Composition Of Sphalerite Research Articles

Silver–base metal epithermal vein and listwaenite types of deposit Crnac, Rogozna Mts., Kosovo. Part I: Ore mineral geochemistry and sulfur isotope study

Pb–Zn–Ag vein and listwaenite types of mineralization in Crnac deposit, Western Vardar zone, were deposited within several stages: (i) the pre-ore stage comprises pyrite, arsenopyrite, pyrrhotite, quartz, kaolinite and is followed by magnetite–pyrite; (ii) the syn-ore stage is composed of galena, sphalerite, tetrahedrite and stefanite; and (iii) the post-ore stage is composed of carbonates, pyrite, arsenopyrite and minor galena. The vein type mineralization is hosted by Jurassic amphibolites and veins terminate within overlying serpentinites. Mineralized listwaenites are developed along the serpentinite–amphibolite interface. The reserves are estimated to 1.7 Mt of ore containing in average 7.6% lead, 2.9% zinc, and 102 g/t silver. Sulfides from the pre- and syn-mineralization assemblage of the vein- and listwaenite-types of mineralization from the Crnac Pb–Zn–Ag deposit have been analyzed using microprobe, crush-leachates and sulfur isotopes. The pre-ore assemblage precipitated under high sulfur fugacities ( f(S 2) = 10 − 8 –10 − 6 bar) from temperatures ranging between 350 °C and 380 °C. Most likely water–rock reactions, boiling and/or increase of pH caused an increase of δ 34S of pyrite toward upper levels within the deposit. The decomposition of pre-ore pyrrhotite to a pyrite-magnetite mixture occurred at a fugacity of sulfur from f(S 2) = 8.7 × 10 − 10 to 9.6 × 10 − 9 bar and fugacity of oxygen from f(O 2) = 2.4 × 10 − 30 to 3.1 × 10 − 28 bars, indicating a contribution of an oxidizing fluid, i.e. meteoric water during pre-ore stages of hydrothermal activity. The crystallization temperatures obtained by the sphalerite-galena isotope geothermometer range from 230 to 310 °C. The δ 34S values of pre- and syn-ore sulfides (pyrite, galena, sphalerite, δ 34S = 0.3–5.9‰) point to magmatic sulfur. Values of δ 34S of galena and sphalerite are decreasing upwards due to precipitation of early formed sulfide minerals. Post-ore assemblage precipitated at temperature below 190 °C. Based on data presented above, we assume two fluid sources: (i) a magmatic source, supported by sulfur isotopic compositions within pre- and syn-ore minerals and a high mol% of fluorine found within pre- and syn-ore leachates, and (ii) a meteoric source, deduced by coincident pyrite–magnetite intergrowth, sulfur isotopic trends within syn-ore minerals and decrease of crystallization temperatures from the pre-ore stage (380–350 °C), towards the syn-ore (310–215 °C) and post-ore stages (< 190 °C). Post-ore fluids are Na–Ca–Mg–K–Li chlorine rich and were modified via water–rock reactions. Simple mineral assemblage and sphalerite composition range from 1.5 to 10.1 mol% of FeS catalog Crnac to a group of intermediate sulfidation epithermal deposit.

Ferric sulphate/chloride leaching of zinc and minor elements from a sphalerite concentrate

Atmospheric leaching of a sphalerite concentrate in sulphate and chloride media was performed and the effect of several variables, such as solid/liquid ratio and oxidant (Fe(III)) concentration were investigated. The behaviour of minor elements, such as Cu, In, As, Sb, Bi, Sn and Pb, was also studied under different conditions. The results showed that using a solid/liquid ratio of 5% (w/v) it was possible to leach 95% of zinc after 2 h, with a solution of 0.5 M H 2SO 4 and Fe 2(SO 4) 3 at 80 °C. The minor elements As, Sb and Bi were also completely leached whereas copper leaching was favoured by the use of chloride medium. The oxidation of Fe(II) during the leaching tests was studied and an improvement of 20% zinc extraction was observed in an oxygenated system. Cross-current leach tests using two/three stages and a solid/liquid ratio of 10% (w/v) were performed to achieve 90% of zinc extraction. The electron microprobe analysis of the leaching residues showed no change on the sphalerite composition after the leaching, which indicates that the leaching of sphalerite involves the break down of the sulphide structure.

Chemical Composition of Sphalerite from the Nebazawa Gold-Silver Mine, Gunma Prefecture, and Its bearing on the Evolution of Ore Fluid

Chemical Composition of Sphalerite from the Nebazawa Gold-Silver Mine, Gunma Prefecture, and Its bearing on the Evolution of Ore Fluid

Sulphur- and lead-isotope geochemistry of the Arapuçandere lead–zinc–copper deposit, Biga Peninsula, northwest Turkey

The Arapuçandere Pb–Zn–Cu ore body is a typical vein-type lead–zinc deposit of the Biga Peninsula, and is currently being mined for lead and zinc. In the study area, Permian–Triassic metamorphic rocks, Triassic metaclastic and metabasic rocks, Oligocene–Miocene granitoids, Miocene volcanic rocks, and Quaternary terrigenous sediments crop out. The ore deposits developed as Pb–Zn–Cu-bearing veins along faults in Triassic metasandstone and metadiabase. Microscopic studies reveal that the veins contain galena, sphalerite, chalcopyrite, pyrite, marcasite, covellite, and specular hematite as ore minerals, and quartz, calcite, and barite as gangue minerals. Analysed sulphur-isotope compositions (δ34SVCDT) of galena, sphalerite, and chalcopyrite range from − 5.9 to − 1.9‰ (average − 3.4‰), from − 5.5 to − 1.7‰ (average − 4.2 ‰), and from − 3.5 to − 0.9‰ (average − 2.6‰), respectively; that of H2S in the hydrothermal fluid was in the calculated range of − 5.8 to +0.1‰ (average − 2.5‰). These isotopic values suggest that magmatic sulphur dominates in sulphides, mixed with minor, isotopically light sulphur. Because no contemporaneous magmatic activity is associated with mineralization, it may be assumed that sulphur was leached from the surrounding Triassic units, mainly from metabasic, partly from metaclastic rocks. Lead-isotope studies indicate a model age of 114–63 Ma for the lead reservoir, in accord with possible sulphur-bearing local source rocks. Thus, the sulphur and lead deposited in the studied ore veins were probably leached from Triassic metabasic and metaclastic rocks some time during the Early Cretaceous to the Palaeocene.

GREENOCKITE AND ASSOCIATED SULFIDE MINERALIZATION FROM THE CALEDONIA GROUP MINES, BLANCA CREEK, LA HUERTA RANGE, SAN JUAN PROVINCE, ARGENTINA

We document various textural occurrences of greenockite associated with several ore minerals in varying parageneses: pyrite + sphalerite + galena + chalcopyrite ± arsenopyrite ± pyrrhotite ± bornite ± tennantite ± tetrahedrite ± covellite ± chalcocite ± aikinite ± wittichenite ± matildite ± berryite ± miharaite ± geocronite ± hematite ± magnetite from mineralized samples of the Caledonia Group mines, Marayes mining district, La Huerta Range, San Juan province, Argentina, based on reflected light and scanning electron microscope investigations. The mineralization is hosted mainly in metacarbonate outcrops in contact with the granulite basement and associated intermediate to mafic meta-intrusive rocks. We recognize three types of greenockite on the basis of textural and chemical criteria. Type-1 greenockite contains less than 1 mol.% ZnS, but its Fe content varies from 0.2 up to 4.5 mol.% FeS. Type 2 is Zn-rich, with 10–25 mol.% ZnS component and <0.1 to 3 mol.% FeS component, and type 3 has a Zn content of 5–7 mol.% ZnS component and FeS <0.1 mol.%. The presence of complex Ag–Cu–Bi–Pb sulfides point to a late-stage, low-temperature overprint of the primary sulfide parageneses, accompanied by the formation of hypogene greenockite, at least partly at the expense of sphalerite. The near-end-member composition of greenockite and late-stage sphalerite also indicates a low temperature of formation.

Mass-dependent cadmium isotopic variations in nature with emphasis on the marine environment

We report a survey of natural mass-dependent cadmium isotope fractionation measured by thermal ionization mass spectrometry using a double-spike technique (DS-TIMS). Over sixty samples of natural terrestrial Cd from diverse environments, including MORB, OIB, continental loess, hydrogenic and hydrothermal ferromanganese deposits, and sphalerites (both oceanic and from major continental ore deposits) were analysed. Our results are expressed in terms of ε 112/110Cd, which are deviations in 112Cd/ 110Cd from our in-house JMC Cd standard in parts per 10 4. The total ε 112/110Cd variation is relatively small, with a range of only 5 ε-units, and is one-to-two orders of magnitude smaller than that previously found in meteorites. The MORB, OIB and loess ε 112/110Cd values are similar and provide a good estimate for the bulk silicate Earth (BSE) value which is − 0.95 ± 0.12 relative to our Cd standard (ε 112/110Cd = + 0.16 relative to Münster JMC Cd). Taken together, these data suggest little Cd isotope fractionation takes place during crust–mantle segregation. Cd isotopic compositions of continental sphalerite (ZnS) deposits worldwide and high-temperature oceanic hydrothermal sulphides show remarkably similar ε 112/110Cd values, consistent with our estimate for the BSE. In contrast, mid-temperature oceanic sulphides from a single extinct hydrothermal chimney display over 4 ε-units variation — along with the most negative values. These variations are most probably caused by precipitation/redissolution of sulphide phases en route within the hydrothermal system. The ε 112/110Cd variability found in worldwide marine Fe–Mn deposits reflects the seawater Cd isotope signal upon precipitation from ambient seawater. A decrease in ε 112/110Cd is observed in passing from shallow-water Fe–Mn deposits to those from deeper waters (> 2000 m depth). This shift is explained by biological fractionation related to the uptake of dissolved seawater Cd by phytoplankton in the upper water column. The relatively uniform ε 112/110Cd values close to zero at great depths are consistent with regeneration and remineralization of Cd at depth. Our data suggest that Cd isotopes – much like the Cd/Ca ratio in foraminifera – could potentially serve as a proxy for past changes in biological productivity. The temporal Cd isotope record in a Fe–Mn crust archive at 2000 m depth from the NE Atlantic suggests no gross long-term changes in Cd cycling took place over the past 8 Ma.

Did the giant Broken Hill (Australia) Zn–Pb–Ag deposit melt?

Recent published research has proposed that some metamorphosed massive sulfide deposits, including the Broken Hill deposit, Australia, have partially melted. Here we discuss the evidence for and against this process. The Broken Hill deposit is located in the Curnamona Craton within a rift in an apparent upward-coarsening sequence of clastic metasediments into which Paleoproterozoic mafic and felsic melts intruded. The deposit was metamorphosed to granulite facies conditions (750–800 °C and 5–6 kbar) and was subjected to at least five periods of deformation. At peak P– T conditions, a melt phase was produced in silicate rocks, especially granitoids and psammopelites, and in ore characterized by the systems Cu–As–S, Ag–Pb–S, Ag–Sb–As–S, Cu–Sb–Pb–S, Sb–As–S, Cu–Sb–S, and Fe–As–S. However, the proportion of minerals in these S-bearing systems is insignificant (< 1 wt.%) when compared to the volume of sulfides in the system SiO 2–FeO–MnO–CaO–Al 2O 3–P 2O 5–CO 2–ZnS–PbS–FeS–(FeAsS/FeAs 2), which is the most relevant system to Broken Hill ores. P– T conditions were never high enough to produce a melt phase in this system. Authors arguing for sulfide rock melting suggest that Mn-rich lithologies (garnetite and quartz garnetite), which are intimately associated with the Pb-rich ores at the Broken Hill deposit, were produced by a reaction between Mn-rich sphalerite and aluminous wall rocks. However, to produce such rocks would require Mn contents of sphalerite compositions that are unrealistically high and yet to be found in nature. Moreover, the implication of the melt model is that wherever Mn-rich quartz garnetite and garnetite are found sphalerite should be located next to them. This is clearly not the case. The presence of polyphase sulfide inclusions within garnet in garnetite has also been considered by some in the literature to be sulfide melt inclusions with the implication that the enclosing garnet was a product of melting. However, we consider these sulfides to be products of hydrothermal processes during retrograde metamorphism. It is impossible to form garnetite and quartz–garnetite in rocks within the Curnamona Craton that formed at upper greenschist–lower amphibolite facies conditions by partial melting. P– T conditions were too low. We consider that the Fe and Mn component of garnetite and quartz garnetite are products of exhalation and inhalation at or near the sea floor, and that these rocks are meta-exhalites or meta-inhalites. Some garnet-rich rocks also formed by metasomatic processes throughout the protracted metamorphic history that affected the Curnamona Craton.

Rates of zinc and trace metal release from dissolving sphalerite at pH 2.0–4.0

Abstract High-Fe and low-Fe sphalerite samples were reacted under controlled pH conditions to determine nonoxidative rates of release of Zn and trace metals from the solid-phase. The release (solubilization) of trace metals from dissolving sphalerite to the aqueous phase can be characterized by a kinetic distribution coefficient, ( D tr ), which is defined as [( R tr / X (tr) Sph )/( R Zn / X (Zn) Sph )], where R is the trace metal or Zn release rate, and X is the mole fraction of the trace metal or Zn in sphalerite. This coefficient describes the relationship of the sphalerite dissolution rate to the trace metal mole fraction in the solid and its aqueous concentration. The distribution was used to determine some controls on metal release during the dissolution of sphalerite. Departures from the ideal D tr of 1.0 suggest that some trace metals may be released via different pathways or that other processes (e.g., adsorption, solubility of trace minerals such as galena) affect the observed concentration of metals. Nonoxidative sphalerite dissolution (mediated by H + ) is characterized by a “fast” stage in the first 24–30 h, followed by a “slow” stage for the remainder of the reaction. Over the pH range 2.0–4.0, and for similar extent of reaction (reaction time), sphalerite composition, and surface area, the rates of release of Zn, Fe, Cd, Cu, Mn and Pb from sphalerite generally increase with lower pH. Zinc and Fe exhibit the fastest rates of release, Mn and Pb have intermediate rates of release, and Cd and Cu show the slowest rates of release. The largest variations in metal release rates occur at pH 2.0. At pH 3.0 and 4.0, release rates show less variation and appear less dependent on the metal abundance in the solid. For the same extent of reaction (100 h), rates of Zn release range from 1.53 × 10 −11 to 5.72 × 10 −10 mol/m 2 /s; for Fe, the range is from 4.59 × 10 −13 to 1.99 × 10 −10 mol/m 2 /s. Trace metal release rates are generally 1–5 orders of magnitude slower than the Zn or Fe rates. Results indicate that the distributions of Fe and Cd are directly related to the rate of sphalerite dissolution throughout the reaction at pH 3.0 and 4.0 because these two elements substitute readily into sphalerite. These two metals are likely to be more amenable to usage in predictive acid dissolution models because of this behavior. The Pb distribution shows no strong relation to sphalerite dissolution and appears to be controlled by pH-dependent solubility, most likely related to trace amounts of galena. The distribution of Cu is similar to that of Fe but is the most-dependent of all metals on its mole fraction ratio (Zn:Cu) in sphalerite. The Mn distributions suggest an increase in the rate of Mn release relative to sphalerite dissolution occurs in low Mn samples as pH increases. The Mn distribution in high Mn samples is nearly independent of pH and sphalerite dissolution at pH 2.0 but shows a dependence on these two parameters at higher pH (3.0–4.0).

Metallic mineralization associated with pillow basalts in the Yaeyama Central Graben, Southern Okinawa Trough, Japan

Mineralogically investigated were massive sulfide samples of chimneys and mounds collected during the SHINKAI 6500 dives (Dive 562 and Dive 567) from two basalt-hosted hydrothermal ore deposits, being at a short distance of about 1 km, occurring in the eastern end of the Yaeyama Central Graben, the Southern Okinawa Trough. In both sample series (#562- and #567- series), ore minerals are dominated by low-iron sphalerite with small amounts of chalcopyrite and galena. Sphalerite in the #567-series samples shows somewhat wide compositional variation, ranging from 0.4 to 3.8 mole%FeS, with the highest mode of about 1 to 2 mole%FeS. Significant amounts of pyrite and secondary marcasite are pres- ent in the #562-series samples with subordinate bornite, while in the #567-series samples, they are lacking, but instead tetra- hedrite-tennantite series with atomic ratios of Sb/(Sb+As) being 0.2 to 0.8 is abundant and contains Ag up to 0.7wt% with mostly less than 0.5 wt%. Neither pyrrhotite nor isocubanite is encountered in the two sample series. Microthermometries of two-phase (liquid + gas) fluid inclusions trapped in calcite (#567-series) have revealed the following results: homogenization temperatures (not corrected for pressure) of 242 to 317 °C and salinities of 5.1 to 7.6 wt.%NaCl equiv., being a little more saline than the seawater. The observed mineral assemblages and sphalerite compositions obtained for the Yaeyama Central Graben deposits lead us to indicate that the basalt-hosted mineralizations might have occurred under rather higher sulfidation state. From the mineral associations observed, it is also indicated that both hydrothermal deposits are characterized by a met al association of essentially Zn-Pb-Cu. The mineral assemblages and ore textures observed are similar to those of the Kuroko deposits rather than the Besshi-type deposits, as well as their metal association (Cu-Pb-Zn).

Geology, petrography, and mineral chemistry of the Vazante non-sulfide and Ambrósia and Fagundes sulfide-rich carbonate-hosted Zn–(Pb) deposits, Minas Gerais, Brazil

The Vazante–Paracatu region represents the most important Zn district known in Brazil and includes the Vazante hypogene non-sulfide Zn deposit composed mainly of willemite (Zn 2SiO 4) and sphalerite-rich carbonate-hosted Zn–(Pb) deposits. Fagundes is a stratabound deposit, characterized by strong silicification, dolomitization and a Fe-rich carbonate alteration halo. The main ore is represented by rhythmically banded, colloform, and zoned pyrite, sphalerite and galena, related to wall rock dissolution and sulfide infilling, which probably occurs late during the burial diagenesis. Later veins and breccia ore types reflect epigenetic mobilization, related to brittle–ductile structures. The Ambrósia mineralization is mainly fault-controlled and related to brecciated dolomite, which is tectonically imbricated with black shales and slates. Typical features include host rock recrystallization, minor silicification, baroque dolomite and ankerite formation. Pyrite, marcasite, sphalerite and minor galena occur in brecciated comb-veins and veinlets, which overprint stylolites and tectonic fractures, suggesting an epigenetic origin for the ore. The Vazante deposit differs from all other deposits of the district due to the presence of willemitic ore, which is composed of willemite, dolomite, siderite, quartz, hematite, Zn-rich chlorite, barite, franklinite and zincite. The willemitic ore occurs tectonically imbricate to small sulfide ore bodies, which comprises sphalerite and galena, metabasites and hydrothermalized dolomites within the Vazante Shear Zone. The relationships between willemite formation and the development of mylonitic structures suggest that willemitic mineralization and deformation are synchronous episodes closely related to the Vazante Shear Zone. Low Zn/Cd ratios in sphalerite from Vazante (64 to 98) and Fagundes colloform (96 to 244) and zoned sphalerite (89 to 305) could reflect the regional role of mineralizing fluids with similar low Zn/Cd ratios and low contents of reduced sulfur (∑S red). High Ge (up to 2200 ppm) and the low Fe, Cu, Mn and Ag contents in late-diagenetic Fagundes sphalerite might suggest that this metal-bearing fluid could be derived from the underlying basin fill, which comprise clastic sediments and organic matter-rich pelitic sequences. Systematic relationships among sphalerite composition and paragenetic evolution of the Fagundes and Ambrósia deposits suggest that progressive fluid mixing processes were important for the genesis of the sulfide-rich deposits in the district. These mixing processes possibly involved the hot metal-bearing fluid with low contents of reduced sulfur and moderate-temperature, highly saline fluids, which could represent an important sulfur supply. The predominance of the highly saline brines in later epigenetic mineralization episodes might be related to episodic migration of hydrothermal fluids mainly derived from reduced shale sequences during the Brasiliano compressive events. The small variation in chemical and sulfur isotopic composition of the Vazante sphalerite could imply that the high-temperature metal-bearing fluid with low Zn/Cd ratios could represent a minor reservoir of reduced sulfur, which permitted only subordinate sphalerite precipitation in the Vazante deposit. The lack of reduced shale sequences above the Vazante deposit could represent a limiting factor for H 2S supply. Additionally, overall mixture between this hot sulfur-deficient metal-bearing fluid and meteoric fluids channeled to the Vazante Shear Zone favor the high fO 2/S 2 conditions responsible for the stability of the Vazante willemitic assemblage.

ZINC ISOTOPE VARIATION IN HYDROTHERMAL SYSTEMS: PRELIMINARY EVIDENCE FROM THE IRISH MIDLANDS ORE FIELD

Little is known about the range or controls on the zinc isotope composition of terrestrial materials and no systematic studies have been carried out on ore-forming systems. We have obtained zinc isotope data from 19 sphalerite samples, formed over a range of well-constrained precipitation conditions, from the Irish Zn-Pb ore field. The results reveal variation in δ 66Zn (where δ 66Zn = [(66Zn/64Zn)sample/(66Zn/64Zn)standard– 1] × 1000), from –0.17 to 1.33 per mil relative to the Lyon JMC 3-0749L zinc standard. This variation is significant compared to the external reproducibility (±0.12‰, 2 σ ), and the data show very good mass-dependency with δ 67Zn and δ 68Zn values. Thus, natural variations in the zinc isotope composition of these ore minerals can be resolved. Our results span the entire range of δ 66Zn values measured on terrestrial geologic samples to date. The data suggest that variations in the primary source rock composition or precipitation temperature are unlikely to be important controls on the zinc isotope composition of sphalerite in the ore field. We suggest that the variation is most likely due to a kinetic fractionation involving the preferential incorporation of light zinc isotopes in sphalerite precipitated rapidly under disequilibrium conditions. However, we cannot rule out the possibility of mixing of zinc derived from two isotopically distinct sources. The significant variation in zinc isotope compositions we have observed in the Irish ore field confirms that such fractionations can provide new insights into mineralizing processes in the Earth’s crust.

Nature of Hydrothermal Fluids at the Shale-Hosted Red Dog Zn-Pb-Ag Deposits, Brooks Range, Alaska

The Red Dog Zn-Pb-Ag district in the western Brooks Range, northern Alaska, contains numerous shale-hosted Zn-Pb sulfide and barite deposits in organic-rich siliceous mudstone and shale, chert, and carbonate rocks of the Carboniferous Kuna Formation. The giant Red Dog shale-hosted deposits consist of a cluster of four orebodies (Main, Qanaiyaq, Aqqaluk, and Paalaaq) that lie within distinct thrust panels that offset a single ore deposit during the Mesozoic Brookian orogeny. These Zn-Pb-Ag-barite orebodies contain one of the world’s largest reserves and resources of zinc. Fluid inclusions in samples of vein sphalerite, which accounts for about 20 percent of the ore in the Main deposit, and quartz that composes the bulk of the extensive silicification in the ore deposit, were studied by microthermometry, Raman spectrometry, and ion chromatography. The study of fluid inclusions in the vein sphalerite was limited by the intense postore deformation of the ore deposits. However, four primary aqueous fluid inclusion assemblages in vein sphalerite yield temperatures of homogenization of 115° to 120°C, 123° to 127°C, 110° to 120°C and 175° to 180°C. More abundant final-melting temperatures indicate that the fluid inclusions in sphalerite have salinities of about 14 to 19 wt percent NaCl equiv. The fluid inclusion electrolyte data show that the ore fluid responsible for the vein sphalerite derived its salinity from the evaporation of seawater. Considering the salinity of the fluid inclusions together with the electrolyte data, it is possible that the evaporative was initially about 30 wt percent saline fluid and that it mixed with a more dilute fluid somewhere along its flow path. The temperature, salinity, and electrolyte composition of vein sphalerite in the Red Dog deposits are remarkably similar to those characteristics in sphalerite veins near the Century zinc deposit, Australia. Together, these data compose the majority of information on the temperature and composition of sphalerite in deposits of this type. On the basis of data describing fluid inclusions in sphalerite and the geologic setting of the ore deposits, a brine model is suggested for the Red Dog deposits. In this model, brines were produced in evaporative environments in supratidal carbonate facies of the Lisburne Group less than 100 km from the Red Dog deposits. These reflux brines may have infiltrated the underlying rocks of Endicott Group or fractured metasedimentary basement rocks. In the absence of a local heat source at the Red Dog deposits, the temperature of the ore fluids (~100° to <200°C) requires that the fluids circulated at depths between ~ 2.4 and 7.4 km. In the Red Dog area, the metalliferous fluids ascended into the organic-rich rocks of the Kuna Formation, probably along zones of active extensional faults or breaches in the shale aquitards overlying the aquifers in the Endicott Group. Fluid inclusions were also studied in the abundant quartz that constitutes the majority of the silica rock in the ore deposits. This postore quartz extensively replaced barite and was traditionally thought to be part of the main ore event. Primary fluid inclusion assemblages contain two-phase aqueous inclusions, single-phase inclusions of dense methane, or both. Primary assemblages that contain single-phase, dense-methane inclusions together with two-phase aqueous inclusions yield consistent homogenization temperatures that provide unequivocal evidence for the coeval trapping of immiscible gas and aqueous fluids. The densities of the methane inclusions, together with the temperature of homogenization of coexisting aqueous fluid inclusions, show that these fluid inclusions were trapped between pressures of 800 and 3,400 bars and temperatures between 187° and 214°C. The pressures obtained provide unequivocal evidence that the quartz formed after ore deposition in the Carboniferous because such high fluid pressures could only have been produced from thrust loading during the Mesozoic Brookian orogeny. The observed large variation in pressure is best explained by transient fluid pressures from hydrostatic to lithostatic conditions during thrust loading. The 3,400 bars pressure corresponds with about 12 km of lithostatic burial, whereas the lower pressures (800 bars) correspond with about 8 km of hydrostatic pressure. Because of their low salinity (0–5 wt % NaCl equiv) the electrolyte compositions of the quartz fluid inclusions do not constrain their origin.

Discussion of “Tectonic controls of Mississippi Valley-type lead-zinc mineralization in orogenic forelands” by D.C. Bradley and D.L. Leach

The interesting and useful study by Bradley and Leach (2003) of tectonic controls on Mississippi Valley-type (MVT) mineralization in orogenic forelands is marred by a surprising disregard for published evidence for the age of these deposits. This problem is of major importance, because the ages of the deposits control their relation to tectonic features. Here are three examples of the problem, showing how they relate to the suggested tectonic models. 1. Bradley and Leach (2003, p. 657 and p. 663) cite the Canadian Rockies as an example of MVT deposits formed in anAndean-type orogen based on Laramideage paleomagnetic poles for Pine Point, KickingHorse and Robb Lake. They ignore Devonian Rb/Sr ages for sphalerite from Pine Point because they ‘‘may date late Devonian clays entrapped in the much younger sphalerite.’’ This statement is referenced to Symons et al. (1998b), which contains nothing about Pine Point. Perhaps they meant to refer to Symons et al. (1998a), which states (p. 79) that ‘‘Garven and Sverjensky (1994) noted that the (Rb-Sr)method is prone to contamination by colloidal clay particles entrapped in sphalerite....’’ However, Garven and Sverjensky (1994, p. 1,150) say only that ‘‘Other workers dated earlystage sphalerite at Pine Point as Devonian using a RbSr method, but this type of analysis may be prone to contamination by clay particles incorporated during rapid precipitation of colloform aggregates of metal sulfides.’’ None of these studies contains any information on clay minerals at Pine Point. Nakai et al. (1990, 1993), which were omitted by Bradley and Leach (2003), deal with this issue specifically and show that inclusions of clay minerals are unlikely to account for Rb-Sr compositions of MVT sphalerite. If Cordilleran MVT deposits are Devonian in age, they could have formed when the western margin of North America was ‘‘...dominated by long-lived regional extension’’ (Nelson et al. 2002). 2. Bradley and Leach (2003, p. 657 and Fig. 5) cite the Cevennes and Maestrat areas as examples of MVT deposits formed in an inversion-type orogen of Santonian to Miocene-age in the Pyrenees. In support of this interpretation, they refer to U-Pb dating by Grandia et al. (2000), isotopic ages on fluorite in Leach et al. (2001), and paleomagnetic measurements by Lewchuk et al. (1998). Sample and analytical data on the fluorite dated by Leach et al. (2001) are, to our knowledge, not published or available for evaluation. However, the U-Pb isochron age of about 62.6 Ma Editorial handling: L. Meinert

The composition of sphalerite and associated sulfides in reactions of the Cu–Fe–Zn–S, Fe–Zn–S and Cu–Fe–S systems at 1 bar and temperatures between 250 and 535 °C

Sphalerites and sulfides in selected reaction assemblages of the Cu–Fe–Zn–S, Cu–Fe–S, and Fe–Zn–S systems were crystallized in molten salt fluxes contained in evacuated capsules at temperatures between ∼250 and ∼535 °C. The combined compositional data for sphalerite indicate large increases in the mol% FeS content and the Fe/Cu ratio with decreasing sulfur fugacity at given temperatures. Pyrite and the associated copper-iron sulfides show parallel increases for Cu/Fe ratios in the Cu–Fe–S system, and increases for Fe/Cu and Fe/Zn ratios in the Cu–Fe–Zn–S system. These compositional changes occur over a range of sulfur fugacity–temperature conditions that include many sulfide deposits. A log fS 2–temperature (1000/K) composition map has been constructed for sphalerite using Lusk and Bray's (2002) data for a sequence of buffer reactions in the Cu–Fe–S and Fe–S systems. Accordingly, sphalerites with mean mol% FeS compositions of ∼1.7 (i.e. for the reaction: pyrite+idaite=bornite+S (vapor)), ∼4.4 (i.e. pyrite+bornite=chalcopyrite+S), ∼14.4 (i.e. pyrite+chalcopyrite=cubanite (iss)+S), 22.9 (i.e. pyrite+cubanite (iss)=pyrrhotite+cubanite (iss)+S), and ∼21.5 (i.e. pyrite=pyrrhotite+S) were obtained. A slight increase in the mol% FeS content of sphalerite with decreasing temperature was obtained for the pyrite=pyrrhotite+S buffer reaction at temperatures below ∼550 °C. The copper contents of sphalerite are generally low, ranging between ∼3 and ∼0.5 mol% CuS, and are highest in sphalerites associated with bornites. The equation: log 10 fS 2=11.01−9.49 (1000/K)+[0.187−0.252 (1000/K)] (mol% FeS)+[0.35−0.2 (1000/K)] (mol% CuS in sphalerite) can be applied to sphalerites that have equilibrated with buffer assemblages containing pyrite, or pyrite and pyrrhotite, in both the Fe–Zn–S and Cu–Fe–Zn–S systems at temperatures (K is in Kelvin) between 250 and 550 °C at 1 bar. This equation incorporates present data and a selection of published experimental data that is fitted to within 10%.

The Setting and Age of the Bermuda Zn-Pb Showing, Grinnell Peninsula, Devon Island: Implications for MVT Mineralization in the Canadian Arctic

Abstract The Bermuda Zn-Pb showing is hosted by dolomitized Blue Fiord Formation of Devonian age, located east of the Ensorcellement dome on the Grinnell Peninsula of Devon Island. Fluid inclusions in hydrothermal calcite indicate that the mineralization precipitated from low-temperature, highly saline brines. The δ34S compositions of sphalerite, galena, and marcasite range from 16.1‰ to 35.2‰. Consequently, the Bermuda showing is interpreted to be Mississippi Valley-type (MVT) mineralization. The apparent age of mineralization has been determined by the Rb-Sr method from sphalerite separates, and a Late Devonian age of 357 ± 50 Ma (MSWD=65), with an initial 87Sr/86Sr ratio of 0.7083 ± 0.0020 is indicated. This date, within error, is the same as the Polaris deposit. This suggests that the mineralization formed during the Ellesmerian Orogen, although it is also possible that mineralization is associated with extension, related to the opening of the Sverdrup basin. The strontium and sulfur isotopes and fluid inclusion analyses indicate the involvement of a brine that acquired its salinity from interaction with evaporites, lower in the stratigraphic succession. The Polaris mine is the type deposit in the Cornwallis Zn-Pb district and is hosted by the Thumb Mountain Formation. The results of this study are significant because they demonstrate that regional brine movement is more widespread than previously recognized, and that MVT mineralization in the Cornwallis Zn-Pb district is not restricted to the Thumb Mountain Formation.

The Perkoa Zinc Deposit, Burkina Faso

The massive sulfide deposit at Perkoa in Burkina Faso is the only one of its kind in the Paleoproterozoic Birimian Supergroup of West Africa. The estimated ore reserves are 5.7 Mt with 18.2 percent Zn, 10 percent BaSO4, 0.06 percent Pb, and 26 ppm Ag (50 ppm Cu). The deposit consists of a main orebody, which contains >90 percent of the reserves, and various satellite orebodies, hosted by tuffite and dioritic to granitic dikes, and structurally overlain by andesite, cumulate-rich basic rocks, and carbonaceous schist. The semitabular main orebody is 400 m long and 450 m wide, and has an average thickness of 10 m, whereas the satellite orebodies have an average thickness of 4 m. The tuffaceous rocks are intruded by a quartz diorite in the structural footwall (stratigraphic hanging wall), which has an U-Pb zircon age of 2175 ± 1 Ma. The main orebody is subparallel to the steeply dipping intrusive contact and is an average distance of 15 m from it. The satellite orebodies are discontinuous but also show a preferred orientation subparallel to the intrusive contact, on average 75 m stratigraphically below the intrusion. Massive sulfide ore is mainly composed of Fe-rich sphalerite (30%), pyrite (25%), barite (10%), hexagonal pyrrhotite (5%), magnetite (5%), and white mica. Quartz is mainly present as host-rock relics. Significant deposition of hydrothermal quartz occurred only in the structural footwall of the deposit, where the main orebody is close to the quartz diorite (0.5 m away). Albite, (Ba, K) feldspar, Ba-rich biotite, chlorite, tourmaline, andradite-rich garnet, ilmenite, rutile, titanite, and galena are subordinate. Chalcopyrite, arsenopyrite, Ag-rich tetrahedrite, and molybdenite occur as trace minerals in the massive sulfide ore. An extremely altered quartz-microdiorite dike with disseminated zincian spinel + magnetite ± Fe-poor sphalerite occurs adjacent to the massive, Fe-rich sphalerite ore of one of the satellite orebodies. The sulfur isotope compositions of sphalerite and pyrite from the main orebody have narrow ranges from –0.5 to 0.4 per mil and 0.4 to 1.0 per mil δ 34S, respectively. These values are close to those of sulfur from mantle-derived rocks. Barite has δ 34S values of 9.8 to 10.6 per mil. Pyrite and pyrrhotite in the sedimentary country rock of the deposit have a typical isotopic range of sulfides deposited in an oceanic environment with an input from sulfate-reducing bacteria ( δ 34S values of –7 to –22‰). A contribution of sedimentary sulfur can be ruled out for the massive sulfide ore. The magmatic rocks in the Perkoa area have a juvenile character according to the Nd-Sm isotopic data, like most Birimian rocks, but the Perkoa basalts show the influence of a subduction component in various discrimination diagrams that incorporate Ta (or Nb) and Th. Birimian basalts from other areas have geochemical signatures of MORB or oceanic, within-plate basalts. The massive sulfide ore has been metamorphosed during a regional thermal event under predominantly isotropic stress, probably 100 to 200 m.y. after the emplacement of the quartz diorite. Sphalerite and magnetite were remobilized, whereas pyrrhotite formed at the expense of pyrite. Peak metamorphic temperatures of >460°C at pressures of >1 kbar are indicated by almandine-rich garnet, the presence of andradite, and geothermometers involving arsenopyrite and pyrrhotite. Due to the metamorphic modifications of the ore and host rock, however, it is not possible to establish the age relationships between the main ore-forming event and granitoid intrusions. As a result, it is not known whether ore deposition took place close to a paleosea floor (i.e., an exhalative massive sulfide environment) or in a subvolcanic environment (i.e., a skarn).

Telluride mineralogy of the low-sulfidation epithermal Emperor gold deposit, Vatukoula, Fiji

¶The epithermal, low sulfidation Emperor gold telluride deposit in Fiji, hosted by Late Miocene-Early Pliocene shoshonitic rocks, is spatially related to a low-grade porphyry Cu system on the western flank of the Tavua Caldera. Gold is largely in the form of “invisible” gold in arsenian pyrite but 10 to 50% of gold is in the form of precious metal tellurides. Gold mineralization occurs in steeply dipping dikes and faults, flat-dipping structures (<45°), referred to locally as “flatmakes,” and at the intersection of two or more structures referred to as “shatter zones.” Petrographic, electron microprobe, and scanning electron microscope analyses of ores from some of the more recently discovered orebodies, Matanagata, Matanagata East, and R1 reveal that tellurium-bearing minerals, sylvanite, calaverite, krennerite, petzite, hessite, coloradoite, melonite, native tellurium, and rare benleonardite, formed during various hydrothermal stages, hosted in quartz, and to a lesser extent arsenian pyrite and tetrahedrite group minerals. Sylvanite followed by krennerite are the two most common tellurides in these orebodies. These tellurides show no systematic spatial distribution within flatmakes but there appears to be a higher concentration of tellurides where the flatmake intersects steep structures. Gold-rich tellurides preceded the formation of silver-rich tellurides and were constrained at 250 °C in log f S2 and log f Te2 space at −12.7 to −10.1 and −9.4 to −7.8, respectively, based on sulfide and telluride stabilities, and the composition of sphalerite. Ore forming components, such as Au, Ag, Te, Cu, V, and S, were likely derived from Late Miocene-Early Pliocene monzonites in and adjacent to the Tavua caldera.

Cadmium in Zinc Deposits: Economic Geology of a Polluting Element

The chief host of cadmium in zinc deposits is sphalerite, the cadmium content of which depends on the type of deposit. Sphalerite from Mississippi Valley-type (MVT) deposits has high cadmium concentrations whereas sphalerite from exhalative deposits has low cadmium concentrations. The Cd content of sphalerite depends on the Cd/Zn ratio, ligand activities, and temperature of the ore-forming fluids. The combined effect of variation of temperature, pH, total activity of reduced sulfur, and activity of Cl- cannot by itself account for either Cd depletion (exhalative deposits) or Cd enrichment (MVT deposits). Variations in the Cd/Zn ratio of the fluid have a significant effect in determining that of sphalerite. Basinal brines, which can considered to be the recent equivalents of MVT fluids, have high Cd/Zn ratios, and active exhalative systems are characterized by low Cd/Zn ratios. Probably the differences in sphalerite composition between the different deposit types are less a function of temperature and ligand activities than Cd/Zn ratio of the ore-forming fluids. In the hydrothermal environment, the Cd/Zn ratio is generally not high enough to allow crystallization of cadmium sulfides (greenockite or hawleyite). The abundance of greenockite in the supergene alteration zone of hydrothermal zinc deposits can be explained on the basis of Zn scavenging by crystallizing smithsonite.

Kinetics of iron depletion near pyrrhotite and chalcopyrite inclusions in sphalerite; the sphalerite speedometer

The textures and chemical compositions of sphalerite containing chalcopyrite and pyrrhotite inclusions from some skarns, together with experimental data on diffusivity, have permitted rates of exsolution (speedometry) and cooling histories of the skarns to be calculated. These calculations are based upon Fe depletion profiles obtained by electron microprobe transverses across those portions of sphalerite grains that are adjacent to chalcopyrite lamellae and pyrrhotite laths. The lamellae and laths occur within and along grain boundaries and polysynthetic twin planes in the host sphalerite. In addition, the samples contain 2- to 10-mu m chalcopyrite blebs that do not have the appearance of chalcopyrite disease textures.Two types of iron depletion profiles are observed in iron-rich sphalerite near sulfide inclusions: one type indicates a slight decrease in the Fe contents adjacent to chalcopyrite blebs, the other exhibits a larger Fe depletion (e.g., 3-6 mole % FeS over 70mu m) near chalcopyrite lamellae and pyrrhotite laths.Experimentally determined tracer diffusion rates of 59 Fe in sphalerite (D Fe = 5.6 x 10 (super -4) exp[-38 + or - 2 kcal/mole/RT], with a pyrrhotite + pyrite buffer) have been applied as a test to determine if Fe depletion can be explained by the simple cooling histories of the skarns. If all the sulfide inclusions are assumed to have been originally dissolved in Cu-bearing sphalerite at a higher temperature, the duration for isothermal precipitation of a chalcopyrite bleb from a sphalerite sphere of 50- mu m radius is calculated to be ca. 870 m.y. at 400 degrees C, the estimated temperature at which such a process may have taken place. Because of the slow Fe diffusivity in sphalerite, this time is unreasonably long, exceeding the geologic age (Mesozoic) of the deposits under consideration, so exsolution of chalcopyrite blebs from homogeneous sphalerite is unlikely. In contrast, the profiles of sharp Fe depletion in sphalerite are presumed to have developed during reequilibration of Fe-rich sphalerite attending the exsolution of pyrrhotite laths in response to decreasing temperature and rising sulfur fugacity. The measured profiles near pyrrhotite laths have been simulated by finite difference approximations to Fick9s diffusion equation. For an exsolution process starting at 350 degrees C and with a cooling rate of 0.5 degrees C/1,000 yr, the observed Fe depletion profiles would have been frozen in within 210,000 yr and with a closure temperature estimated to be 245 degrees C.

福岡県三ノ岳地域における接触交代鉱床の金属鉱化作用

The Yokozuru, Suisho, Tokoya, Sotan, Zurine and Usagida deposits are distributed around Mt. San-notake. They are contact metasomatic hydrothermal ore deposits formed in the contact between the Paleozoic limestone and the Kawara granodiorite that intruded in the Cretaceous period. Skarn zones are divided into two main zones, clinopyroxene zone and garnet zone. Based on the mineral assemblages the former is subdivided into three zones, A-, B- and C-zones, and the latter into two zones, D- and E-zones. A-zone is composed of plagioclase, alkali feldspar, quartz, hornblende, clinopyroxene, actinolite and opaque minerals, B-zone of quartz, clinopyroxene, garnet and opaque minerals, and C-zone of clinopyroxene, garnet, calcite and opaque minerals. D-zone includes garnet, clinopyroxene, chlorite, epidote and opaque minerals, and E-zone garnet, calcite, epidote, actinolite, quartz and opaque minerals. Sulfide mineralization occurred in the C-, D- and E-zones, and limestone zone. The main ore minerals are chalcopyrite, pyrite, pyrrhotite and sphalerite. Many kinds of Bi-, Te- and Ag-minerals were also found in a small quantity. They are bismuthinite, native bismuth, tetradymite, hessite, wittichenite, gustavite, aleksite, cuprobismutite, cosalite, argentite and so on. The FeS content of sphalerite coexisting with pyrite and chalcopyrite ranges from 13.6 to 19.8 mol.%, and the composition of pyrrhotite coexisting with pyrite varies from 46.9 to 47.4 Fe at.%. Based on the main mineral assemblages and the chemical compositions of sphalerite and pyrrhotite, it is suggested that the temperature-sulfur fugacity condition of the mineralization in the San-notake district locates in the pyrite-stability field near pyrite-pyrrhotite univariant curve in high temperature (400°-500°C) and cuts the univariant curve between 350° and 300°C. At later stage many Bi-Te-Ag-Pb-Cu-Fe Ni-Co-S minerals were produced, and the temperature-sulfur fugacity condition is estimated to be around the bismuthinite-bismuth univariant curve.