65. The Voisey’s Bay and Mushuau intrusions were, therefore, not only separated by 16 to 20 m.y. in time but also have different magma sources and/or styles of contamination. The isotopic data imply that the primary magmas of the Voisey’s Bay intrusion were either derived from an enriched continental mantle or contaminated by a small amount of crustal material during ascent through lower-middle crust. Subsequent contamination with Tasiuyak gneisses in the upper crust was volumetrically minor, probably about 8 to 13 percent. No late-stage contamination with Archean crustal material is detected. The amount of assimilated enderbitic gneisses is difficult to evaluate because their difference in isotopic composition from the Voisey’s Bay intrusion is small. Observed isotopic variations in the Mushuau intrusion can be explained by assimilation of 15 to 35 percent of a U-depleted Archean crust similar to Nain gneisses. Some rocks from both the Voisey’s Bay and Mushuau intrusions were either modified by contact metamorphism caused by emplacement of the Voisey’s Bay syenite or gained excessive local contamination. The Voisey’s Bay intrusion occupies a unique position in the Nain Plutonic Suite, being the oldest and the least-contaminated mafic intrusion. We suggest that these may be essential conditions for the origin of the Voisey’s Bay deposit. The Voisey’s Bay parental magmas were only moderately affected by contamination during ascent through the crust and probably did not undergo early sulfide separation. In contrast, the parental magmas of the Mushuau intrusion, as well as most other basic magmas in the Nain Plutonic Suite coeval with voluminous granitoids, were probably extensively contaminated with hot restites from intracrustal melting and/or granitoid melts, reached early sulfide saturation, and lost much of their Ni and Cu content.

The Evolution of Troctolitic and High Al Basaltic Magmas inProterozoic Anorthosite Plutonic Suites and Implications for the Voisey's BayMassive Ni-Cu Sulfide Deposit

The Voisey's Bay massive Ni-Cu sulfide deposit is contained within troctolitic (plagioclase + olivine) intrusions of the 1.34 to 1.29 Ga Nain Plutonic Suite, coastal Labrador, Canada, and represents a new setting for magmatic sulfide deposits. Proterozoic anorthosite plutonic suites consist of vast quantities (as much as 20,000 km 2) of cumulate anorthosite-leucotroctolite-leuconorite, intrusions of troctolite and Fe-enriched dioritic rocks, and large granitic batholiths. Crystallization ages span nearly 1,200 m.y. within the Middle Proterozoic (2.12-0.92 Ga). Many Proterozoic anorthosite plutonic suites were emplaced into relatively young Proterozoic crust and some were intruded along Archean-Proterozoic boundaries (e.g., Nain, Labrador; Laramie, Wyoming; Korosten, Ukraine). Nearly all of the largest plutonic suites contain significant areas (thousands of km 2) of plagioclase-rich cumulate rocks that are olivine bearing (olivine anorthosite-leucotroctolite). Troctolitic rocks (20-30% modal olivine) occur in Proterozoic anorthosite plutonic suites, principally as cumulates in layered or massive intrusions (e.g., Kiglapait, Hettasch, Jonathon-Nain) intruded into anorthositic rocks. With the notable exception of the Voisey's Bay deposit, nearly all troctolitic rocks crystallized late in the evolution of individual plutonic suites. Many of the troctolitic intrusions have fine-grained olivine-rich marginal rocks that closely represent parental magma compositions. Numerous fine-grained olivine-normative dikes with high Al basaltic compositions also occur within many plutonic suites (Nain, Harp Lake, Laramie, Adirondack) and share compositional characteristics with the troctolitic magmas. These olivine-normative high Al basaltic magmas record the least evolved compositions in Proterozoic anorthosite plutonic suites (Mg = 0.43-0.64; An = 45-60), have high Al 2O 3 (15-19 wt %) and FeO(total) (9-15 wt %) contents, and typically show the least contaminated Sr and Nd isotope compositions in a given plutonic suite. Many dikes and marginal rocks have strong positive Eu anomalies, a feature of the melt composition that is not related to plagioclase accumulation. Except at Laramie, the least evolved troctolitic magmas appear to have about 8 wt percent MgO and corresponding Ni and S concentrations between 100 to 150 ppm and 0.02 to 0.05 wt percent, respectively. Based on closed-system fractionation in the Kiglapait intrusion, sulfide saturation occurs late in the crystallization history of troctolitic magmas, after Ni concentrations have been severely depleted by protracted olivine fractionation. The optimal conditions for formation of a Ni-Cu sulfide deposit associated with a Proterozoic anorthosite plutonic suite require a relatively high MgO parental magma that has not crystallized much prior olivine and an external source of S to promote early sulfide saturation, probably through desulfurization reactions as hot troctolitic magmas ascend through the crust. An economically important massive Ni-Cu sulfide deposit on the scale of Voisey's Bay clearly requires large volumes of magma, probably involving continuous replenishment of relatively Ni rich, unfractionated parental magma in staging chambers or mixing between magmas in conduits during ascent. A model for the formation of troctolite-associated massive Ni-Cu sulfide deposits is proposed. Depending on the relative degrees of sulfide saturation and fractionation of the mixed magmas, mixing between troctolitic magmas in a conduit or magma chamber may either poison the system, stopping sulfide segregation, or enhance sulfide segregation.

The role of evaporites in the genesis of base metal sulphide mineralisation in the Northern Platform of the Pan-African Damara Belt, Namibia: geochemical and fluid inclusion evidence from carbonate wall rock alteration

The Otavi Mountain Land is a base metal sulphide ore province in northern Namibia where deposits are hosted by platform carbonates of the Otavi Group in a foreland fold-and-thrust belt on the northern edge of the Pan-African Damara Belt. Deposits have been classified as the Berg Aukas- or Tsumeb-types, based on differences in ore association, stratigraphic position and geochemistry of ores and gangue carbonates. Mineralisation at these deposits is accompanied by carbonate alteration in the form of dolomite and calcite veins, carbonate recrystallisation, calcitisation and carbonate silicification. Based on cathodoluminescence imaging, trace and rare earth element (REE), O and C isotope, and fluid inclusion data, a series of carbonate generations, constituting wall rock alteration around the Tsumeb and Kombat (Tsumeb-type) and Berg Aukas (Berg Aukas-type) deposits, was established. Similar data obtained on the recently discovered Khusib Springs deposit indicate a strong affinity to Tsumeb-type deposits. Tsumeb-type deposits are distinguished from Berg Aukas-type deposits by having trace element and REE concentrations that are significantly higher in the alteration products compared to the carbonate host rocks. Only around Tsumeb-type deposits a relative enrichment in light REE is noted for the hydrothermal carbonate generations that are cogenetic with the main stage of mineralisation. Microthermometric results from fluid inclusions in carbonate alteration phases and associated quartz indicate relatively high salinity (17–23 wt% NaCl equivalent) for the main mineralising and subsequent sulphide remobilisation stages at the deposits investigated. Estimated mineralisation temperatures are significantly higher for Tsumeb-type deposits (370–405 °C) with early sulphide remobilisation in Tsumeb at 275 °C, whereas they are lower at Berg Aukas (up to 255 °C). Fluid inclusion leachate analysis suggests that most of the observed salinity can be ascribed to dissolved, predominantly Ca- and Mg-carbonates and chlorides with subordinate NaCl. Na-Cl-Br leachate systematics indicate a derivation of the fluid salinity from the interaction with evaporitic rocks en route. Tsumeb-type mineralisation is interpreted to be derived from fluids expelled during Pan-African orogeny in the more intensely deformed internal zones of the Damara Belt further south. When the high salinity fluids reached the carbonate platform after having scavenged high concentrations of base metals, base metal sulphide precipitation occurred in zones of high porosity, provided by karst features in the carbonate sequence. Results obtained for the Berg Aukas-type deposits emphasise their derivation from basinal brines, similar to Mississippi Valley-type deposits, and confirm that mineralisation of the Berg Aukas- and Tsumeb-types are both spatially and temporally distinct.

Geology and origin of shale-hosted Zn-Pb-Ag mineralization at the Century Deposit, Northwest Queensland, Australia

The Century deposit represents an important new member of the major sediment-hosted Zn-Pb-Ag deposits of the Mount Isa inlier and McArthur basin. The deposit is hosted by shales and siltstones of the Mesoproterozoic Lawn Hill Formation. Mineralization comprises fine-grained sphalerite with minor galena and pyrite. Most sulfides (80-90%) occur as delicate replacive lamellae in black shale units, separated by siderite-rich silt-stone horizons. The remainder are present as progressively coarser-grained and more discordant fracture-filling forms, strata-bound within the ore sequence. The mineralization envelope transgresses stratigraphy, with the position of the highest-grade material migrating upward within the mineralized sequence from southeast to northwest. Despite systematic lateral variation in grades from 3 to 5 percent Zn to greater than 25 percent Zn in individual units, this grade variation occurs without concomitant changes in thickness of the host shales, suggesting that the mineralization is dominantly of replacement origin. The host sediments show no lateral chemical or textural changes suggestive of exhalative facies within the preserved portions of the deposit.Two principal textural varieties of strata-bound sphalerite are recognized: porous, which has a high pyrobitumen content, and nonporous, which has a relatively low pyrobitumen content. These appear to be cogenetic and almost co-abundant. Porous sphalerite is interpreted as the product of oil-mediated thermochemical sulfate reduction (TSR) and sulfide deposition. Nonporous sphalerite is the result of gas mediated TSR. A paleosource reservoir-type hydrocarbon trap is proposed as a model to explain the grade distribution trends and zoning of the mineralization.Paragenetically early sulfides have delta 34 S between 5 and 10 per mil, while later fracture-filling and replacement styles have values up to 20 per mil. This trend appears to follow through into more widespread syndeformational vein-style lodes in a 100- to 200-km 2 area surrounding the deposit. In the regional lodes, early sphalerite has delta 34 S values from 20 to 25 per mil while later sphalerite generations have progressively higher values, reaching a maximum of 25 to 30 per mil in the final stages of vein mineralization. This progression suggests that the total mineralizing fluid system was a large closed-system reservoir, with progressive enrichment in heavy sulfur over the life of the deformation and mineralization event.Migration of the mineralizing fluids was triggered by the early stages of basin inversion and regional deformation, continuing through into the development of gentle north-south-trending folds. As deformation proceeded, the regional-scale fluid system was progressively redistributed into late fractures and faults. This is recorded by the initial development of a dense network of hairline fractures throughout the Century orebody. These fractures were mineralized with progressively more discordant sphalerite, galena, and siderite. As tectonism continued, vein-style lode deposits were emplaced at a more regional scale in fault structures. The process culminated in the reactivation of major regional structures such as the Termite Range fault.Genetic links between shale-hosted Pb-Zn mineralization and classical Mississippi Valley-type deposits have often been alluded to, but comparisons have been obscured by specific geological attributes assigned to the classic sedex model for mineralization; for example, rifting and high heat flow. The Century deposit appears to represent a shale-hosted mineralization style formed during tectonically driven migration of basin fluids into an overpressured/undercompacted shale sequence. This scenario is much more reminiscent of some models of Mississippi Valley styles of mineralization than those traditionally associated with sedimentary exhalative processes. Century demonstrates an intimate relationship, becoming increasingly identified, in exploration, between petroleum reservoirs and traps and sites of base metal accumulations formed during deep burial. The recognition of this style of mineralization greatly expands the framework in which to explore for shale-hosted zinc and lead in the Proterozoic Mount Isa inlier and McArthur basin.

Effect of Various Bonding Agents on the Sulfidation of Brass-Plated Steel Cords Immersed in Squalene Mixtures

Abstract Brass-plated steel cords have been immersed at elevated temperatures in various squalene mixtures containing sulfur curatives and commercially available bonding agents. Copper sulfide growth was characterized by scanning electron microscopy and energy dispersive x-ray analysis. The bonding agents cause early sulfide growth, even prior to “scorch,” but mitigate sulfide overgrowth upon humid aging. Both effects are expected to improve joint durability.

Geology and structure of a sulfide-rich gold deposit; an example from the Mouska gold mine, Bousquet District, Canada

The Mouska mine in the Bousquet mining district represents a sulfide-rich gold deposit hosted by the Archean mafic metavolcanites of the Blake River Group, southern Abitibi greenstone belt. Mineralization shows both lithological and structural control and coincides with variably oriented high angle-reverse ductile and brittle-ductile shear zones. Host shear zones are superimposed on the regional foliation, and both appear to have formed from the same bulk stress field. Gold mineralization is composed of sulfide and quartz lodes and is associated with three distinct ore zones; each differs from the others by its style of veining and the nature of tectono-metamorphic overprint. Sulfide mineralization in this area varies from semimassive to massive sulfide lodes and dissemination. The strain features and age relationships of ore-bearing rocks and vein material throughout the ore zones display evidence of earlier sulfide mineralization with gold. This mineralization is pretectonic (with respect to the development of the foliation) and is interpreted as hydrothermal-synvolcanic. The auriferous quartz sulfide veins correspond to a multistage syn- to late tectonic mineralization and clearly crosscuts massive sulfide bodies. In the Mouska deposit, strain features and structural setting of gold mineralization show interactions between lithological factors, such as contacts, competency contrast and rock-body geometry, and the presence of pretectonic sulfides. In several parts of the mine volcanic sequence, the shear zone nucleation and development were controlled by both rock heterogeneities and the presence of soft sulfide bodies. The sulfides, in particular, were responsible for the structural instability recorded in some volcanic contacts and their activation as shear zones. In other parts, the sulfides have variably influenced the local slip movement. The shapes of ore shoots throughout the ore zones are variable and interpreted as reflecting the initial geometry of anisotropic interfaces involved in the deformation history.

Proterozoic gold mineralization in NW burundi

In Burundi two types of primary gold mineralization can be distinguished: sulfide vein assemblages with gold related to the post-Kibaran G4-granite magmatism (900–1000 Ma) and Pan-African ferruginous breccia zones with gold dated around 640 Ma. The post-Kibaran sulfide assemblages have a characteristic bismuth component and are accompanied by the early gangue assemblage of quartz-tourmaline-rutile-magnetite. The elevated Sn and W contents and the spatial and temporal relationships with highly fractionated granitic rocks point to a magmatic input into the Au-bearing hydrothermal system. The Pan-African gold-bearing ferruginous breccia zones have relatively high Ba contents and are characterized by an early sulfide stage and subsequent extensive hydrothermal leaching and transformation into hematite/limonite in an oxidizing environment. The hematite/limonite assemblage is host to native gold mineralization with lenticular gold aggregates up to several millimeters in size. The breccia-style gold mineralization is thought to represent redistributed gold from the post-Kibaran sulfide stage, which was remobilized during the Pan-African structural reactivation.

Thrust-controlled gold mineralisation at the Elandshoogte Mine, Sabie-Pilgrim's Rest goldfield, South Africa

Stratiform quartz-sulphide-gold veins, locally termed “reefs” are hosted within the Proterozoic Transvaal Sequence sedimentary succession, in the Sabie-Pilgrim's Rest goldfield, eastern Transvaal. These deposits have produced about 180 tonnes of gold and share many characteristics with those of Telfer, Western Australia. Detailed examination of the Elandshoogte Mine shows that gold deposition occurred in two stages, both linked to bedding-parallel thrust faulting within the sedimentary pile. Deformation being concentrated within incompetent shale beds, interlayered within more competent units. The majority of gold was introduced in the second stage of mineralisation and occurs within fractures in early-formed sulphide minerals. Deposition of competent quartz veins accompanying early sulphide and gold mineralisation resulted in a change in deformation style within the reef zone, from early shearing in shales to later duplex faulting of the quartz-reef. Fluids accompanying faulting are implied to have transported gold, and a magmatic source of mineralisation is suggested.

Multistage carbonatization and mineral geochemistry of a skarn-type copper deposit at Tongshan, Guichi

The Tongshan copper deposit at Guichi can be considered as a skarn-type copper deposit. Multi-stage carbonatization was well developed in the skarn zone and copper-bearing orebodies. Mineralogy, mineral chemistry and stable isotope data allow it to be divided into five stages: (1) carbonatization of single calcite crystals in the skarn stage; (2) calcite carbonatization in the oxide stage; (3) carbonatization in the early sulfide stage; (4) carbonatization in the late sulfide stage; and (5) carbonatization in the post-sulfide stage. Carbonatization in the early sulfide stage is, among other things, closely related to copper mineralization and is one of the alteration indicators of copper mineralization of this type.

Ore-lead isotopic composition of early Proterozoic volcanogenic sulfide ores at Garpenberg, Bergslagen, south-central Sweden

Ore-lead isotopic composition of early Proterozoic volcanogenic sulfide ores at Garpenberg, Bergslagen, south-central Sweden