Thermodynamics of arsenates, selenites, and sulfates in the oxidation zone of sulfide ores: V. Chalcomenite and its synthetic analog, properties, and formation conditions

Understanding the mechanisms of selenium behavior under near-surface conditions is a topical problem of modern mineralogy and geochemistry that is very important in solving some environmental problems. The objective of this study is to develop techniques of synthesizing a chalcomenite analog and to study its speciation and properties. The synthesis was performed by boiling-dry aqueous Cu₂(CO₃)(OH)₂ solutions and selenium acid H₂SeO₃. The obtained samples were identified by X-ray diffraction and IR spectroscopy. The Eh-pH diagrams were calculated using the Geochemist’s Workbench (GMB 7.0) software package. The database comprises the thermodynamic parameters of 46 elements, 47 main particles, 48 redox pairs, 551 particles in solution, 624 solid phases, and 10 gases. The Eh-pH diagrams have been calculated for the Cu-Se-CO₂-H₂O system for the average content of these elements in underground waters and their contents in acidic waters in the oxidation zones of sulfide deposits. The formation of chalcomenite and malachite under near-surface conditions is discussed.     

Speciation of noble metals and conditions of their concentration in massive sulfide ores of the Urals

The distribution of noble metals has been studied in ores and sulfide concentrates from the Gai, Uchaly, Uzel’ga, Aleksandrinsky, Degtyarsk, and Saf’yanovka deposits. The ores, technological products, and hand-picked monofractions were analyzed with INAA; PGE were determined with kinetic and chromatographic methods after their preliminary chemical separation. The ultraheavy fractions from Au-rich samples were used for examining minerals of noble metals. Phase relations and compositions of ore minerals were studied with an X-ray microprobe and electron microscope equipped with an energy dispersive X-ray analyzer. Gold is associated largely with Fe and Cu minerals (pyrite, chalcopyrite, fahlore) and has been detected as an admixture in Pb, Bi, and Ag tellurides. Pyrite—the major mineral of massive sulfide ores—is the main gold concentrator (up to 20 ppm, ～1 ppm on average). As follows from the results of rational analysis, the concentration of finely dispersed gold in sulfide ores from the studied deposits ranges from 0.8 to 5.0 ppm, i.e., is less than the bulk Au content in the respective samples (0.93–21.2 ppm). Formation conditions of Au-enriched massive sulfide ores were estimated from the homogenization temperature of fluid inclusions in minerals and on the basis of the electrum-argentite-pyrite-sphalerite and electrum-hessite geothermometers, taking into account the sulfur and tellurium fugacities. The appearance of visible gold and tellurides in ores is caused by recrystallization of their fine-grained intergrowths with ore-forming minerals and, likely, by release of isomorphic admixtures contained in sulfides during epigenetic hydrothermal alteration.     

Polymetamorphism of ores in Precambrian stratiform massive sulfide deposits at Ambaji-Deri,Western India

Massive stratiform zinc-lead-copper sulfide ores, in association with cordierite-anthophyllite rocks, occur in adjacent localities of Ambaji and Deri, in Western India. The metasedimentary country rocks, interlayered with amphibolites and intruded by acidic to intermediate plutonic rocks, belong to the Precambrian Delhi Supergroup. The ore minerals identified by detailed mineragraphic studies include: sphalerite, galena, chalcopyrite, pyrite, pyrrhotite (both monclinic and hexagonal phases), magnetite, ilmenite, rutile, arsenopyrite, molybdenite, cubanite, mackinawite, boulangerite, gudmundite, meneghinite, lautite, tenantite, native bismuth, native silver, chalcocite and covellite. The common sulfide-silicate schistosity in the ores, flowage of sulfide streaks and tails around rotated poikiloblasts and in their pressure shadow region developed during early folding (F₁) and regional metamorphism of the rocks under green schist facies condition. These were superimposed by a pervasive hornfelsic fabric involving sulfides and silicates and including microfabrics due to annealing and grain growth in sulfides, during a subsequent phase of low pressure thermal metamorphism and related tectonism (F₂). Finally certain deformation features and some uncommon fabrics like martensitic lamellae in galena and subgrains in sphalerite developed during a mild deformation episode (F₃) in the waning stages of tectonism in the area. Compositional change in the ores during thermal metamorphism was minimal.     

Polymetallic sulfide ores hosted in Late Permian carbonate at the Alanish locality,northern Iraq: petrography and mineral chemistry

Polymetallic sulfide ores (Zn, Pb, Fe, Cu, Ag, and Cd) found in the Alanish locality of northern Iraq are hosted by dolostone in the Late Permian Chia Zairi Formation. The Alanish locality is one of several Zn–Pb deposits that are widespread in northern Iraq, situated along the northern passive margin of the Arabian plate. This paper describes the ore deposit classification, mineral chemistry, and paragenetic sequence of the area and proposes an ore formation model. We report the presence of acanthite and greenockite for the first time in Iraq. A brine solution derived from the sedimentary basin formed the primary sulfide ore minerals (sphalerite, galena, acanthite, pyrite, chalcopyrite, greenockite, and marcasite). The pre-tectonic mineralization is characterized by replacement textures including (1) high-Fe, low-Zn, dark-colored, coarse-grained sphalerite; (2) deformed anisotropic coarse-grained galena; and, (3) idiomorphic cubes of crushed pyrite. Conversely, the post-tectonic mineralization is characterized by open-space filling textures, including (1) low-Fe, high-Zn, light-colored, fine aggregated sphalerite; (2) fine-grained galena; and, (3) the existence of acanthite and marcasite. Although galena is an Ag carrier, both mineralization phases contained non-argentiferous galena. Non-sulfides (smithsonite, cerussite, and goethite) have replaced older sulfides in many areas due to supergene process. Gangue minerals present are dolomite, calcite, barite, and siderite. Open spaces and cavity filling of small paleo-karsts, replacement, veins, and veinlets are common features of the ore body. Metals were sourced from brines generated in the sedimentary basin, whereas sulfur was derived from nearby evaporates. Sediment compaction and tectonic activity, probably during Late Cretaceous, were the driving forces that squeezed and moved ore-bearing fluids derived from the sedimentary basin. Multiple stages of ore-bearing fluids were epigenetically intruded into the Late Paleozoic dolostone, forming an epigenetic strata-bound Mississippi Valley-type deposit precipitated under a temperature of 120 °C, as indicated by the cadmium fractionation in sphalerite and galena. Dolomitization and tectonic activity provided the necessary permeability for accumulating ores. The main ore body is directly connected to a fault plane and to adjacent dolostone that is frequently fractured and brecciated.     

Komatiites and nickel sulfide ores of the Black Swan area,Yilgarn Craton,Western Australia. 4. Platinum group element distribution in the ores,and genetic implications

The Black Swan komatiite sequence, in the Eastern Goldfields province of the Archaean Yilgarn Craton in Western Australia, is a body of dominantly olivine-rich cumulates with lesser volumes of spinifex textured rocks, interpreted as a section through an extensive komatiite lava flow field. The sequence hosts a number of nickel sulfide orebodies, including the Silver Swan massive shoot and the Cygnet and Black Swan disseminated orebodies. The massive sulfide orebodies of the Black Swan Succession are pervasively depleted in all platinum group elements (PGEs), particularly Pt and Pd, despite very high Ni contents. This depletion cannot be explained by R-factor variations, which would also require relatively low Ni tenors. The PGE depletion could be explained in part if the ores are enriched in a monosulfide solid solution (MSS) cumulate component, but requires some additional fractional segregation of sulfide melt upstream from the site of deposition. The Silver Swan orebody shows a remarkably consistent vertical zonation in PGE contents, particularly in Ir, Ru, Rh, Os, which increase systematically from very low levels at the stratigraphic base of the sulfide body to maxima corresponding roughly with the top of a lower layer of the orebody rich in silicate inclusions. Platinum shows the opposite trend, but is somewhat modified by remobilisation during talc carbonate alteration. A similar pattern is also observed in the adjacent White Swan orebody. This zonation is interpreted and modelled as the result of fractional crystallisation of MSS from the molten sulfide pool. The strong IPGE depletion towards the base of the orebody may be a consequence of sulfide liquid crystallisation in an inverted thermal gradient, between a thin rapidly cooling upper rind of komatiite lava and a hot substrate.Electronic Supplementary Material Supplementary material is available in the online version of this article at Editorial handling: Peter Lightfoot     

Thermochemistry of sulfide liquids. I. the system O-S-Fe at 1 bar

Recrystallized globules representing former immiscible sulfide liquids are found in a variety of igneous environments. Relatively little is known about the physical properties and thermochemistry of sulfide liquids, despite their importance in igneous systems. This study presents results of a series of experiments designed to calibrate a thermodynamic model for sulfide liquids in the system O-S-Fe at one atmosphere pressure. Sulfide liquids were equilibrated under controlled oxygen and sulfur fugacities at temperatures between 1100 and 1350 ° C in equilibrium with a silica mineral and a silicate melt. Experiments were quenched in a high-speed double-roller “splat” quencher in order to assure that measured compositions were as close to equilibrium liquid values as possible. Sulfide liquids are not stable in equilibrium with a silica-saturated silicate melt at log₁₀(f _O2) > FMQ-1 at 1250 °C and log₁₀(f _S2)=−3. Iron content of the sulfide changes little with variations in oxygen and sulfur fugacity at a given temperature. Consequently, oxygen and sulfur contents are inversely correlated in these liquids. Sulfur is present entirely as sulfide. Iron appears to be present in both its ferric and ferrous states. Data from this study were combined with data compiled from the literature to calibrate an asymmetric regular solution thermodynamic mixing model for O-S-Fe liquids. This model reproduces miscibility gaps and data from this study quite well, but exhibits minor but systematic errors at the O-Fe binary. The observed inverse correlation between sulfur and oxygen is reflected in the predicted free-energy surface by a sharp energy valley running along a line of constant Fe content. Received: 10 April 1996 / Accepted: 15 November 1996     

PGE mineralization in marginal sulfide ores of the Chineisky layered intrusion, Russia

Summary We have undertaken a detailed study of platinum group element (PGE) mineralogy and geochemistry of disseminated sulfides associated with the marginal zone of the Chineisky layered mafic intrusion. Towards an intrusive contact the marginal zone reveals a gradual progression from gabbro-gabbronorites towards monzodiorite. Sulfides occur in all the rocks of the marginal zone including exocontact sandstone. They occur mainly as pyrrhotite, chalcopyrite, and pentlandite and show progressive enrichment in Cu towards the intrusive contact. In the same direction, PGE mineralogy reveals the following systematic changes: (1) size of PGE mineral grains decreases from 50 μm up to 1 μm; (2) the association of Pd minerals with Ni and Co arsenide and sulfarsenides becomes stronger; and (3) the composition of PGE minerals changes for palladium: Pd-Sn → Pd-As → Pd-Sb → Pd-Te → Pd-Bi; for platinum: Pt-Fe + PtAs₂ → PtS + PtAs₂ → PtAs_2. This zoning pattern is interpreted as the result of fractional crystallization of an immiscible sulfide melt, with the residual liquid, enriched in Cu, PGE, and volatile elements, being expelled towards the periphery of the intrusive body. PGE minerals also decompose in an oxidation zone. The most stable of them are paolovite and sperrylite, which both accumulated in placers derived from the massif. Pd is removed from the decomposed minerals, and then absorbed by brown iron and goethite in the oxidation zone. Author’s address: Nadezhda Tolstykh, Institute of Geology and Mineralogy SB RUS, pr. Ak. Koptyga 3, Novosibirsk 630090, Russia     

Mineralogical and geochemical zoning and genesis of massive sulfide ores at the Oktyabr’sky deposit

Fractional crystallization and emanation differentiation of sulfide magma and related mineralogical and geochemical zoning are exemplified in massive sulfide ores of the Oktyabr’sky deposit, Noril’sk district. The mineralogical zoning is expressed in the change of mineral types of ore from pyrrhotite (Po) to chalcopyrite (Cp) (from the flanks to the center of the ore lode). In terms of geochemistry, the Cu content, Cu/(Cu + Ni) ratio, and contents of noble metals incompatible with Mss (Pt, Pd, and Au) increase in this direction, while the S and Fe contents decrease. The distribution of elements compatible with Mss (Ir, Os, Rh, and Ru) is more complex. Their contents decrease from Po to high-Cu Cp ore, although there is a second maximum for Cb-type ore. The distribution of ore elements in the vertical and horizontal sections of massive ores at the deposit is different. The upper outer contact zone and frontal parts of massive ore lodes are enriched in all ore elements and a light sulfur isotope. The succession of enrichment is correlated with the relative affinity for sulfur and remains independent of the affinity of these elements for Mss (Pd-Rh, Os-Au). The possible role of liquid immiscibility of sulfide magma in the development of the mineralogical and geochemical zoning of massive ore is discussed.     

Localization conditions of lenticular layers of polymetallic ores in Irtysh zone of buckling,as in Belousovskoye ore deposit

Morphology of ore bodies is relatively simple along the strike but characteristically intricate across the strike (table 4) in the regional type of pyrite-polymetallic ore deposit (C₃-P₁). Positions of the ore bodies are structurally-lithologically controlled. Historically, every aspect of the deposit's modern structure may be related to geological evolution of the region, from the Devonian sedimentation to terminations of the Upper Paleozoic tectonic movements.     

Gold potential of massive sulfide ores in the Kamennoozero structural unit,Eastern Karelia

Two types of massive sulfide ores have been identified in the Kamennoozero segment of the green-stone belt: (1) hydrothermal volcanic-sedimentary strata-bound ores with massive, banded, and disseminated structures and (2) massive, brecciated, and stringer-disseminated Au-bearing base-metal ores, crosscutting the rocks of the Vozhmozero Group. The strata-bound, slightly metamorphosed orebodies are located at several levels along the contact between the Kamennoozero and Kumbuksa groups in the deep fault zones of the same names. These ores are composed of pyrite and pyrrhotite, small amounts of chalcopyrite and sphalerite, and distinguished by low grades of base metals and not higher than 0.06 g/t Au. In the Lebyazhino and Svetloozero areas, close to the sulfide Cu-Ni ore hosted in ultramafic rocks, the strata-bound bodies contain pentlandite and are enriched in Co, Ni, Cu, Zn, and up to 2.0–9.2 g/t Au. Brecciated and recrystallized pyrite ores contain up to 0.08–0.4% Sb and As, and up to 0.6–1 g/t Au in the Kumbuksa Fault Zone near Zolotye Porogi. The North Vozhma and Upper Vozhma base-metal massive sulfide occurrences, composed of pyrite, chalcopyrite, sphalerite, pyrrhotite, galena, bornite, and chalcocite, are considered to be promising Au-bearing prospects. Some samples from the North Vozhma occurrence contain up to 1.2–2.8 g/t Au and up to 167 g/t Ag. A gold grade of up to 20 g/t has been detected in the Upper Vozhma occurrence. The potential gold resources of the North Vozhma occurrence are estimated at about 600 kg.     

西藏北部舍索与拉屋铜矿床硫化物铅同位素特征

本文在系统的野外地质工作基础上,对舍索与拉屋矿床的矿石硫化物铅同位素组成进行综合分析,进而示踪其成矿物质来源。结果显示,舍索矿区矿石硫化物铅的²⁰⁶Pb/²⁰⁴Pb值为18.517~18.776,²⁰⁷Pb/²⁰⁴Pb值为15.671~15.756,²⁰⁸Pb/²⁰⁴Pb值为38.955~39.33;拉屋矿区矿石硫化物铅的²⁰⁶Pb/²⁰⁴Pb值为18.651~18.757,²⁰⁷Pb/²⁰⁴Pb值为15.707~15.823,²⁰⁸Pb/²⁰⁴Pb值为39.183~39.561。研究表明,舍索与拉屋矿床矿石硫化物铅同位素含量比值具有明显的上地壳特征,指示两个矿床成矿物质主要来自上地壳。其中舍索矿床成矿物质富集受燕山期岩浆作用影响,而拉屋矿床部分成矿物质由晚石炭纪地幔物质的喷流沉积作用提供。     

Komatiites and nickel sulfide ores of the Black Swan area,Yilgarn Craton,Western Australia. 2: Geology and genesis of the orebodies

The Black Swan Ultramafic Succession hosts a number of magmatic Fe–Ni–Cu–PGE sulfide ore shoots, ranging from high grade massive ore to low grade disseminated sulfides. Of these, the most economically significant is the Silver Swan massive sulfide orebody, associated with the basal contact of the succession. The deposit varies in thickness between 5 and 20 m, reaches a N–S strike length of 75 m, extends for at least 1.2 km of vertical plunge and is open at depth. Overlying matrix (net-textured) ore is rare. Inclusions of dacite are abundant within the lower 5 m of the massive sulfide. They range from angular fragments through smooth sinuous and plumose morphologies to fine lace-like intergrowths with the sulfide matrix, and comprise variable proportions of cores of porphyritic dacite and carapaces with skeletal plagioclase phenocrysts. Dynamic crystallisation and kinetic melting textures in the carapaces indicate that the inclusions have been heated to various temperatures, some well above their liquidus temperature. The composition of the inclusions ranges from a perfect match with the immediate footwall dacites to mixtures of dacite with up to 30% komatiite. The consistent thickness of the inclusion-bearing basal layer within the massive sulphide is interpreted as the extent of 3-D physical connectivity between the inclusions and a partially molten underlying hybrid layer. Primary contacts between the Silver Swan massive sulfide orebody and overlying ultramafic rocks are marked by thin rinds containing coarse-grained chevron-textured chromites with skeletal textures. Compositions of these chromites match those from Kambalda, Perseverance and other localities, and are inconsistent with a metamorphic origin. They are interpreted as markers of primary magmatic contacts. The combination of this feature with the general paucity of matrix ore implies that the massive ore accumulated and solidified before the accumulation of the overlying thick sequence of olivine cumulates. Taken together with observations on the internal fractionation of platinum group elements within the massive ores, these observations are consistent with a model where the massive ore were emplaced at the floor of a small partially drained lava tube. The floor of the tube had been previously heated by passage of large volumes of lava, such that it had reached its melting range. The felsic inclusions within the ore are the result of buoyant ascent of partially molten substrate into the ore magma. This constitutes strong evidence for the operation of thermo-mechanical erosion during ore emplacement. The disseminated Cygnet and Black Swan orebodies show a number of distinctive features. Cygnet contains a assemblage of clasts and inclusions which are interpreted as the result of rip-up, transport and redeposition of sulfides from a pre-existing massive sulfide orebody, of which Black Duck may be a remnant. The Black Swan orebody, by contrast, does not show xenolithic features, but is characterised by an association of sulfide blebs with segregation vesicles, and by unusually coarse-grained olivine. The Black Swan orebody is interpreted as the result of transport of sulfide droplets within a lava charged with a suspended load of coarse olivine crystals.Editorial handling: Peter Lightfoot     

Relative contribution of magmatic and post-magmatic processes in the genesis of the Thompson Mine Ni-Co sulfide ores,Manitoba, Canada

The Ni-Co-(PGE) sulfide deposits of the Thompson Nickel Belt (TNB) in Northern Manitoba, Canada are part of the fifth largest nickel camp in the world based on contained nickel; past production from the TNB deposits is 2500 kt Ni. The Thompson Deposit is located on the eastern and southern flanks of the Thompson Dome structure, which is a re-folded nappe structure formed during collision of the Trans-Hudson Orogen with the Canadian Shield at 1.9–1.7 Ga. The Thompson Deposit is almost entirely hosted by P2 member sulfidic metasedimentary rocks of the Paleoproterozoic Ospwagan Group. Variably serpentinised and altered dunites, peridotites and pyroxenites contain disseminated sulfides and have a spatial association with sediment-hosted Ni sulfides which comprise the bulk of the ore types. These rocks formed from rift-related komatiitic magmas that were emplaced at 1.88 Ga, and subsequently deformed by boudinage, thinning, folding, and stacking.Disseminated sulfide mineralization in the large serpentinised peridotite and dunite intrusions that host the Birchtree and Pipe Ni-Co sulfide deposits typically has 4–6 wt% Ni in 100% sulfide. The disseminated sulfides in the less abundant and much smaller boudinaged serpentinised peridotite and dunite bodies associated with the Thompson Deposit have 7–10 wt% Ni in 100% sulfide. The majority of Thompson Mine sulfides are hosted in the P2 member of the Pipe Formation which is a sulfidic schist developed from a shale prololith; the mineralization in the schist includes both low Ni tenor (<1 wt% Ni in sulfide) and barren sulfide (<200 ppm Ni) and a Ni-enriched sulfide with 1–18 wt% Ni in 100% sulfide. The semi-massive and massive sulfide ores show a similar range in Ni tenor to the metasediment-hosted mineralization, but there are discrete populations with maximum Ni tenors of ∼8, 11 and 13 wt% Ni in 100% sulfide. The variations in Ni tenor are related to the Ni/Co ratio (high Ni/Co correlates with high Ni tenor sulfide) and this relationship is produced by the different Ni/Co ratios in sulfides with a range in proportions of pyrrhotite and pentlandite. Geological models of the ore deposit, host rocks, and sulfide geochemical data in three dimensions reveal that the Thompson Deposit forms an anastomosing domain on the south and east flanks of a first order D3 structure which is the Thompson Dome. In detail, a series of second order doubly-plunging folds on the eastern and southern flank control the geometry of the mineral zones. The position of these folds on the flank of the Thompson Dome is a response to the anisotropy of the host rocks during deformation; ultramafic boudins and layers of massive quartzite in ductile metasedimentary rocks control the geometry of the doubly-plunging F3 structures. The envelope of mineralization is almost entirely contained within the P2 member of the Pipe formation, so the deposit is clearly folded by the first order and second order D3 structures. The sulfides with highest Ni tenor (typically >13 wt% Ni in sulfide) define a systematic trend that mirrors the configuration of the second order doubly-plunging F3 structures on the flanks of the Dome. Although moderate to high Ni tenor mineralization is sometimes localized in fold hinges, more typically the highest Ni tenor mineralization is located on the flanks of the fold structures.There is no indication of the mineralogical and geochemical signatures of sedimentary exhalative or hydrothermal processes in the genesis of the Thompson ores. The primary origin of the mineralization is undoubtedly magmatic and this was a critical stage in the development of economic mineralization. Variations in metal tenor in disseminated sulfides contained in ultramafic rock indicate a higher magma/sulfide ratio in the Thompson parental magma relative to Birchtree and Pipe. The variation in Ni tenor of the semi-massive and massive sulfide broadly supports this conclusion, but the variations in metal tenor in the Thompson ores was likely created partly during deformation. The sequence of rocks was modified by burial and loading of the crust (D2 events) to a peak temperature of 750 °C and pressure of 7.5 kbar. The third major phase of deformation (D3) was a sinistral transpression (D3 event) which generated the dome and basin configuration of the TNB. These conditions allowed for progressive deformation and reformation of pyrrhotite and pentlandite into monosulfide solid solution as pressure and temperature increased; this process is termed sulfide kinesis. Separation of the ductile monosulfide solid solution from granular pentlandite would result in an effective separation of Ni during metamorphism, and the monosulfide solid solution would likely be spread out in the stratigraphy to form a broad halo around the main deposit to produce the low Ni tenor sulfide. Reformation of pentlandite and pyrrhotite after the peak D2 event would explain the broad footprint of the mineral system. The effect of the D3 event at lower pressure and temperature would have been to locally redistribute, deform, and repeat the lenses of sulfide.The understanding of the relationships between petrology, stratigraphy, structure, and geochemistry has assisted in formulating a predictive exploration model that has triggered new discoveries to the north and south of the mine, and provides a framework for understanding ore genesis in deformed terrains and the future exploration of the Thompson Nickel Belt.     

Formation conditions of some iron ores in the Uda-Shantar basin,Far East

The first results of the SEM study of banded iron ores of the Uda-Shantar basin in the Russian Far East are described. They show that the ores contain various microbial remains with Fe-Mn mineralization.     

Mineralogy and formation conditions of ores in the Bereznyakovskoe ore field,the Southern Urals,Russia

The Bereznyakovskoe ore field is situated in the Birgil’da-Tomino ore district of the East Ural volcanic zone. The ore field comprises several centers of hydrothermal mineralization, including the Central Bereznyakovskoe and Southeastern Bereznyakovskoe deposits, which are characterized in this paper. The disseminated and stringer-disseminated orebodies at these deposits are hosted in Upper Devonian-Lower Carboniferous dacitic-andesitic tuff and are accompanied by quartz-sericite hydrothermal alteration. Three ore stages are recognized: early ore (pyrite); main ore (telluride-base-metal, with enargite, fahlore-telluride, and gold telluride substages); and late ore (galena-sphalerite). The early and the main ore stages covered temperature intervals of 320–380 to 180°C and 280–300 to 170°C, respectively; the ore precipitated from fluids with a predominance of NaCl. The mineral zoning of the ore field is expressed in the following change of prevalent mineral assemblages from the Central Bereznyakovskoe deposit toward the Southeastern Bereznyakovskoe deposit: enargite, tennantite, native tellurium, tellurides, and selenides → tennantite-tetrahedrite, tellurides, and sulfoselenides (galenoclausthalite) → tetrahedrite, tellurides, native gold, galena, and sphalerite. The established trend of mineral assemblages was controlled by a decrease in $ f_{S_2 } $ f_{S_2 } , $ f_{Te_2 } $ f_{Te_2 } and $ f_{O_2 } $ f_{O_2 } and an increase in pH of mineral-forming fluids from early to late assemblages and from the Central Bereznyakovskoe deposit toward the Southeastern Bereznyakovskoe deposit. Thus, the Central Bereznyakovskoe deposit was located in the center of an epithermal high-sulfidation ore-forming system. As follows from widespread enargite and digenite, a high Au/Ag ratio, and Au-Cu specialization of this deposit, it is rather deeply eroded. The ore mineralization at the Southeastern Bereznyakovskoe deposit fits the intermediate- or low-sulfidation type and is distinguished by development of tennantite, a low Au/Ag ratio, and enrichment in base metals against a lowered copper content. In general, the Bereznyakovskoe ore field is a hydrothermal system with a wide spectrum of epithermal mineralization styles.     

Komatiites and nickel sulfide ores of the Black Swan area,Yilgarn Craton,Western Australia. 3: Komatiite geochemistry,and implications for ore forming processes

The Black Swan komatiite sequence is a package of dominantly olivine-rich cumulates with lesser volumes of spinifex textured rocks, interpreted as a section through an extensive komatiite lava flow field. The sequence hosts a number of nickel sulfide orebodies, including the Silver Swan massive shoot and the Cygnet and Black Swan disseminated orebodies. A large body of whole rock analyses on komatiitic rocks from the Black Swan area has been filtered for metasomatic effects. With the exception of mobile elements such as Ca and alkalis, most samples retain residual igneous geochemistry, and can be modelled predominantly by fractionation and accumulation of olivine. Whole rock MgO–FeO relationships imply a relatively restricted range of olivine compositions, more primitive than the olivine which would have been in equilibrium with the transporting komatiite lavas, and together with textural data indicate that much of the cumulus olivine in the sequence was transported. Flow top compositions show evidence for chromite saturation, but the cumulates are deficient in accumulated chromite. Chromite compositions are typical of those found in compound flow-facies komatiites, and are distinct from those in komatiitic dunite bodies. Incompatible trace element abundances show three superimposed influences: control by the relative proportion of olivine to liquid; a signature of crustal contamination and an overprint of metasomatic introduction of LREE, Zr and Th. This overprint is most evident in cumulates, and relatively insignificant in the spinifex rocks. Platinum and palladium behaved as incompatible elements and are negatively correlated with MgO. They show no evidence for wholesale depletion due to sulfide extraction, which was evidently restricted to specific lava tubes or pathways. The lack of correspondence between PGE depletion and contamination by siliceous material implies that contamination alone is insufficient to generate S-saturation and ore formation in the absence of sulfide in the assimilant. Contamination signatures in spinifex-textured rocks may be a guide to Ni-sulfide mineralisation, but are not entirely reliable in the absence of other evidence. The widespread vesicularity of the sequence may be attributable to assimilated water rather than to primary mantle-derived volatiles, and cannot be taken as evidence for primary volatile-rich magmas. The characteristic signature of the Black Swan Succession is the presence of highly localised disseminated sulfide within a sequence showing more widespread evidence for crustal contamination and interaction with its immediate substrate. This has important implications for the applicability of trace element geochemistry in exploration for komatiite-hosted nickel deposits.Electronic Supplementary Material Supplementary material is available in the online version of this article at Editorial handling: Peter Lightfoot