A fluid inclusion and light element stable isotope study of the gold-bearing quartz vein system,Falun, Sweden

The Falun gold quartz vein mineralization is located ca 230 km NW of Stockholm, Sweden, within the Early Proterozoic volcano-sedimentary sequence of Bergslagen. The mineralization consists of a system with subparallel quartz veins that crosscut the alteration zone to the Falun massive sulphide deposit. Early barren and late gold-bearing quartz veins follow tectonic structures postdating the formation of the massive sulphide ore. Both generations of veins are epigenetic to the massive sulphide ore and were formed by hydrothermal processes. Fluid inclusion study of the gold-bearing quartz veins indicates a low-moderately saline fluid (0.3 to 17.4 equiv wt% NaCl). Heterogeneous trapping is indicated by coexisting inclusions showing a variable CO₂ content from 100% CO₂ ± CH₄ to 100% aqueous fluid. Temperatures of total homogenization also show a wide spread from 116–350°C with a slightly bimodal distribution with peaks at ca 180°C and 280°C. MeasuredδD values — 69 to — 63%0 (SMOW), of inclusion fluid and calculatedδ ¹⁸O values of hydrothermal fluids — 7.5 to — 1.4%0 (SMOW), strongly suggest a meteoric origin for the fluids. The quite consistentδD values and the range inδ ¹⁸O values indicate that major water-rock interaction led to the evolution inδ18O of the hydrothermal fluids.     

The role of prokaryotes in subsurface weathering of hydrothermal sediments: A combined geochemical and microbiological investigation

A detailed geochemical and microbiological study of a ∼2 m sediment core from the inactive Alvin mounds within the TAG hydrothermal field was conducted to examine, for the first time, the role of prokaryotes in subsurface weathering of hydrothermal sediments. Results show that there has been substantial post-depositional remobilisation of metal species and diagenetic overprinting of the original high-temperature hydrothermal minerals, and aspects have involved prokaryotic processes. Prokaryotic enumeration demonstrates the presence of a population smaller than the average for deep sea sediments, probably due to the low organic carbon content, but not inhibited by (and hence adapted to) the metal rich environment. There was a small but significant increase in population size associated with the active redox boundary in an upper metal sulphide layer (50-70 cm) around which active metal remobilisation was concentrated (Cu, Au, Cd, Ag, U, Zn and Zn). Hence, subsurface prokaryotes were potentially obtaining energy from metal metabolism in this near surface zone. Close association of numbers of culturable Mn and Fe reducing prokaryotes with subsurface Fe²⁺ and Mn²⁺ pore water profiles suggested active prokaryotic metal reduction at depth in core CD102/43 (to ∼175 cm). In addition, a prokaryotic mechanism, which is associated with bacterial sulphate reduction, is invoked to explain the U enrichment on pyrite surfaces and Zn and Pb remobilisation in the upper sediment. Although prokaryotic populations are present throughout this metalliferous sediment, thermodynamic calculations indicated that the inferred low pH of pore waters and the suboxic/anoxic conditions limits the potential energy available from Fe(II) oxidation, which may restrict prokaryotic chemolithotrophic biomass. This suggests that intense prokaryotic Fe oxidation and weathering of seafloor massive sulphide deposits may be restricted to the upper portion of the deposit that is influenced by near neutral pH and oxic seawater unless there is significant subsurface fluid flow.     

Iron-formation and glaciogenic rocks of the Rapitan Group,Northwest Territories,Canada

In northwestern Canada, iron-formation occurs as part of the Rapitan Group, a dominantly sedimentary succession of probable Late Precambrian age. The Rapitan Group contains abundant evidence of glaciogenic deposition. It includes massive mixtites which contain numerous faceted and striated clasts. Finely bedded and laminated sedimentary rocks of the Lower Rapitan contain many large isolated (ice-rafted?) intra- and extra-basinal clasts. The Lower and Middle Rapitan are interpreted as products of a glacial marine regime. The iron-formation is interbedded with thin mixtite beds and contains large exotic clasts which are probably indicative of the existence of floating ice at the time of deposition of at least part of the iron-formation. If the apparently low paleolatitudes are confirmed, then glacial marine interpretation of the Rapitan, and the probably correlative Toby Conglomerate of southern British Columbia, support the postulate of a very extensive Late Precambrian ice sheet in North America.Similar iron-formations of similar age are present in South America (Jacadigo Series), in South-West Africa (Damara Supergroup) and in South Australia (Yudnamutana Sub-Group). All of these iron-formations are associated with glaciogenic rocks. In addition to the iron-formations, dolostones, limestones and evaporites (?) are intimately associated with Late Precambrian mixtites, considered by many to be glaciogenic.Huronian (Early Proterozoic) and correlative sequences of North America, and rocks of similar age in South Africa also contain closely juxtaposed undoubted glaciogenic rocks, iron-formations, dolostones and aluminous quartzites. The dolostones and aluminous sedimentary rocks have been interpreted as having formed under warm climatic conditions, but might also be explained by invoking higher P_CO2 levels in the Early Proterozoic atmosphere. By analogy with the Huronian succession, preservation of “warm climate” indicators in mixtite-bearing Late Precambrian sequences does not preclude a glacial origin for the mixtites.     

Sulphur isotope geochemistry of carbonatites

Sulphur isotopic data for sulphides and barite from several carbonatites (Mountain Pass, Oka, Magnet Cove, Bearpaw Mountains, Phalabora) show that individual carbonatites have different mean sulphide or barite isotopic compositions which deviate from the meteoritic mean $\text{δ}{}^{34}\text{S(0‰)}$.Classification of carbonatites in terms of T,?O₂ and pH during formation of the sulphur-bearing assemblages indicates that with decreasing T and increasing relative ?O₂ the mean δ³⁴S sulphide becomes increasing negative relative to the mean magma δ³⁴S. Only barite-free high temperature carbonatites (Phalabora) in which the mean δ^34S sulphide approaches the mean magmaδ³⁴S as a consequence of the paucity of oxidized anionic sulphur species in the magma can be used to directly estimate the mean isotopic composition of the source material.Barites from the Mountain Pass carbonatite show an increase in δ³⁴S with sequence of intrusion of the carbonatite units; dolomitic carbonatite (mean δ³⁴S, + 5.4‰), calcitic carbonatite (+ 4.8%.), silicified carbonatite (+ 6.9‰), tabular carbonatite dikes (+ 8.7‰), mineralized shear zones (+ 9.5‰). Within each of these units a spread of 6.8%. is evident. Isotopic trends in this low temperature (300°C) carbonatite are evaluated by treating the system as a hydrothermal fluid. The observed isotopic variations can be explained by removal of large amounts of sulphur from a fluid whose mean δ³⁴S is 0 to + 1‰     

The mobility of radium-226 and trace metals in pre-oxidized subaqueous uranium mill tailings

The exchange of ²²⁶Ra and trace metals across the tailings-water interface and the mechanisms governing their mobility were assessed via sub-centimetre resolution profiling of dissolved constituents across the tailings–water interface in Cell 14 of the Quirke Waste Management Area at Rio Algom's Quirke Mine, near Elliot Lake, Ontario, Canada. Shallow zones (<1.5 m water depth) are characterized by sparse filamentous vegetation, well-mixed water columns and fully oxygenated bottom waters. Profiles of dissolved O₂, Fe and Mn indicate that the tailings deposits in these areas are sub-oxic below tailings depths of ∼3 cm. These zones exhibit minor remobilization of Ra in the upper 5 cm of the tailings deposit; ²²⁶Ra fluxes at these sites are relatively small, and contribute negligibly to the water column activity of ²²⁶Ra. The shallow areas also exhibit minor remobilization of Ni, As, Mo and U. The release of these elements to the water cover is, however, limited by scavenging mechanisms in the interfacial oxic horizons. The presence of thick vegetation (Chara sp.) in the deeper areas (>2 m water depth) fosters stagnant bottom waters and permits the development of anoxia above the benthic boundary. These anoxic tailings are characterized by substantial remobilization of ²²⁶Ra, resulting in a relatively large flux of ²²⁶Ra from the tailings to the water column. The strong correlation between the porewater profiles of ²²⁶Ra and Ba (r²=0.99), as well as solubility calculations, indicate that the mobility of Ra is controlled by saturation with respect to a poorly ordered and/or impure barite phase [(Ra,Ba)SO₄]. In the anoxic zones, severe undersaturation with respect to barite is sustained by microbial SO₄ reduction. Flux calculations suggest that the increase in ²²⁶Ra activity in the water cover since 1995 (from <0.5 to 2.5 Bq l⁻¹) can be attributed to an increase in the spatial distribution of anoxic bottom waters caused by increased density of benthic flora. The anoxic, vegetated areas also exhibit minor remobilization with respect to dissolved As, Ni and Zn. The removal of trace metals in the anoxic bottom waters appears to be limited by the availability of free sulphide. Collectively, the data demonstrate that while the water cover over the U mill tailings minimizes sulphide oxidation and metal mobility, anoxic conditions which have developed in deeper areas have led to increased mobility of ²²⁶Ra.     

The carbonate-hosted willemite prospects of the Zambezi Metamorphic Belt (Zambia)

Zambian willemite (Zn₂SiO₄) deposits occur in the metasedimentary carbonate rocks of the Proterozoic Katangan Supergroup. The most important orebodies are located around Kabwe and contain both sulphides and willemite in dolomites of low metamorphic grade. The Star Zinc and Excelsior prospects (Lusaka area), discovered in the early 1920s, occur in the metamorphic lithotypes of the late Proterozoic Zambezi Supracrustal sequence, which were deposited in a transtensional basin formed during the oblique collision of the Kalahari and Congo cratons. The deposits are hosted by the limestone and dolomitic marbles of the Cheta and Lusaka Formations. Structural analysis indicates that several fracture sets host the deposits, which may be genetically related to the Pan-African Mwembeshi dislocation zone (a major geotectonic boundary between the Lufilian Arc and the Zambezi Belt). In both prospects, willemite replaces the marbles and is found along joints and fissures with open-space filling textures and locally may develop colloform and vuggy fabrics as well. Silver as well as traces of germanium and cadmium have been detected within the willemite ore, and lead or zinc sulphides are scarce or absent. Calcite locally replaces willemite. Willemite is associated with specular hematite and franklinite and post-dates the Zn-spinel gahnite in the paragenesis. Genthelvite [Zn₄Be₃(SiO₄)₃S] occurs as a minor phase in irregular aggregates. The willemites from the Lusaka area, though Mn-poor, show green cathodoluminescence colours and bright green fluorescence in short-wave UV (as the high-temperature willemites in USA). Thermometric analyses of primary fluid inclusions in willemite yield homogenization temperatures that range from 160°C to 240°C and salinities of 8–16 wt.% equiv. NaCl. The homogenization temperatures suggest a hypogene–hydrothermal origin for the willemite concentrations. The geochemistry of fluid inclusion leachates suggests that the hydrothermal fluids were brines derived from highly evaporated seawater. Precise age constraints are currently lacking for the Lusaka area deposits, though the deposits are not deformed, indicating that they post-date the Lufilian orogeny (~520 Ma). The possibility of precursor ores exists; the gahnite–franklinite–willemite deposits could have been derived from a metamorphosed primary sulphide (or even nonsulphide) concentration that has subsequently been completely destroyed. However, there is no real evidence of such a primary source for the willemite mineral association. The Lusaka zinc ores may have been produced by an extensive hydrothermal system, with fluids discharging along basinal fracture zones controlled by the pre-Pan-African rifting stage. A paragenesis similar to that of the Lusaka prospects has been proposed to be a vector towards massive sulphide ores in several parts of the world; therefore, it is possible that these small willemite showings in Zambia may be part of a much bigger, and still unexplored, zinc province.     

Paleomagnetism of the 1.88-Ga Sokoman Formation in the Schefferville–Knob Lake area,Québec,Canada, and implications for the genesis of iron oxide deposits in the central New Québec Orogen

Thirty-nine oriented block samples of iron-formation were collected at 13 sites, including opposite limbs of major folds, from the 1.88-Ga Sokoman Formation (Knob Lake Group) in the Schefferville–Knob Lake area of the central New Québec Orogen, northern Québec. The samples assayed up to 80.24% Fe₂O₃T (54.08% Fe), implying Fe-enrichment of the iron-formation up to ore grade. Anisotropy of magnetic susceptibility measurements on 245 standard specimens indicate a well preserved bedding-parallel fabric in the iron-formation, suggesting minimal alteration of the magnetic mineralogy since deposition and/or a mimetic secondary magnetic mineralogy. The iron-formation has not been internally deformed since the magnetic mineralogy was established. Analyses by variable-field translation balance and X-ray diffraction showed that the predominant magnetic mineral is hematite but a small amount of magnetite also is present in most samples. Following low-temperature pre-treatment as appropriate, stepwise thermal and alternating-field demagnetization of 218 specimens revealed a low-temperature, post-folding component (maximum T_ub≈400 °C, D=27.1°, I=20.1°, α₉₅=10.9°, from seven sites; pole position of 40.6°S, 257.0°E), and components carried by magnetite (maximum T_ub≈580 °C, D=35.8°, I=3.9°, α₉₅=9.1°, from 10 sites; pole position of 29.6°S, 250.9°E) and hematite (maximum T_ub≈680 °C, D=40.0°, I=1.6°, α₉₅=18.6°, from seven sites; pole position of 26.8°S, 247.0°E). The components carried by magnetite and hematite are pre-, syn- and post-folding depending on the sampling site, indicating that the magnetization was acquired continuously with deformation in the New Québec Orogen at 1.84–1.83 Ga. No evidence was found for acquisition of magnetization during the Mesozoic, when many of the iron oxide orebodies in the Schefferville–Knob Lake area are thought to have formed. Our findings imply that an episode of Fe-enrichment of iron-formation in the Sokoman Formation involved the circulation of hydrothermal fluids related to late Paleoproterozoic orogenesis. Such orogenic circulation of fluids may have contributed to the development of hematitic orebodies in the central New Québec Orogen.     

Iodine diagenesis in non-pelagic deep-sea sediments

Measurements of sediment geochemistry and porewater speciation have been made using eight cores containing turbidite sections from the Madeira and Nares Abyssal Plains. The results have been used to evaluate how the diagenetic chemistry of iodine in these sediments compares with that in sediments undergoing steady-state diagenesis. The behaviour of iodine is related to the development of a redox front within the turbidite, between the organic-rich anoxic sediment and its oxic cap, and the downward migration of the front through the turbidite with time. In contrast to the steady-state case, sediment I contents and I/ C ratios increase downwards through the oxidised section reaching a maximum at the redox front (up to ~ 100 μ/g I; molar I/C~ 20 × 10⁻⁴) below which values drop dramatically (I/C ~ 5 × 10⁻⁴). A strong iodate enrichment (up to ~3 μmol kg⁻¹) is observed in the oxidised section of the sediment. At the front interconversion of I⁻ and IO⁻₃ species occur and below the front porewater IO⁻₃ is absent and I~ concentrations increase with depth (as in other cases of anoxic diagenesis) up to ~ 10 μmol kg⁻. In the oxidised section of the sediment the I enrichment has been supplied by upward transport of iodide with the increasing I content, with depth being accounted for by progressive diagenetic enrichment with time.     

Sulphur cycling in organic-rich marine sediments from a Scottish fjord

In this study, the biogeochemical transformations of sulphur in organic‐rich marine sediments in a Scottish fjord are investigated by a combination of pore water and sediment geochemistry with sulphide diffusive gradient thin‐film probes and sulphate isotopic data (δ³⁴S and δ¹⁸O). Particular attention is paid to sulphur cycling in the upper sediment profile where sulphate reduction occurs but free sulphide is below the detection limits of conventional pore water geochemical analysis but quantifiable by sulphide diffusive gradient thin film. In the uppermost part of the sediment core, δ¹⁸O sulphate decreased from near‐sea water values to +7‰, indicating that anoxic sulphide oxidation dominated during this interval. Sulphate δ³⁴S remained unchanged as there was no net sulphate reduction (i.e. reduction was balanced by re‐oxidation). Below 4 cm depth, there was a slight increase in sulphate δ³⁴S from 20‰ to 23‰ associated with minor accumulation of iron sulphide. The δ¹⁸O of the sulphate also increased, to around +10‰ at 10 cm depth, as a result of the isotopic exchange of sulphate–oxygen with pore water and/or sulphur disproportionation reactions mediated during sulphur cycling. These processes continued to increase the δ¹⁸O of the sulphate to 14‰ at 20 cm depth with no further change in the δ³⁴S of the sulphate. Below 20 cm depth, free sulphide is detectable in pore waters and both the δ³⁴S of the sulphate and sulphide increase with depth with an offset controlled by kinetic fractionation during bacterial sulphate reduction. The δ³⁴S of the sedimentary organic fraction shifted towards lower, more bacteriogenic, values with depth in the profile, without any increase in the size of this sulphur pool. Thus, the organic sulphur fraction was open to interaction with bacteriogenic sulphide without the occurrence of net addition. Therefore, caution should be exercised when using sulphur isotopic compositions to infer simple net addition of bacteriogenic sulphide to the organic sulphur fraction.     

The sulphur isotope systematics of the Taolin lead-zinc ore deposit,China

Sulphur isotope data from coexisting sulphides and sulphates from the Taolin Pb-Zn ore deposit have been used to estimate the temperatures of sulphur mineral precipitation. The data indicate that sulphide was the dominant sulphur species in solution at high temperatures and that sulphate was dominant at low temperatures. Also the data show that the δ³⁴S value of total sulphur in solution was close to zero at high temperatures (~325°C) but had high positive values (+15%.) at low temperatures (~250°C). We interpret this phenomenon in terms of the effects of mineral precipitation on the isotopic composition of the solution. The increase in the δ³⁴S value of total sulphur with decreasing temperature was brought about by the removal from the system, by precipitation, of isotopically light sulphides.     

Lead isotope systematics of the giant massive sulphide deposits in the Iberian Pyrite Belt

Lead isotope analyses were performed on 26 polymetallic massive sulphide deposits of the Iberian Pyrite Belt, as well as on overlying gossans and associated volcanic rocks. All the massive sulphide deposits (except for Neves-Corvo), and nearly all the volcanic rocks show very similar isotopic compositions grouped around 18.183 (²⁰⁶Pb/²⁰⁴Pb), 15.622 (²⁰⁷Pb/²⁰⁴Pb) and 38.191 (²⁰⁸Pb/²⁰⁴Pb), indicating that most of the ore deposit lead was derived from the same continental crust environment as the associated volcanic rocks. The isotopic compositions are representative of the average south Iberian crust during the Devonian to Early Carboniferous (Dinantian), and their constancy implies a homogenization of the mineralizing fluids before the deposition of the massive sulphides from hydrothermal fluids circulating through interconnected regional fracture systems. This isotopic constancy is incompatible with multiple, small, independent hydrothermal cells of the East Pacific Rise type, and fits much better with a model of hydrothermal convections driven by “magmatic floor heating”. Neves-Corvo is the only south Iberian massive sulphide deposit to have a heterogeneous isotopic composition with, in particular, a highly radiogenic stanniferous ore (²⁰⁶Pb/²⁰⁴Pb of the cassiterite is >18.40). A model of lead mixing with three components is proposed to explain these variations: (1) one derived from the Devonian to Early Carboniferous (Dinantian) continental crust that generated all the other massive ores; (2) an Eohercynian stanniferous mineralization partly remobilized during the formation of the massive sulphides, but independent of them; and (3) a Precambrian continental crust component. The juxtaposition of three different sources places Neves-Corvo in a specific paleogeographic situation that could also explain its mineralogical specificity. The geodynamic context that best explains all the obtained isotopic results is one of an accretionary prism. The fact that lead isotope signatures of the gossans are almost identical to those of the underlying massive sulphides means that this technique could be a useful exploration tool for the Iberian Pyrite Belt.     

Boron,sulphur and copper isotope systematics in the orogenic gold deposits of the Archaean Hattu schist belt,eastern Finland

The Hattu schist belt is located in the western part of the Archaean Karelian domain of the Fennoscandian Shield. The orogenic gold deposits with Au–Bi–Te geochemical signatures are hosted by NE–SW, N–S and NW–SE oriented shear zones that deform 2.76–2.73 Ga volcanic and sedimentary sequences, as well as 2.75–2.72 Ga tonalite–granodiorite intrusions and diverse felsic porphyry dykes. Mo–W mineralization is also present in some tonalite intrusions, both separate from, and associated with Au mineralization. Somewhat younger, unmineralized leucogranite intrusions (2.70 Ga) also intrude the belt. Lower amphibolite facies peak metamorphism at 3–5 kbar pressures and at 500–600 °C temperatures affected the belt at around 2.70 Ga and post-date hydrothermal alteration and ore formation. In this study, we investigated the potential influence of magmatic-hydrothermal processes on the formation of orogenic gold deposits on the basis of multiple stable isotope (B, S, Cu) studies of tourmaline and sulphide minerals by application of in situ SIMS and LA ICP MS analytical techniques.Crystal chemistry of tourmaline from a Mo–W mineralization hosted by a tonalite intrusion in the Hattu schist belt is characterized by Fe^3 +–Al^3 +-substitution indicating relatively oxidizing conditions of hydrothermal processes. The range of δ¹¹B data for this kind of tourmaline is from − 17.2‰ to − 12.2‰. The hydrothermal tourmaline from felsic porphyry dyke swith gold mineralization has similar crystal chemistry (e.g. dravite–povondraite compositional trend with Fe^3 +–Al^3 + substitution) and δ¹¹B values between − 19.0‰ and − 9.6‰. The uvite–foitite compositional trend and δ¹¹B ‰ values between − 24.1% and − 13.6% characterize metasomatic–hydrothermal tourmaline from the metasediment-hosted gold deposits. Composition of hydrothermal vein-filling and disseminated tourmaline from the gold-bearing shear zones in metavolcanic rocks is transitional between the felsic intrusion and metasedimentary rock hosted hydrothermal tourmaline but the range of average boron isotope data is essentially identical with that of the metasediment-hosted tourmaline. Rock-forming (magmatic) tourmaline from leucogranite has δ¹¹B values between − 14.5‰ and − 10.8‰ and the major element composition is similar to that of the metasediment-hosted tourmaline.The range of δ³⁴S_VCDT values measured in pyrite, chalcopyrite and pyrrhotite is from − 9.1 to + 8.5‰, which falls within the typical range of sulphur isotope data for Archaean orogenic gold deposits. In the Hattu schist belt, positive δ³⁴S_VCDT values characterize metasediment-hosted gold ores with sulphide parageneses dominated by pyrrhotite and arsenopyrite. The δ³⁴S_VCDT values are both positive and negative in ore mineral parageneses within felsic intrusive rocks in which variable amounts of pyrrhotite are associated with pyrite. Purely negative values were only recorded from the pyrite-dominated gold mineralization within metavolcanic units. Therefore the shift of δ³⁴S_VCDT values to the negative values reflects precipitation of sulphide minerals from relatively oxidizing fluids. The range of measured δ⁶⁵Cu_NBS978 values from chalcopyrite is from − 1.11 to 1.19‰. Positive values are common for mineralization in felsic intrusive rocks and negative values are more typical for deposits confined to metasedimentary rocks. Positive and negative δ⁶⁵Cu_NBS978 values occur in the ores hosted by metavolcanic rocks. There is no correlation between sulphur and copper isotope data obtained in the same chalcopyrite grains.Evaluation of sulphur and boron isotope data together and comparisons with other Archaean orogenic gold provinces supports the hypothesis that the metasedimentary rocks were the major sources of sulphur and boron in the orogenic gold deposits in the Hattu schist belt. Variations in major element and boron isotope compositions in tourmaline, as well as in the δ³⁴S_VCDT values in sulphide minerals are attributed to localized involvement of magmatic fluids in the hydrothermal processes. The results of copper isotope studies indicate that local sources of copper in orogenic gold deposits may potentially be recognized if the original, distinct signatures of the sources have not been homogenized by widespread interaction of fluids with a large variety of rocks and provided that local chemical variations have been too small to trigger changes in the oxidation state of copper during hydrothermal processes.     

Geological and C–O–S–Pb–Sr isotopic constraints on the origin of the Qingshan carbonate-hosted Pb–Zn deposit,Southwest China

Located in the western Yangtze Block, the Qingshan Pb–Zn deposit, part of the Sichuan–Yunnan–Guizhou Pb–Zn metallogenic province, contains 0.3 million tonnes of 9.86 wt.% Pb and 22.27 wt.% Zn. Ore bodies are hosted in Carboniferous and Permian carbonate rocks, structurally controlled by the Weining–Shuicheng anticline and its intraformational faults. Ores composed of sphalerite, galena, pyrite, dolomite, and calcite occur as massive, brecciated, veinlets, and disseminations in dolomitic limestones.

The C–O isotope compositions of hydrothermal calcite and S–Pb–Sr isotope compositions of Qingshan sulphide minerals were analysed in order to trace the sources of reduced sulphur and metals for the Pb–Zn deposit. δ¹³C_PDB and δ¹⁸O_SMOW values of calcite range from –5.0‰ to –3.4‰ and +18.9‰ to +19.6‰, respectively, and fall in the field between mantle and marine carbonate rocks. They display a negative correlation, suggesting that CO₂ in the hydrothermal fluid had a mixed origin of mantle, marine carbonate rocks, and sedimentary organic matter. δ³⁴S values of sulphide minerals range from +10.7‰ to +19.6‰, similar to Devonian-to-Permian seawater sulphate (+20‰ to +35‰) and evaporite rocks (+23‰ to +28‰) in Carboniferous-to-Permian strata, suggesting that the reduced sulphur in hydrothermal fluids was derived from host-strata evaporites. Ores and sulphide minerals have homogeneous and low radiogenic Pb isotope compositions (²⁰⁶Pb/²⁰⁴Pb = 18.561 to 18.768, ²⁰⁷Pb/²⁰⁴Pb = 15.701 to 15.920, and ²⁰⁸Pb/²⁰⁴Pb = 38.831 to 39.641) that plot in the upper crust Pb evolution curve, and are similar to those of Devonian-to-Permian carbonate rocks. Pb isotope compositions suggest derivation of Pb metal from the host rocks. ⁸⁷Sr/⁸⁶Sr ratios of sphalerite range from 0.7107 to 0.7136 and (⁸⁷Sr/⁸⁶Sr)_200Ma ratios range from 0.7099 to 0.7126, higher than Sinian-to-Permian sedimentary rocks and Permian Emeishan flood basalts, but lower than Proterozoic basement rocks. This indicates that the ore strontium has a mixture source of the older basement rocks and the younger cover sequence. C–O–S–Pb–Sr isotope compositions of the Qingshan Pb–Zn deposit indicate a mixed origin of the ore-forming fluids and metals.     

Continentally-derived solutes in shallow Archean seawater: Rare earth element and Nd isotope evidence in iron formation from the 2.9 Ga Pongola Supergroup, South Africa

The chemical composition of surface water in the photic zone of the Precambrian ocean is almost exclusively known from studies of stromatolitic carbonates, while banded iron formations (IFs) have provided information on the composition of deeper waters. Here we discuss the trace element and Nd isotope geochemistry of very shallow-water IF from the Pongola Supergroup, South Africa, to gain a better understanding of solute sources to Mesoarchean shallow coastal seawater. The Pongola Supergroup formed on the stable margin of the Kaapvaal craton ∼2.9 Ga ago and contains banded iron formations (IFs) that represent the oldest documented Superior-type iron formations. The IFs are near-shore, pure chemical sediments, and shale-normalized rare earth and yttrium distributions (REY_SN) exhibit positive La_SN, Gd_SN, and Y_SN anomalies, which are typical features of marine waters throughout the Archean and Proterozoic. The marine origin of these samples is further supported by super-chondritic Y/Ho ratios (average Y/Ho = 42). Relative to older Isua IFs (3.7 Ga) from Greenland, and younger Kuruman IFs (2.5 Ga) also from South Africa, the Pongola IFs are depleted in heavy rare earth elements (HREE), and appear to record variations in solute fluxes related to sea level rise and fall. Sm-Nd isotopes were used to identify potential sediment and solute sources within pongola shales and IFs. The ?_Nd(t) for Pongola shales ranges from −2.7 to −4.2, and ?_Nd(t) values for the coeval iron-formation samples (range −1.9 to −4.3) are generally indistinguishable from those of the shales, although two IF samples display ?_Nd(t) as low as −8.1 and −10.9. The similarity in Nd isotope signatures between the shale and iron-formation suggests that mantle-derived REY were not a significant Nd source within the Pongola depositional environment, though the presence of positive Eu anomalies in the IF samples indicates that high-T hydrothermal input did contribute to their REY signature. Isotopic mass balance calculations indicate that most (?72%) of the Nd in these seawater precipitates was derived from continental sources. If previous models of Fe-Nd distributions in Archean IFs are applied, then the Pongola IFs suggest that continental fluxes of Fe to Archean seawater were significantly greater than are generally considered.     

A new sulphur isotopic study of some Iberian Pyrite Belt deposits: evidence of a textural control on sulphur isotope composition

The sulphide deposits of the Iberian Pyrite Belt (IPB) represent an ore province of global importance. Our study presents 113 new sulphur isotope analyses from deposits selected to represent the textural spectrum of ores. Measured ³⁴S values range from −26 to +10‰ mostly for massive and stockwork ores, in agreement with data previously published. In situ laser ³⁴S analyses reveals a close correlation of ³⁴S with texture. Primary diagenetic textures are dominated by relatively low ³⁴S (−8‰ to −2‰), whereas stockwork feeder textures are dominated by higher ³⁴S (∼+3‰ to +5‰). Intermediate textures (mainly coarse textures in stratiform zones) have intermediate ³⁴S, although they are mostly dominated by the high ³⁴S component. Rare barite has a homogeneous ³⁴S around +18‰, which is consistent with direct derivation from Lower Carboniferous seawater sulphate. A dual source of sulphide sulphur in the IPB deposits has been considered. A hydrothermal source, derived from reduction of coeval seawater sulphate in the convective systems, is represented by sulphide in the feeder zones. Here variations in ³⁴S are caused by variations in the extent of the sulphate reduction, which governs the SO₄:H₂S ratio. The second end-member was derived from the bacterial reduction of coeval seawater sulphate at or near the surface, as reflected in the primary textures. A distinct geographical variation in ³⁴S and texture from SW (more bacteriogenic and primary textures) to NE (more hydrothermal textures and ³⁴S) which reflects a variation in the relative input of each source was likely controlled by local geological environments. Given that the sulphur isotope characteristics of the IPB deposits are unlike most VMS and Kuroko deposits, and noting the dominance of a mixed reduced sedimentary and volcanic environment, we suggest that the IPB could represent an ore style which is intermediate between volcanic and sedimentary hosted massive sulphide types. Received: 8 October 1997 / Accepted: 14 May 1998     

Draa Sfar, Morocco: A Visean (331 Ma) pyrrhotite-rich, polymetallic volcanogenic massive sulphide deposit in a Hercynian sediment-dominant terrane

Draa Sfar is a Visean, stratabound, volcanogenic massive sulphide ore deposit hosted by a Hercynian carbonaceous, black shale-rich succession of the Jebilet terrane, Morocco. The ore deposit contains 10 Mt grading 5.3 wt.% Zn, 2 wt.% Pb, and 0.3 wt.% Cu within two main massive sulphides orebodies, Tazakourt (Zn-rich) and Sidi M'Barek (Zn–Cu rich). Pyrrhotite is by far the dominant sulphide (70 to 95% of total sulphides), sphalerite is fairly abundant, chalcopyrite and galena are accessory, pyrite, arsenopyrite and bismuth minerals are rare. Pyrrhotite is monoclinic and mineralogical criteria indicate that it is of primary origin and not formed during metamorphism. Its composition is very homogeneous, close to Fe₇S₈, and its absolute magnetic susceptibility is 2.10^− 3 SI/g. Ar–Ar dating of hydrothermal sericites from a coherent rhyolite flow or dome within the immediate deposit footwall indicates an age of 331.7 ± 7.9 Ma for the Draa Sfar deposit and rhyolite volcanism.The Draa Sfar deposit has undergone a low-grade regional metamorphic event that caused pervasive recrystallization, followed by a ductile–brittle deformation event that has locally imparted a mylonitic texture to the sulphides and, in part, is responsible for the elongated and sheet-like morphology of the sulphide orebodies. Lead isotope data fall into two compositional end-members. The least radiogenic end-member, (²⁰⁶Pb/²⁰⁴Pb = 18.28), is characteristic of the Tazakourt orebody, whereas the more radiogenic end-member (²⁰⁶Pb/²⁰⁴Pb  18.80) is associated with the Sidi M'Barek orebody, giving a mixing trend between the two end-members. Lead isotope compositions at Draa Sfar testify to a significant continental crust source for the base metals, but are different than those of the Hajar and South Iberian Pyrite Belt VMS deposits.The abundance of pyrrhotite versus pyrite in the orebodies is attributed to low fO₂ conditions and neither a high temperature nor a low aH₂S (below 10^− 3) is required. The highly anoxic conditions required to stabilize pyrrhotite over pyrite are consistent with formation of the deposit within a restricted, sediment-starved, anoxic basin characterized by the deposition of carbonaceous, pelagic sediments along the flank of a rhyolitic flow-dome complex that was buried by pelitic sediments. Deposition of sulphides likely occurred at and below the seafloor within anoxic and carbonaceous muds.Draa Sfar and other Moroccan volcanogenic massive sulphide deposits occur in an epicontinental volcanic domain within the outer zone of the Hercynian belt and formed within a sedimentary environment that has a high pelagic component. In spite of the diachronous emplacement between the IPB deposits (late Devonian to Visean) and Moroccan deposits (Dinantian), all were formed around 340 ± 10 Ma following a major phase of the Devonian compression.     

The geology and genesis of the Tarcoola gold deposits,South Australia

The Tarcoola goldfield is located in the Gawler Craton in northwestern Eyre Peninsula, South Australia. The gold deposits are hosted in the Middle Proterozoic Tarcoola Formation, comprised of the fluviatile Peela Coglomerate Member, the shallow marine Fabian Quartzite Member, and the marine Sullivan Shale Member. Mineralization in the goldfield consists of north-northeast to north-northwest trending gold-bearing quartz veins with associated hematite, pyrite, arsenopyrite, sphalerite, chalcopyrite, galena, electrum and gold. Adamellite in contact with the Tarcoola Formation has previously been included in the Middle Proterozoic Hiltaba Suite granitoids, on the basis of an apparent intrusive relationship with the Tarcoola Formation, and the gold-quartz veins were interpreted as being genetically related to the cooling pluton. However, detailed field and petrographic studies have demonstrated that the contact between the Tarcoola Formation and adamellite is a nonconformity. Hence, there is no genetic relationship between the mineralization and the adamellite. Oxygen isotope data indicate that an oreforming fluid, derived from convective circulation of meteoric or seawater, or from formation water, underwent isotope exchange with sediments of the Tarcoola Formation. A magmatic heat source for the hydrothermal system is suggested by the presence of intrusive igneous rocks, including dykes of aplite, quartz monzonite and microdiorite. Sulphur isotope characteristics of the mineralization can be explained by reduction of seawater sulphate or dissolution of disseminated sulphides in the sedimentary sequence. Metals were probably derived from rocks of the Tarcoola Formation. A complex paragenetic sequence involved deposition of minerals in several stages separated by episodes of fracturing. Fluid inclusions in quartz and fluorite show that deposition took place over a temperature range of about 340° to 110°C from a low salinity fluid. Analyses of chlorite coexisting with sulphide minerals suggest deposition of sulphides from about 300° to 170°C. Gold was transported as Au(HS) ₂ ^– and deposition appears to have coincided with a sudden decrease in fO₂ at around 260° to 250 °C.     

Brine-pool deposition for the Zn–Pb–Cu massive sulphide deposits of the Bathurst mining camp, New Brunswick, Canada. II. Ocean anoxia during mineralisation

Current models of massive sulphide ore genesis in the Bathurst mining camp, New Brunswick, involve settling of sulphide particles from a stagnating, low-salinity hydrothermal plume spreading laterally in an anoxic ocean layer with minimal sulphate content. There is fragmentary evidence of ocean anoxia in the form of local fine lamination in the shales that host some of the deposits but the total organic carbon, S, Fe, U/Th, Ni/Co, V/Ni and V/Cr relationships indicate deposition under oxic or dysoxic conditions. Vanadium and Mn values range from oxic to anoxic and sulphate-reducing to non-sulphate reducing but Mn may be anomalously low due to derivation by erosion of acidic volcanic rocks. The somewhat equivocal physical and chemical data, combined with the likely disturbing effects of penecontemporaneous volcanism, considerably weaken the case for an anoxic bottom layer in a static ocean. The presence of barite with ambient seawater ³⁴S values in Brunswick no. 12 ore, and the abundance of sulphate in modern euxinic basin waters, make a sulphate-free layer unlikely, even if anoxic. Sulphate-bearing, low-salinity fluids mixing with seawater would lead to growth of barite-bearing chimneys and baritic rubble mounds, which are not observed. A model involving brine-pool deposition better explains the major features of the Bathurst ores.     

Experimental determination of the stability and stoichiometry of sulphide complexes of silver(I) in hydrothermal solutions to 400°C

The solubility of silver sulphide (acanthite/argentite) has been measured in aqueous sulphide solutions between 25 and 400°C at saturated water vapour pressure and 500 bar to determine the stability and stoichiometry of sulphide complexes of silver(I) in hydrothermal solutions. The experiments were carried out in a flow-through autoclave, connected to a high-performance liquid chromatographic pump, titanium sampling loop, and a back-pressure regulator on line. Samples for silver determination were collected via the titanium sampling loop at experimental temperatures and pressures. The solubilities, measured as total dissolved silver, were in the range 1.0 × 10⁻⁷ to 1.30 × 10⁻⁴ mol kg⁻¹ (0.01 to 14.0 ppm), in solutions of total reduced sulphur between 0.007 and 0.176 mol kg⁻¹ and pH_T,p of 3.7 to 12.7. A nonlinear least squares treatment of the data demonstrates that the solubility of silver sulphide in aqueous sulphide solutions of acidic to alkaline pH is accurately described by the reactions0.5Ag₂S(s) + 0.5H₂S(aq) = AgHS(aq) K_s,1110.5Ag₂S(s) + 0.5H₂S(aq) + HS⁻ = Ag(HS)2− K_s,122Ag₂S(s) + 2HS⁻ = Ag₂S(HS)22− K_s,232where AgHS(aq) is the dominant species in acidic solutions, Ag(HS)2− under neutral pH conditions and Ag₂S(HS)22− in alkaline solutions. With increasing temperature the stability field of Ag(HS)2− increases and shifts to more alkaline pH in accordance with the change in the first ionisation constant of H₂S(aq). Consequently, Ag₂S(HS)22− is not an important species above 200°C. The solubility constant for the first reaction is independent of temperature to 300°C, with values in the range logK_s,111 = −5.79 (±0.07) to −5.59 (±0.09), and decreases to −5.92 (±0.16) at 400°C. The solubility constant for the second reaction increases almost linearly with inverse temperature from logK_s,122 = −3.97 (±0.04) at 25°C to −1.89 (±0.03) at 400°C. The solubility constant for the third reaction increases with temperature from logK_s,232 = −4.78 (±0.04) at 25°C to −4.57 (±0.18) at 200°C. All solubility constants were found to be independent of pressure within experimental uncertainties. The interaction between Ag⁺ and HS⁻ at 25°C and 1 bar to form AgHS(aq) has appreciable covalent character, as reflected in the exothermic enthalpy and small entropy of formation. With increasing temperature, the stepwise formation reactions become progressively more endothermic and are accompanied by large positive entropies, indicating greater electrostatic interaction. The aqueous speciation of silver is very sensitive to fluid composition and temperature. Below 100°C silver(I) sulphide complexes predominate in reduced sulphide solutions, whereas Ag⁺ and AgClOH⁻ are the dominant species in oxidised waters. In high-temperature hydrothermal solutions of seawater salinity, chloride complexes of silver(I) are most important, whereas in dilute hydrothermal fluids of meteoric origin typically found in active geothermal systems, sulphide complexes predominate. Adiabatic boiling of dilute and saline geothermal waters leads to precipitation of silver sulphide and removal of silver from solution. Conductive cooling has insignificant effects on silver mobility in dilute fluids, whereas it leads to quantitative loss of silver for geothermal fluids of seawater salinity.     

Evolution of a Carboniferous carbonate-hosted sphalerite breccia deposit,Isle of Man

A newly discovered, extensive sphalerite-bearing breccia (~7.5 wt.% Zn) is hosted in dolomitised Carboniferous limestones overlying Ordovician–Silurian metasedimentary rocks on the Isle of Man. Although base metal sulphide deposits have been mined historically on the island, they are nearly all quartz vein deposits in the metamorphic basement. This study investigates the origin of the unusual sphalerite breccia and its relationship to basement-hosted deposits, through a combination of petrographic, cathodoluminescence, fluid inclusion, stable isotope and hydrogeologic modelling techniques. Breccia mineralisation comprises four stages, marked by episodes of structural deformation and abrupt changes in fluid temperature and chemistry. In stage I, high-temperature (T _h > 300°C), high-salinity (20–45 wt.% equiv. NaCl) fluid of likely basement origin deposited a discontinuous quartz vein. This vein was subsequently dismembered during a major brecciation event. Stages II–IV are dominated by open-space filling sphalerite, quartz and dolomite, respectively. Fluid inclusions in these minerals record temperatures of ~105–180°C and salinities of ~15–20 wt.% equiv. NaCl. The δ³⁴S values of sphalerite (6.5–6.9‰ Vienna-Canyon Diablo troilite) are nearly identical to those of ore sulphides from mines in the Lower Palaeozoic metamorphic rocks. The δ¹⁸O values for quartz and dolomite indicate two main fluid sources in the breccia’s hydrothermal system, local Carboniferous-hosted brines (~0.5–6.0‰ Vienna standard mean ocean water) and basement-involved fluids (~5.5–11.5‰). Ore sulphide deposition in the breccia is compatible with the introduction and cooling of a hot, basement-derived fluid that interacted with local sedimentary brines.