Chromite-PGM Mineralization in the Lherzolite Mantle Tectonite of the Kraka Ophiolite Complex (Southern Urals, Russia)

The mantle tectonite of the Kraka ophiolite contains several chromite deposits. Two of them consisting of high-Cr podiform chromitite—the Bolshoi Bashart located within harzburgite of the upper mantle transition zone and Prospect 33 located in the deep lherzolitic mantle—have been investigated. Both deposits are enveloped in dunite, and were formed by reaction between the mantle protolith and high-Mg, anhydrous magma, enriched in Al2O3, TiO2, and Na2O compared with boninite. The PGE mineralization is very poor (<100 ppb) in both deposits. Laurite (RuS2) is the most common PGM inclusion in chromite, although it is accompanied by erlichmanite (OsS2) and (Ir,Ni) sulfides in Prospect 33. Precipitation of PGM occurred at sulfur fugacity and temperatures of logƒS2 = (−3.0), 1300–1100 °C in Bolshoi Bashart, and logƒS2 = (−3.0/+1.0), 1100–800 °C in Prospect 33, respectively. The paucity of chromite-PGM mineralization compared with giant chromite deposits in the mantle tectonite in supra-subduction zones (SSZ) of the Urals (Ray-Iz, Kempirsai) is ascribed to the peculiar petrologic nature (low depleted lherzolite) and geodynamic setting (rifted continental margin?) of the Kraka ophiolite, which did not enable drainage of the upper mantle with a large volume of mafic magma.

Origin of the Pd-Rich Pentlandite in the Massive Sulfide Ores of the Talnakh Deposit, Norilsk Region, Russia

Pd-rich pentlandite (PdPn) along with ore-forming pentlandite (Pn) occurs in the cubanite and chalcopyrite massive sulfide ores in the EM-7 well of the Southern-2 ore body of the Talnakh deposit. PdPn forms groups of small grains and comprises marginal areas in large crystals of Pn. The palladium content in PdPn reaches up to 11.26 wt.%. EDS elemental mapping and a contour map of palladium concentrations indicate distinct variations in the palladium content within and between individual grains. Palladium distribution in the large grains is uneven and non-zoned. PdPn was formed as the result of a superimposed process, which is not associated with either the sulfide liquid crystallization or the subsolidus transformations of sulfides. Deming regression calculations demonstrated the isomorphic substitution character of Ni by 0.71 Pd and 0.30 Fe (apfu), leading to PdPn occurrence. The replacement of Ni by Fe may also indicate a change in sulfur fugacity, compared to that taking place during the crystallization of the primary Pn. The transformation of Pn into PdPn could have occurred under the influence of a Pd-bearing fluid, which separated from the crystallizing body of the massive sulfide ores.

Distribution of sulfides and PGE minerals in the picritic and taxitic gabbro-dolerites of the Norilsk 1 intrusion

ABSTRACT Disseminated ores in the Norilsk 1 intrusion were studied to elucidate the typomorphic features of sulfides and noble metal mineralizations in picritic and taxitic (or lower olivine) gabbro-dolerites. The former are characterized by the development of a low-sulfur sulfide association (troilite, Fe-rich pentlandite, talnakhite, chalcocite, native copper) while the latter exhibits a high-sulfur association (monoclinic pyrrhotite, Ni-rich pentlandite, pyrite, heazlewoodite). The contact between these types of rocks is geochemically and mineralogically contrasting. The mineralogical and geochemical zoning directed from the roof to the base of each layer is expressed by an increase in the Cu content (and chalcopyrite) in ores, an increase in the concentration of Ni in pentlandite and S in pyrrhotite in line with a decrease of the crystallization temperature, and an increase in sulfur fugacity in the same direction. Zoning of Pd(Pt) mineralization in picritic and taxitic (olivine) gabbro-dolerites is uniform and characterized by the distribution of Pd-Sn compounds in the upper parts (together with Pd-Pb minerals in picritic rocks) and Pd-As compounds in the lower parts of the sections according to a drop in temperature. Such reverse zoning contradicts the typical mechanism of differentiation by fractional crystallization, and possibly suggests a fluid-magmatic nature. Mineralogical and geochemical features in platinum group element-Cu-Ni-bearing rocks are consistent with the idea that different stages of multi-pulse intrusions of mafic-ultramafic magmas with different compositions formed the picritic and taxitic gabbro-dolerites of the Norilsk region.

Coexisting Tetrahedrite–(Zn) and Sphalerite at the Teremki Gold-Ore Deposit (East Transbaikalia): Chemical Composition and Formation Conditions

Paragenetic associations of tetrahedrite-(Zn) and sphalerite are distinguished in the Teremki gold-ore deposit. The chemical composition of coexisting minerals of this association is determined. The Sb/(Sb + As) and Fe/(Fe + Zn) ratios in tetrahedrite-(Zn) vary from 0.66 to 0.97 and from 0.28 to 0.40, respectively. A negative correlation was established between Sb/(Sb + As) and Fe/(Fe + Zn) ratios. Contents of Fe in sphalerite change from 0.88 to 1.43 wt % (1.5–2.5 mol % FeS). Temperature and sulfur fugacity when precipitation of tetrahedrite-(Zn)–paragenesis were estimated: they range from 130 to 280°C and from 10–13.2 to 10–8.1 bars, respectively.

PGE-Cu-Ni Mineralization of Mafic-Ultramafic Massifs of the Khangai Upland, Western Mongolia

The mafic-ultramafic massifs with the PGE-Cu-Ni mineralization located in North-Central Mongolia: Oortsog, Dulaan, Mankhan, Yamat, and Nomgon were investigated. For the first time we consider these massifs as a single magmatic association and as fragments of Khangai batholith caused by the action of the plume responsible for the formation Permian Khangai LIP. The massifs fractionated from peridotite to gabbro have a similar typomorphic ore mineralogical and geochemical features, which change depending on the degrees of fractionation of magma and evolution of the sulfide melt. The least fractionated Oortsog massif originated from Ni-rich high-Mg basaltic magma. It is characterized by predominance of pyrrhotite mineralization due to exsolution of monosulfide solid solution (MSS). The most fractionated is the Nomgon massif originated from Cu-rich basaltic magma with bornite-chalcopyrite mineralization, formed as an exsolution of intermediate solid solution (ISS). The rest of the massifs have a medium characteristics between these two. The compositions of sulfides in the studied massifs change in accordance with the increase in sulfur fugacity from peridotite to gabbro: enrichment of pentlandite in Ni and pyrrhotite in S. The composition of PGM changes from Pt minerals in Oortsog massif to Pd minerals in Nomgon massif in the same direction. These massifs can be considered as potential for the PGE.

Geochemical Features and Mineral Associations of Differentiated Rocks of the Norilsk 1 Intrusion

The purpose of this study is to show the patterns of distribution of disseminated sulfide in layered rocks based on the numerous geochemical and mineralogical data obtained for eight boreholes of the Norilsk intrusion (southern part of the Norilsk 1 deposit). There is a common trend of sulfide liquid fractionation in the Main Ore Horizon, which is composed of picritic and taxite (or olivine) gabbro-dolerites: the Ni/Cu in both rock types decreases down all sections, indicating an increase in the degree of fractionation of the sulfide liquid from top to bottom. On the contrary, the Ni/Fe ratios in pentlandite increase in this direction due to an increase in sulfur fugacity. However, picrite and taxite/olivine gabbro-dolerites are very distinctly separated by Ni/Cu values: these values are >1 in picritic gabbro-dolerite while they are always <1 in taxite/olivine gabbro-dolerite. These rock types are distinguished by sulfide assemblages. The first includes troilite, Fe-rich pentlandite, chalcopyrite, cubanite, talnahite, bornite and copper (low sulfur association); the second one is composed of monoclinic pyrrhotite, chalcopyrite, Ni-rich pentlandite and pyrite (high sulfur association). A two-stage magma injection with different ore specializations is supposed for picritic and taxite/olivine gabbro-dolerites.

Zoned Laurite from the Merensky Reef, Bushveld Complex, South Africa: “Hydrothermal” in Origin?

Laurite, ideally (Ru,Os)S2, is a common accessory mineral in podiform and stratiform chromitites and, to a lesser extent, it also occurs in placer deposits and is associated with Ni-Cu magmatic sulfides. In this paper, we report on the occurrence of zoned laurite found in the Merensky Reef of the Bushveld layered intrusion, South Africa. The zoned laurite forms relatively large crystals of up to more than 100 µm, and occurs in contact between serpentine and sulfides, such as pyrrhotite, chalcopyrite, and pentlandite, that contain small phases containing Pb and Cl. Some zoned crystals of laurite show a slight enrichment in Os in the rim, as typical of laurite that crystallized at magmatic stage, under decreasing temperature and increasing sulfur fugacity, in a thermal range of about 1300–1000 °C. However, most of the laurite from the Merensky Reef are characterized by an unusual zoning that involves local enrichment of As, Pt, Ir, and Fe. Comparison in terms of Ru-Os-Ir of the Merensky Reef zoned laurite with those found in the layered chromitites of the Bushveld and podiform chromitites reveals that they are enriched in Ir. The Merensky Reef zoned laurite also contain high amount of As (up to 9.72 wt%), Pt (up to 9.72 wt%) and Fe (up to 14.19 wt%). On the basis of its textural position, composition, and zoning, we can suggest that the zoned laurite of the Merensky Reef is “hydrothermal” in origin, having crystallized in the presence of a Cl- and As-rich hydrous solution, at temperatures much lower than those typical of the precipitation of magmatic laurite. Although, it remains to be seen whether the “hydrothermal” laurite precipitated directly from the hydrothermal fluid, or it represents the alteration product of a pre-existing laurite reacting with the hydrothermal solution.

Podiform Chromitites and PGE Mineralization in the Ulan-Sar’dag Ophiolite (East Sayan, Russia)

In this paper, we present the first detailed study on the chromitites and platinum-group element mineralization (PGM) of the Ulan-Sar’dag ophiolite (USO), located in the Central Asian Fold Belt (East Sayan). Three groups of chrome spinels, differing in their chemical features and physical–chemical parameters, under equilibrium conditions of the mantle mineral association, have been distinguished. The temperature and log oxygen fugacity values are, for the chrome spinels I, from 820 to 920 °C and from (−0.7) to (−1.5); for chrome spinels II, 891 to 1003 °C and (−1.1) to (−4.4); and for chrome spinels III, 738 to 846 °C and (−1.1) to (−4.4), respectively. Chrome spinels I were formed through the interaction of peridotites with mid-ocean ridge basalt (MORB)-type melts, and chrome spinels II were formed through the interaction of peridotites with boninite melts. Chrome spinels III were probably formed through the interaction of andesitic melts with rocks of an overlying mantle wedge. Chromitites demonstrate the fractionated form of the distribution of the platinum-group elements (PGE), which indicates a high degree of partial melting at 20–24% of the mantle source. Two assemblages of PGM have been distinguished: The primary PGE assemblage of Os-Ir-Ru alloys-I, (Os,Ru)S2, and IrAsS, and the secondary PGM assemblage of Os-Ir-Ru alloys-II, Os0, Ru0, RuS2, OsS2, IrAsS, RhNiAs with Ni, Fe, and Cu sulfides. The formation of the secondary phases of PGE occurred upon exposure to a reduced fluid, with a temperature range of 300–700 °C, log sulfur fugacity of (−20), and pressure of 0.5 kbar. We have proposed a scheme for the sequence of the formation and transformation of the PGMs at various stages of the evolution of the Ulan-Sar’dag ophiolite.

“Invisible” gold in synthetic and natural arsenopyrite crystals (Vorontsovka deposit, Northern Urals)

In many types of hydrothermal ore deposits Au occurs in invisible state in most common minerals of the Fe-As-S system. It is supposed that the state of theinvisible Au may beeither non-structural (nano-sized inclusions of metal and its compounds) or chemically bound (isomorphous solid solution). Here we report results of investigation of the state and the concentration range ofinvisible Au in synthetic and natural arsenopyrites FeAsS (Vorontsovka deposit, North Urals, type Carlin). Conditions that favor the formation of Au-bearing arsenopyrite were identified. The synthesis experiments were carried out in Au-saturated system by means of salt flux method with a stationary temperature gradient. The temperature at the cold end of the ampole was 400500С. The chemical composition of arsenopyrite was determined by electron probe microanalysis. The composition of the synthesized arsenopyrite varied within [at.%]: Fe from 32.6 to34.4, As from 30.0 to 36.5, S from 29.4 to36.0. The Au content in arsenopyrite varied from the detection limit ( 45ppm) to 3wt.%. A strong negative correlation between the concentrations of Au and Fe was observed in the synthesized arsenopyrite grains. The slope of the correlation lines corresponds to the formation of the Au-bearing solid solution where Au isomorphically substitutes for Fe at the parameters of the synthesis experiments. In addition, there is a weaker positive correlation between Au and As: higher Au concentrations are characteristic of arsenic-rich compositions (As/S [at.%] 1) and those close to stoichiometric arsenopyrite, whereas in sulfur-rich arsenopyrite the concentration of Au is lower and does not exceed 0.25wt.%. The positive Au-As correlation appears not only on a local level within a single crystal of synthetic and natural arsenopyrite, but is valid on the Vorontsovka deposit scale: As-rich arsenopyrite formed at lower temperature and sulfur fugacity (t= 250370C, logfS2= 1217) contains more Au than the As-poor early arsenopyrite (t= 270400C, logfS2= 79). Comparison of these results with the literature data shows that the positive correlation between the concentrations of Au and As in arsenopyrite and the negative correlation between the concentrations of Au and Fe are the common features of ores of the Carlin-type deposits. We suggest that, in contrast to negative correlation Au-Fe, the positive correlation Au-As cannot be explained in terms of crystal chemistry, but can result from the effect of external factors among which are the difference in composition of ore-forming hydrothermal fluids and the sulfur fugacity.

The Gersdorffite-Bismuthinite-Native Gold Association and the Skarn-Porphyry Mineralization in the Kamariza Mining District, Lavrion, Greece

Vein-type Pb-Ni-Bi-Au-Ag mineralization at the Clemence deposit in the Kamariza and “km3” in the Lavrion area, was synchronous with the intrusion of a Miocene granodiorite body and related felsic and mafic dikes and sills within marbles and schists in the footwall of (and within) the Western Cycladic detachment system. In the Serpieri deposit (Kamariza area), a porphyry-style pyrrhotite-arsenopyrite mineralized microgranitic dike is genetically related to a garnet-wollastonite bearing skarn characterized by a similar base metal and Ni (up to 219 ppm) enrichment. The Ni–Bi–Au association in the Clemence deposit consists of initial deposition of pyrite and arsenopyrite followed by an intergrowth of native gold-bismuthinite and oscillatory zoned gersdorffite. The zoning is related to variable As, Ni, and Fe contents, indicating fluctuations of arsenic and sulfur fugacity in the hydrothermal fluid. A late evolution towards higher sulfur fugacity in the mineralization is evident by the deposition of chalcopyrite, tennantite, enargite, and galena rimming gersdorffite. At the “km3” locality, Ni sulfides and sulfarsenides, vaesite, millerite, ullmannite, and polydymite, are enclosed in gersdorffite and/or galena. The gersdorffite is homogenous and contains less Fe (up to 2 wt.%) than that from the Clemence deposit (up to 9 wt.%). Bulk ore analyses of the Clemence ore reveal Au and Ag grades both exceeding 100 g/t, Pb and Zn > 1 wt.%, Ni up to 9700 ppm, Co up to 118 ppm, Sn > 100 ppm, and Bi > 2000 ppm. The “km3” mineralization is enriched in Mo (up to 36 ppm), Ni (>1 wt.%), and Co (up to 1290 ppm). Our data further support a magmatic contribution to the ore-forming fluids, although remobilization and leaching of metals from previous mineralization and/or host rocks, through the late involvement of non-magmatic fluid in the ore system, cannot be excluded.