High Sulfide Intrusion in Five Temperate Seagrasses Growing Under Contrasting Sediment Conditions

Five temperate seagrasses (Amphibolis antartica, Halophila ovalis, Posidonia australis, Posidonia sinuosa and Zostera nigricaulis) were surveyed along the south-west coast of Western Australia. These morphological different seagrasses grow in contrasting sediments with large variations in sedimentary organic matter, carbonate and iron contents. We tested if sulfur composition in the plants responded to sulfur dynamics in the sediments and if plant morphology affected the sulfur composition of the plants. The sediments were characterized by low sulfate reduction rates (<9 mmol m^?2day^?1), low concentrations of dissolved sulfides in the pore waters (<74 μM) and low burial of sulfides (total reducible sulfur <0.8 mol m^?2) in the sediments. However, all seagrasses showed high intrusion in the below-ground parts with up to 84 % of the sulfur derived from sedimentary sulfides. There were no direct links between sulfur in the plants and sulfur dynamics in the sediments, probably due to low iron contents in the sediments limiting the buffering capacity of the sediments and exposing the plants to sulfides despite low rates of production and low pools of sulfides. The intrusion was linked between plant compartments (roots, rhizomes and leaves) for the two small species (H. ovalis and Z. nigricaulis), whereas the intrusion into the leaves was limited for the larger species (P. australis and P. sinuosa) and for A. antarctica, where extensive rhizomes and roots and the long stem for A. antarctica separate the leaves from the sediment compartment. Elevated intrusion was observed at two study locations, where natural deposition of organic matter or nutrient enrichment may be contributing factors to enhanced sulfide pressure.     

Sulfides in mantle peridotites from Penghu Islands, Taiwan: Melt percolation, PGE fractionation, and the lithospheric evolution of the South China block

Major elements, highly siderophile elements (HSE) and Re-Os isotope ratios were analysed in situ on individual sulfide grains in spinel peridotite xenoliths hosted by Miocene intraplate basalts from the Penghu Islands, Taiwan. The xenoliths represent texturally and compositionally different mantle domains, and the geochemical characteristics of the sulfides show changes in HSE distribution and Re-Os isotope systematics, produced as their host rocks were metasomatised by percolating fluids/melts. In prophyroclastic and partly metasomatised peridotites from the Kueipi (KP) locality, the sulfides have subchondritic to superchondritic ¹⁸⁷Re/¹⁸⁸Os and ¹⁸⁷Os/¹⁸⁸Os ratios. Many of these sulfides reflect fluid/melt interaction with residual MSS and/or crystallization of fractionated sulfide melts, which produced high contents of Cu and PPGEs and high Re/Os; inferred melt/rock ratios are low. In contrast, sulfides in equigranular and extensively metasomatised peridotites from the Tungchiyu (TCY) locality are mainly more sulfur-rich Ni-(Co)-rich MSS, with subchondritic to chondritic ¹⁸⁷Os/¹⁸⁸Os and subchondritic ¹⁸⁷Re/¹⁸⁸Os. These sulfides are interpreted as products of interaction between pre-existing MSS and percolating silicate melts. Melt/rock ratios were high and the percolating melt was less differentiated than the melt that percolated the KP peridotites. Sulfides in a TCY pyroxenite are mainly MSS; they have the lowest HSE contents, subchondritic to superchondritic ¹⁸⁷Os/¹⁸⁸Os and subchondritic ¹⁸⁷Re/¹⁸⁸Os, and may have precipitated from sulfide melts that segregated from basaltic melts under S-saturated conditions. In most sulfides melt percolation appears to have induced fractionation among the HSEs and disturbed Re-Os isotope compositions. Despite the metasomatic effects, rare residual MSS, sulfides that from crystallised sulfide melts and sulfides modified by addition of Re (with no evidence for Os addition) can still provide useful chronological information. Such sulfides yield T_RD age peaks of 1.9, 1.7-1.6, 1.4-1.3 and 0.9-0.8 Ga, which may record the timing of melt extraction and/or metasomatic events in the mantle. These periods are contemporaneous with the major crustal events recorded by U-Pb dates and Nd and Hf model ages in the overlying crust. This close correspondence indicates that the sulfide T_RD ages reflect the timing of lithosphere-scale tectonothermal events (such as melting and metasomatism) that affected both the lithospheric mantle and the overlying crust. The sulfide T_RD ages, taken together with the crustal data, suggest that most of the Cathaysia block had formed at least by Paleo-Proterozoic time, and that some domains are Archean in age.     

The Kabanga Ni sulfide deposit, Tanzania: I. Geology, petrography, silicate rock geochemistry, and sulfur and oxygen isotopes

The Kabanga Ni sulfide deposit represents one of the most significant Ni sulfide discoveries of the last two decades, with current indicated mineral resources of 23.23 Mt at 2.64% Ni and inferred mineral resources of 28.5 Mt at 2.7% Ni (Nov. 2008). The sulfides are hosted by a suite of ∼1.4 Ga ultramafic–mafic, sill-like, and chonolithic intrusions that form part of the approximately 500 km long Kabanga–Musongati–Kapalagulu igneous belt in Tanzania and Burundi. The igneous bodies are up to about 1 km thick and 4 km long. They crystallized from several compositionally distinct magma pulses emplaced into sulfide-bearing pelitic schists. The first magma was a siliceous high-magnesium basalt (approximately 13.3% MgO) that formed a network of fine-grained acicular-textured gabbronoritic and orthopyroxenitic sills (Mg# opx 78–88, An plag 45–88). The magma was highly enriched in incompatible trace elements (LILE, LREE) and had pronounced negative Nb and Ta anomalies and heavy O isotopic signatures (δ¹⁸O +6 to +8). These compositional features are consistent with about 20% contamination of primitive picrite with the sulfidic pelitic schists. Subsequent magma pulses were more magnesian (approximately 14–15% MgO) and less contaminated (e.g., δ¹⁸O +5.1 to +6.6). They injected into the earlier sills, resulting in the formation of medium-grained harzburgites, olivine orthopyroxenites and orthopyroxenites (Fo 83–89, Mg# _opx 86–89), and magmatic breccias consisting of gabbronorite–orthopyroxenite fragments within an olivine-rich matrix. All intrusions in the Kabanga area contain abundant sulfides (pyrrhotite, pentlandite, and minor chalcopyrite and pyrite). In the lower portions and the immediate footwall of two of the intrusions, namely Kabanga North and Kabanga Main, there occur numerous layers, lenses, and veins of massive Ni sulfides reaching a thickness of several meters. The largest amount of high grade, massive sulfide occurs in the smallest intrusion (Kabanga North). The sulfides have heavy S isotopic signatures (δ³⁴S wr = +10 to +24) that broadly overlap with those of the country rock sulfides, consistent with significant assimilation of external sulfur from the Karagwe–Ankolean sedimentary sequence. However, based partly on the relatively homogenous distribution of disseminated sulfides in many of the intrusive rocks, we propose that the Kabanga magmas reached sulfide saturation prior to final emplacement, in staging chambers or feeder conduits, followed by entrainment of the sulfides during continued magma ascent. Oxygen isotope data indicate that the mode of sulfide assimilation changed with time. The heavy δ¹⁸O ratios of the early magmas are consistent with ingestion of the sedimentary country rocks in bulk. The relatively light δ¹⁸O ratios of the later magmas indicate less bulk assimilation of the country rocks, but in addition the magmas selectively assimilated additional S, possibly through devolatization of the country rocks or through cannibalization of magmatic sulfides deposited in the conduits by preceding magma surges. The intrusions were tilted at ca. 1.37 Ga, during the Kibaran orogeny and associated synkinematic granite plutonism. This caused solid-state mobilization of ductile sulfides into shear zones, notably along the base of the intrusions where sulfide-hornfels breccias and lenses and layers of massive sulfides may reach a thickness of >10 m and can extend for several 10 s to >100 m away from the intrusions. These horizons represent an important exploration target for additional nickel sulfide deposits.     

Sulfur isotopic composition of sulfides at the Mangazeya silver deposit,Eastern Sakha-Yakutia,Russia

The succession of the formation of ore zones and sulfur isotope ratio of sulfides at the Mangazeya Ag deposit have been studied. The deposit is located in the Nyuektame Fault Zone in the eastern limb of the Endybal Anticline. The ore zones are hosted in the Middle Carboniferous to Middle Jurassic terrigenous sequences of the Verkhoyansk Complex intruded by the Endybal subvolcanic stock and felsic and mafic dikes. Three ore stages are distinguished: (I) gold-rare metal, (II) cassiterite-sulfide, and (III) silver-base-metal. Products of these stages are spatially isolated. The δ³⁴S of sulfides ranges from −6.4 to +8.0‰. In the sulfides of the gold-rare metal assemblage, this value varies from −1.8 to +4.7‰; in the sulfides of the cassiterite-sulfide stage, −6.4 to +6.6‰; and in the sulfides of the silver-base-metal assemblage, -5.6 to +8.0‰. A sulfur isotope thermometer indicates the temperature of mineral deposition at 315–415°C for the first stage and 125–280°C for the third stage. Possible causes of variable sulfur isotopic composition in sulfides are discussed. The data on the sulfur isotope ratio is interpreted in terms of involvement of magmatic fluid (δ³⁴S ∼ 0) in the mineralizing process along with low-temperature fluid taking sulfur from host rocks (δ³⁴S ≫ 0). Boiling and mixing of magmatic fluid with heated meteoric water were important at the last stage of the deposit formation.     

Geology, Geochemistry, and Genesis of the Tongcun Reduced Porphyry Mo (Cu) Deposit, NW Zhejiang Province, China

The Tongcun Mo(Cu) deposit in Kaihua city of Zhejiang Province,eastern China,occurs in and adjacent to the Songjiazhuang granodiorite porphyry and is a medium-sized and important porphyry type ore deposit.Two irregular Mo(Cu) orebodies consist of various types of hydrothermal veinlets.Intensive hydrothermal alteration contains skarnization,chloritization,carbonatization,silicification and sericitization.Based on mineral assemblages and crosscutting relationships,the oreforming processes are divided into five stages,i.e.,the early stage of garnet + epidote ± chlorite associated with skarnization and K-feldspar + quartz ± molybdenite veins associated with potassicsilicic alteration,the quartz-sulfides stage of quartz + molybdenite ± chalcopyrite ± pyrite veins,the carbonatization stage of calcite veinlets or stockworks,the sericite + chalcopyrite ± pyrite stage,and the late calcite + quartz stage.Only the quartz-bearing samples in the early stage and in the quartzsulfides stage are suitable for fluid inclusions(FIs) study.Four types of FIs were observed,including1) CO_2-CH_4 single phase FIs,2) CO_2-bearing two- or three-phase FIs,3) Aqueous two-phase FIs,and4) Aqueous single phase FIs.FIs of the early stages are predominantly CO_2- and CH_4-rich FIs of the CO_2-CH4-H_2O-NaCl system,whereas minerals in the quartz-sulfides stage contain CO_2-rich FIs of the CO_2-H_2O-NaCl system and liquid-rich FIs of the H_2O-NaCl system.For the CO_2-CH_4 single phase FIs of the early mineralization stage,the homogenization temperatures of the CO_2 phase range from 15.4 ℃ to 25.3 ℃(to liquid),and the fluid density varies from 0.7 g/cm~3 to 0.8 g/cm~3;for two- or three-phase FIs of the CO_2-CH_4-H_2O-NaCl system,the homogenization temperatures,salinities and densities range from 312℃ to 412℃,7.7 wt%NaCl eqv.to 10.9 wt%NaCl eqv.,and 0.9 g/cm~3 to 1.0 g/cm~3,respectively.For CO_2-H_2O-NaCI two- or threephase FIs of the quartz-sulfides stage,the homogenization temperatures and salinities range from255℃ to 418℃,4.8 wt%NaCl eqv.to 12.4 wt%NaCl eqv.,respectively;for H_2O-NaCl two-phase FIs,the homogenization temperatures range from 230 ℃ to 368 ℃,salinities from 11.7 wt%NaCl eqv.to16.9 wt%NaCl eqv.,and densities from 0.7 g/cm~3 to 1.0 g/cm~3.Microthermometric measurements and Laser Raman spectroscopy analyses indicate that CO_2 and CH_4 contents and reducibility(indicated by the presence of CH_4) of the fluid inclusions trapped in quartz-sulfides stage minerals are lower than those in the early stage.Twelve molybdenite separates yield a Re-Os isochron age of 163 ± 2.4 Ma,which is consistent with the emplacement age of the Tongcun,Songjiazhuang,Dayutang and Huangbaikeng granodiorite porphyries.The S18OSMow values of fluids calculated from quartz of the quartz-sulfides stage range from 5.6‰ to 8.6‰,and the JDSMOw values of fluid inclusions in quartz of this stage range from-71.8‰ to-88.9‰,indicating a primary magmatic fluid source.534SV-cdt values of sulfides range from+1.6‰ to +3.8‰,which indicate that the sulfur in the ores was sourced from magmatic origins.Phase separation is inferred to have occurred from the early stage to the quartz-sulfides stage and resulted in ore mineral precipitation.The characteristics of alteration and mineralization,fluid inclusion,sulfur and hydrogen-oxygen isotope data,and molybdenite Re-Os ages all suggest that the Tongcun Mo(Cu) deposit is likely to be a reduced porphyry Mo(Cu) deposit associated with the granodiorite porphyry in the Tongcun area.     

Geology and geochemistry of the Changba SEDEX Pb-Zn deposit,Qinling orogenic belt,China

The Changba Pb-Zn SEDEX deposit occurs in the Middle Devonian sequence of the Anjiaca Formation of the Western Qinling Hercynian Orogen in the Gansu Province, China. The Changba-II orebody is hosted in biotite quartz schist and is the largest of 143 stratiform orebodies that are hosted either in biotite quartz schist or marble. The Changba-II comprises two types of mineralization: a bedded facies and an underlying breccia lens. The bedded section exhibits three sulfide sub-facies zoned from bottom to top: 1) banded sphalerite intercalated with quartz albitite; 2) interbedded massive pyrite and sphalerite ore; and 3) banded sphalerite ore intercalated with banded baritite. Major metallic minerals are sphalerite, pyrite, galena, with minor arsenopyrite, pyrrhotite, boulangerite, and rare chalcopyrite. The bedded sulfides are underlain by a lens of brecciated and albitized biotite-quartz schists cemented by sulfides and tourmaline.Massive and bedded sulfide ³⁴S values range from 8.1 to 29.3, whereas barite ³⁴S values range from 20.8 to 31.5. Disseminated pyrite in footwall schists has ³⁴S values ranging from 8.1 to 10.6, and increase to values ranging from 11.1 to 14.7 in the hangingwall. The lower ³⁴S values for massive and bedded sulfides are interpreted to be derived from progressive bacterial sulfate reduction (BSR) of Devonian seawater in a sulfate-restricted sub-basin. The higher ³⁴S values for massive and bedded sulfides could be a product of quantitative BSR but this is incompatible with barite being more abundant above the bedded sulfides. Instead, it is more likely that thermochemical sulfate reduction of seawater sulfate or of evaporite was the source of heavy hydrothermal sulfur. Heavy hydrothermal sulfur was injected into a sulfate-restricted sub-basin where it mixed with low ³⁴S BSR sulfide to form the massive and bedded sulfides. The REE patterns of sulfide layers and associated quartz albitite and baritite are similar to those of the host biotite quartz schists, suggesting that the hydrothermal fluids leached REE from the underlying rocks. Pb isotope ratios in galena form an array between the Upper Crust and the Mantle reservoir curves, which indicates that the lead is derived from upper crustal rocks comprising mafic igneous units. The Sr⁸⁷/Sr⁸⁶ ratio of 0.7101 for carbonate within the sulfide layers also suggests that Sr is derived from the mixing of Sr leached from upper crustal rocks with Middle Devonian seawater Sr. A Rb-Sr isochron age of 389.4 ± 6.4 Ma for sulfide layers and the interbedded hydrothermal sediments is consistent with the age of host Mid-Devonian strata. Ar³⁹/Ar⁴⁰ plateau age at 352.8 ± 3.5 Ma and Ar³⁹-Ar⁴⁰ isochron age of 346.6 ± 6.4 Ma for albite in the quartz albitite intercalated with sulfide layers indicate either albite formation after the sulfides or thermal resetting of the Rb-Sr system at about 350 Ma, the age of collision between the North China and Yangtze cratons.Editorial handling: E. Frimmel     

S,Pb, C and O isotopic compositions and ore genesis of the strata-bound polymetallic sulfide deposits in central Inner Mongolia,China

The strata-bound Cu−Pb−Zn polymetallic sulfide deposits occur in metamorphic rocks of greenschist phase of the middle-upper Proterozoic Langshan Group in central Inner Mongolia. δ³⁴S values for sulfides range from −3.1‰ to +37.3‰, and an apparent difference is noticed between vein sulfides and those in bedded rocks. For example, δ³⁴S values for bedded pyrite range from +10.6‰ to +20.0‰, while those for vein pyrite vary from −3.1‰ to +14.1‰. δ³⁴S of bedded pyrrhotite is in the range +7.9‰–+23.5‰ in comparison with +6.5‰–+17.1‰ for vein pyrrhotite. The wide scatter of δ³⁴S and the enrichment of heavier sulfur indicate that sulfur may have been derived from H₂S as a result of bacterial reduction of sulfates in the sea water. Sulfur isotopic composition also differs from deposit to deposit in this area because of the difference in environment in which they were formed. The mobilization of bedded sulfides in response to regional metamorphism and magmatic intrusion led to the formation of vein sulfides. δ¹⁸O and δ¹³C of ore-bearing rocks and wall rocks are within the range typical of ordinary marine facies, with the exception of lower values for ore-bearing marble at Huogeqi probably due to diopsidization and tremalitization of carbonate rocks. Pb isotopic composition is relatively stable and characterized by lower radio-genetic lead. The age of basement rocks was calculated to be about 23.9 Ma and ore-forming age 7.8 Ma.²⁰⁷Pb/²⁰⁴Pb−²⁰⁶Pb/²⁰⁴Pb and²⁰⁸Pb/²⁰⁴Pb−²⁰⁶Pb/²⁰⁴Pb plots indicate that Pb may probably be derived from the lower crust or upper mantle. It is believed that the deposits in this region are related to submarine volcanic exhalation superimposed by later regional metamorphism and magmatic intrusion.     

Proterozoic copper deposits in NW Queensland,Australia: Sulfur isotopic data

Sulfur isotopic disequilibrium is commonly observed between associated pyrite and copper sulfides in NW Queensland. A sulfur isotopic study of copper mineralization in dolomites at Paradise Valley and arenites at Mammoth has allowed the significance of such disequilibrium to be evaluated. Copper mineralization at Paradise Valley is characterized by a greater enrichment in ³⁴S, with δ³⁴S values often greater than +30‰, for both copper sulfides and associated syngenetic/diagneetic pyrite. At Mammoth, copper sulfides have isotopic compositions (δ³⁴S=?15.9 to ?0.3‰) transitional between disseminated syngenetic/diagenetic pyrite (δ³⁴S=?5.7 to ?1.7‰) and epigenetic vein pyrite (δ³⁴S=?17.9 to ?7.1‰) suggesting progressive reaction and replacement of syngenetic/diagenetic pyrite by a copper-bearing mineralizing fluid under oxidizing conditions. The isotopic data, within the constraints imposed by geological and geochemical factors, support a model of reaction between copper-bearing mineralizing fluids and pre-existing syngenetic/diagenetic pyrite for both the carbonate- and arenite-hosted deposits.     

Time variability of pore water chemistry in recent carbonate sediments,Devil's Hole,Harrington Sound,Bermuda

Chemical analyses of pore waters from recent marine carbonate sediments, Devil's Hole, Harrington Sound, Bermuda, have been obtained at intervals over a four year period. Interstitial waters were systematically analyzed for pH, titration alkalinity, dissolved sulfides, NH₄⁺, Ca²⁺, Mg²⁺ and Na⁺ or Cl^?. Additional analyses on some cores included SO₄^2?, PO₄^3?, dissolved CH₄ and N₂, and C:N:H ratios in the detrital organic material. The following general trends with depth (to ～ 1 m) are observed: (1) major cations show little or no change; (2) pH decreases; (3) alkalinity, sulfides, NH₄⁺ and PO₄^3? increase; (4) dissolved CH₄ is consistently low. The chemical changes with depth can be modelled theoretically and are consistent with experimental data.Significant seasonal changes in pore water chemistry are observed. The data suggest an annual exchange between the pore waters (to a depth of ～ 1 m) and the overlying water of Harrington Sound; the exchange occurs between August and January. The nutrient flux out of the sediments during this process may be a major factor in the plankton ecology of Harrington Sound.     

The sulfur isotopic composition of sulfides from ores and host rocks of the Upper Kolyma region,Magadan Oblast

More than 200 analyses of the sulfur isotopic composition of sulfides from various terrigenous and intrusive host rocks, metasomatically altered wall rocks, and gold lodes of the Upper Kolyma region are presented. In accessory pyrite of the metaterrigenous rocks, δ³⁴S varies from ?23.1 to +5.7‰ δ³⁴S of pyrite and arsenopyrite from gold-quartz mineralization is within the range ?10.6 to ?0.4‰ and is close to the average δ³⁴S of pyrite from the metaterrigenous rocks (?4.4‰). In the intrusive rocks, δ³⁴S of pyrite varies from ?3.8 to +2.6‰ (+0.7‰, on average) and drastically differs from δ³⁴S of arsenopyrite from postmagmatic gold-rare-metal mineralization (?7.9 to ?2.7‰; ?5.2‰, on average). The comparison of the δ³⁴S of accessory sulfides from the host rocks with δ³⁴S of sulfides from the gold deposits suggests that sulfur mobilized from the terrigenous sequences participated in the hydrothermal process. The results obtained are consistent with the metamorphic model of the formation of gold-quartz deposits in the Upper Kolyma region.     

Calculation of the visible-UV absorption spectra of hydrogen sulfide,bisulfide, polysulfides,and As and Sb sulfides,in aqueous solution

Recently we showed that visible-UV spectra in aqueous solution can be accurately calculated for arsenic (III) bisulfides, such as As(SH)₃, As(SH)₂S^- and their oligomers. The calculated lowest energy transitions for these species were diagnostic of their protonation and oligomerization state. We here extend these studies to As and Sb oxidation state III and v sulfides and to polysulfides S _n ^2- , n = 2–6, the bisulfide anion, SH^-, hydrogen sulfide, H₂S and the sulfanes, S _n H₂, n = 2–5. Many of these calculations are more difficult than those performed for the As(iii) bisulfides, since the As and Sb(v) species are more acidic and therefore exist as highly charged anions in neutral and basic solutions. In general, small and/or highly charged anions are more difficult to describe computationally than larger, monovalent anions or neutral molecules. We have used both Hartree-Fock based (CI Singles and Time-Dependent HF) and density functional based (TD B3LYP) techniques for the calculations of absorption energy and intensity and have used both explicit water molecules and a polarizable continuum to describe the effects of hydration. We correctly reproduce the general trends observed experimentally, with absorption energies increasing from polysulfides to As, Sb sulfides to SH^- to H₂S. As and Sb(v) species, both monomers and dimers, also absorb at characteristically higher energies than do the analogous As and Sb(III)species. There is also a small reduction in absorption energy from monomeric to dimeric species, for both As and Sb III and v. The polysufides, on the other hand, show no simple systematic changes in UV spectra with chain length, n, or with protonation state. Our results indicate that for the As and Sb sulfides, the oxidation state, degree of protonation and degree of oligomerization can all be determined from the visible-UV absorption spectrum. We have also calculated the aqueous phase energetics for the reaction of S₈ with SH^- to produce the polysulfides, S _n H^-, n = 2–6. Our results are in excellent agreement with available experimental data, and support the existence of a S₆ species.     

Geology and Genesis of the Bajiazi Polymetallic Sulfide Deposits,Liaoning, China

The Bajiazi deposits of western Liaoning Province, northeastern China, are aligned approximately west-east in an 8-× 15-km district. The Zn-Pb-Ag-FeS₂ sulfide ores are hosted in intensively folded and faulted Early Proterozoic carbonates and minor clastic rocks, near their contact with Late Jurassic-Early Cretaceous Bajiazi granite. The ore-bearing strata on both limbs of the Bajiazi syncline can be correlated throughout the district. Orebodies are stratigraphically conformable with the host rocks and mostly parallel the contact metamorphic halo of the granite, in the skarn mineralizing zone. The ores are composed of stratabound and stratiform sulfide lenses and layers, are associated with sediments formed in local depressions of a tectonically active, shallow-marine environment, and are limited to dolomitized tidal flat and lagoonal facies with cryptalgal lamination and to adjacent oolitic grainstones of barrier facies. The ores consist of fine- to medium-grained intergrowths of sulfides in dolomite and locally skarn gangues. Sulfides are intensely recrystallized and annealed in the contact metamorphic halo. Isotopes of sulfur, carbon, oxygen, and lead were examined for genetic interpretation of the sulfides and host rocks. Sulfur isotope values range from ?9.1 to 13.0 per mil for sulfides, suggesting sedimentary exhalative origin, though they were, to some extent, homogenized during late overprinting events. The δ¹⁸O values range between ?6 and ?15 per mil PDB, and the δ¹³C values between +1 and ?5 per mil PDB. Late crystallization generations are enriched in the light isotopes of oxygen and carbon compared to primary ones. Lead isotope ratios in galenas from all Bajiazi ores have means of ²⁰⁶Pb/²⁰⁴Pb = 16.23, ²⁰⁷Pb/²⁰⁴Pb = 15.23, and ²⁰⁸Pb/²⁰⁴Pb = 36.51. These isotope ratios, characterized by a model age of about 1350 Ma, suggest an early Proterozoic, crustal source for the lead in the ores, unrelated to the lead in the Mesozoic granite. All evidence identified in this study suggests that the Bajiazi polymetallic sulfide deposits formed as sedimentary exhalative accumulations in a Lower Proterozoic, shallow-marine sedimentary basin. The present distribution of the ores and their distinctive textures indicate overprinting by. contemporaneous and/or post-depositional tectonic processes, as well as contact metamorphism during the emplacement of the Mesozoic Bajiazi granite.     

Sulfur isotopic composition of the Bainiuchang super-large Ag polymetallic deposit, Yunnan Province, China： Implications for the source of sulfur in ore-forming fluids

The Bainiuchang deposit in Yunnan Province, China, is located geographically between the Gejiu ore field and the Dulong ore field. In addition to >7000 t Ag reserves, the deposit possesses large-scale Pb, Zn, Sn reserves and a mass of dispersed elements (i.e., In, Cd, Ge, Ga, etc.). Based on systematic studies of sulfur isotopic composition, the authors conclude: The Bainiuchang deposit experienced two epochs of metallogenesis, i.e., the Middle-Cambrian sea-floor exhalative sedimentary metallogenic epoch and the Yanshanian magmatic hydrothermal superimposition metallogenic epoch. In the two metallogenic epochs, the δ³⁴S values of sulfides were all near 0, showing a tendency of being enriched slightly in heavy sulfur. The δ³⁴S values of sulfides in the early metallogenic epoch are within the range of 2‰–5‰ with a peak value range of 2‰–3‰ and an average of 3.0‰, and those of sulfides in the late metallogenic epoch are within the range of 2‰–6‰ with a peak value of 3‰–4‰ and an average of 3.9‰. For the single metallogenic epoch, sulfur in the ore-forming fluids in the early epoch already reached isotopic equilibrium and was derived mainly from underneath the magma chamber or basement metamorphic igneous rocks. Sulfur in the sulfides in the late epoch was derived mainly from magmatic hydrothermal fluids formed in the process of remelting of the basement metamorphic igneous rocks.     

Genesis of copper mineralisation,Myall Creek prospect,South Australia

The Myall Creek copper prospect is in unmetamorphosed carbonaceous dolosiltstone and sandstone at the base of the late Proterozoic (Adelaidean) Tapley Hill Formation. It contains disseminated, fine-grained chalcopyrite, zincian tennanite, bornite, chalcocite, pyrite, sphalerite and galena, and irregular to straight chalcopyrite-rich veinlets. Some ore minerals rim and/or partially replace pyrite or clastic grains. There is no evidence of hydrothermal activity. The ³⁴S_CDT values of pyrite and the other sulfides fall in the wide range –3.6 to +44.2. Dolomite in both mineralised and unmineralised samples has ¹³C_PDB values concentrated around –3, and ¹⁸O_SMOW values around +25. It is concluded that the mineralising fluids were near-neutral brines which leached metals from the basement and early Adelaidean rocks. They entered the Tapley Hill sediments at moderately low temperatures via permeable strata and faults. The metals were precipitated by biogenic H₂S, and also fixed by reaction with iron sulfides and, possibly, organic matter. Continuing ascent of brines into the mineralised strata caused breakdown of detrital feldspars and Fe-Ti oxides, and some solution-remobilisation of early-formed sulfides.     

Variable initial Pb isotopic compositions of rocks associated with the UG2 chromitite, eastern Bushveld Complex

An investigation of the Pb isotopic compositions of plagioclase and sulfide in a stratigraphic interval including the UG2 chromitite of the eastern Bushveld Complex has been conducted to determine the Pb isotopic composition(s) of the magma(s) that crystallized to form this part of the intrusion, gain a better understanding of why coexisting plagioclase and sulfide commonly exhibit widely different Pb isotopic compositions, and explore the use of Pb isotopes in deducing post-accumulation history. Analyses were obtained in situ with a NuPlasma multicollector ICP-MS coupled with 193 nm Excimer or 213 nm lasers.Most plagioclase compositions fall on the ²⁰⁷Pb/²⁰⁴Pb vs ²⁰⁶Pb/²⁰⁴Pb geochron of 2.06 Ga, which is the solidification age of the intrusion. The measured ratios have not been affected by radiogenic ingrowth, and plagioclase generally remained closed to Pb exchange after initial cooling. The array of plagioclase compositions on the geochron is significantly larger than that defined by analytical error. This indicates that in terms of Pb at least two different magma compositions were present. The composition of the least radiogenic magma was approximated by that of the contemporaneous BSE with μ (²³⁸U/²⁰⁴Pb) and ω (²³²Th/²⁰⁴Pb) values of ≈9.0 and 35, respectively, suggesting a mantle derivation with little or no involvement of the continental crust, while the second magma possessed a Pb isotopic composition similar to the upper crust with μ ≈ 9.6.Compared to plagioclase, sulfides generally possess slightly higher ²⁰⁶Pb/²⁰⁴Pb ratios for equivalent ²⁰⁷Pb/²⁰⁴Pb ratios such that their compositions fall between the 2.06 and 1.86 Ga geochrons. The latter age is much younger than the cooling age. The data are interpreted to mean that the Bushveld Complex remained buried in the crust at temperatures of several hundred °C for about 200 Ma after solidification, and that any sulfides accessible to fluid continued to re-equilibrated during this time with more radiogenic Pb. The sulfide Pb may have been transported into the Bushveld Complex by fluids from an external reservoir when the rocks were still partially molten and thus permeable. Alternatively, the sulfide Pb may have originated mainly from radiogenic decay of U and Th present in minerals other than the sulfides in the immediately surrounding Bushveld rocks, followed by local redistribution of Pb by whatever fluid was present. Indeed, some sulfides are characterized by ²⁰⁸Pb/²⁰⁴Pb ratios sufficiently high that an external source is unlikely. This observation and the fact that the sulfides display small-scale heterogeneity suggest that most, if not all, of the radiogenic sulfide Pb was locally derived. It also implies that during the post-solidification, re-equilibration period there was no large-scale fluid-flow through the microfracture network because otherwise the isotopic heterogeneities would not have been preserved. The minerals in 2 of the 19 samples studied contain young Pb, presumably introduced by meteoric waters that permeated the macrofracture network.     

New lead isotope determination from five Proterozoic sulfide deposits,Skellefte district,Sweden

Lead isotope analyses of 25 sulfide samples (galenas, iron sulfides, and sulfosalts) from five different mines of the Skellefte district, northern Sweden, demonstrate that the Pb-isotopic composition of galenas and other sulfides rich in lead varies between individual deposits within the district. This contrasts with many other base-metal districts, where ore lead is isotopically homogeneous on a regional basis. Although all of the Skellefte leads are depleted in ²⁰⁷Pb relative to average global lead evolution models, thus suggesting a large mantle-derived component in their sources, the Nasliden deposit lying at the contact of the host volcanic rocks and the overlying metasediments contains a significant component of crustal lead. It is concluded that while the Pb-isotope data are consistent with a volcanic exhalative origin of the ores of the Skellefte district, they also demonstrate that older crustal lead was incorporated into the sulfides during their emplacement and the subsequent period of magmatic and metamorphic activity which followed their deposition.     

Ion activity products of iron sulfides in groundwaters: Implications from the Choshui fan-delta, Western Taiwan

Precipitation of iron sulfides is an important process in groundwater geochemistry because it reduces iron mobility in anaerobic aquifers. Iron sulfides occur in various allotropic forms such as amorphous FeS and pyrite, and their solubility products differ up to 13 orders of magnitude. However, few data for ion activity products (IAP) of iron sulfides defined by the equation: H⁺ + FeS_(S) = Fe²⁺ + HS^- in groundwater have been reported in the literature. We computed IAP values of iron sulfides for 46 groundwater samples from the Choshui fan-delta of Taiwan and 65 samples from other areas of the world. The mean of -log(IAP) values obtained for the 46 samples is 3.07 ± 0.34 (1σ), which is consistent with the solubility constant 3.00 ± 0.12 (Davison et al., 1999) of amorphous FeS, implying that the anaerobic aquifers in the Choshui fan-delta are still undergoing active sulfate-reduction processes and keeping the groundwater saturated with amorphous FeS.We suggest that the −logKsp value 3.91 of amorphous FeS adopted in the databases for WATEQF and PHREEQC computer programs ought to be revised to 3.00. Otherwise, the saturation indices (SI) calculated by the two computer programs will be an order of magnitude too high.     

Lead isotope heterogeneity in sulfides from different assemblages at the Verninskoe gold deposit (Baikal-Patom Highland,Russia)

Using the high-precision technique of MC-ICP mass spectrometry, the isotope composition of lead was studied for the first time in sulfides of different mineral associations at the Verninskoe deposit that belong to large gold deposits of the Lena Province. In 23 monofractions of sulfides (pyrite, arsenopyrite, galena, and sphalerite), the Pb-Pb data showed a pronounced heterogeneity of the isotope composition of ore lead (²⁰⁶Pb/²⁰⁴Pb = 18.21–18.69, ²⁰⁷Pb/²⁰⁴Pb = 15.59–15.67, and ²⁰⁸Pb/²⁰⁴Pb = 37.98–38.63) for the deposit as a whole. This heterogeneity is also seen to a lesser degree within individual samples. In this case, a correlation takes place between the isotope composition of ore Pb and the type of mineral association: the sulfides in earlier associations are characterized by lower contents of the ²⁰⁶Pb, ²⁰⁷Pb, and ²⁰⁸Pb radiogenic isotopes compared to the minerals of later parageneses. The comparison of Pb-Pb isotope characteristics of ore mineralization of the Verninskoe deposit to those of the Sukhoi Log deposit (the greatest in the Lena Province) testifies to the geochemical similarity of the sources of ore Pb involved in the formation of these deposits. The sources as such were terrigenous rocks of the Bodaibo synclinorium formed mainly as a result of the disintegration of Precambrian rocks of the Siberian craton.     

Sulfur isotope study of a modern intertidal environment,and the interpretation of ancient sulfides

Extensive sulfur isotope distribution data for sulfides precipitated in an intertidal environment show no distinctive features when compared with isotope values for other marine, sedimentary sulfides. The fractionation ranges from α = 1.030 to α = 1.048. The pattern is characteristic for a system essentially open to sulfate, and isotope analyses of interstitial sulfates are corroborative. A population of sulfate-reducing bacteria of the order of 10⁹ organisms per cc of interstitial water is indicated. Seasonal variation of the isotope distribution reflects a transient sulfide composition and a bacterial population in which the fractionation effect is indirectly controlled by temperature. The data presented for this modern shallow water environment are at variance with an earlier assessment of isotopic distributions in ancient sulfides which linked shallow water environments with limited fractionation (α ? 1.025) in a closed system.     

A sulfur isotope study on pyrite deposits of Southern Tuscany,Italy

The S-isotope composition (δ³⁴S_CDT) of 213 samples of sulfides, sulfates and native sulfur from the pyrite mineralizations of southern Tuscany and associated country rocks were determined. The sulfur isotopic composition of pyrite is quite homogeneous and similar for all studied ore bodies, with an average δ³⁴S value near +9,5‰. Pyrite disseminated within the Filladi di Boccheggiano formation, and thought to be authigenic, shows a much larger range of δ³⁴S values (-13.1 to +14.5‰). The isotopic compositions of other sulfides associated with pyrite in the deposits show that isotopic equilibrium among sulfides was approached on a regional scale, but seldom fully attained. Isotopic data suggest that sedimentary marine sulfate was the ultimate source of sulfur in ores. Sulfates (mostly anhydrite) from the sulfate-carbonate lenses associated with both the Filladi di Boccheggiano and the Calcare Cavernoso formations also have similar and homogeneous compositions (average δ³⁴S=+15–16‰). Coexisting sulfates and sulfides are not in isotopic equilibrium. In the light of the isotopic data, among the many proposed genetic models for the largest stratabound pyrite bodies the two following alternatives appear the most likely: 1) in agreement with recently suggested hypotheses, the ore bodies are older than the emplacement of the Mio-Pliocenic granitoids in the area, and are probably hydrothermal-sedimentary in origin, coeval with associated country rocks; 2) the ore bodies were formed as a consequence of bacterial reduction of anhydrite in low-temperature convection systems related to the early stages of the Mio-Pliocenic thermal anomaly. In both cases, the emplacement of the Mio-Pliocenic granitoids caused metamorphism and remobilization of the pre-existing ores, producing smaller discordant mineralized bodies.