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The Gaching high-sulfidation epithermal deposit in the Maletoyvayam ore field features a wide range of Se-containing minerals and selenides, as well as complex gold oxides, Au tellurides (calaverite, krennerite) and native gold typical for epithermal deposits. Pyrite included in quartzites and quartz-alunite rocks was probably formed during an early stage of the ore-forming process. During the following Au-rich stage, the $f_{{\rm S}{\rm e}_{\rm 2}}$ / $f_{{\rm S}_{\rm 2}}$ increased with $f_{{\rm O}_{\rm 2}}$ being relatively high, resulting in the formation of very rare compounds that have not been previously described in nature. These include Au2Te4(Se,S)3, Se3Te2, AuSe and Au(Te,Se,S) phases. The Au2Te4(Se,S)3 compounds have some variations in composition: the complete isomorphic series between Au2Te4Se3 and Au2Te4S3 was observed. The gold and Au-minerals at the main ore stage can be stable within a range of log $f_{{\rm O}_{\rm 2}}$ of −27.3 and atmospheric oxygen (?); log $f_{{\rm S}{\rm e}_{\rm 2}}$ between −12.4 and −5.7; log $f_{{\rm T}{\rm e}_{\rm 2}}$ between −10.5 and −7.8; and log $f_{{\rm S}_{\rm 2}}$ between −12.8 and −6.8 (at 250°C). The increasing oxygen fugacity during the final stage of mineralization resulted in the formation of complex Sb,As,Te,S-bearing Au oxides. Gold-oxide formation occurs due to oxidation of Au-tellurides. The final products of this process are newly-formed secondary mustard gold and Te–Se solid solutions. [ABSTRACT FROM AUTHOR]

The formation conditions of the parental magmas of Gorely volcano, which is located behind a volcanic front in Southern Kamchatka, have been evaluated using the modern methods of micro-element thermobarometry. These magmas contained 1.7 ± 0.8 (2σ) wt % of HO, the majority (82%) of which has been lost from inclusions. They crystallized at 1121 ± 17°C and an oxygen fugacity of ΔQFM 1.2 ± 0.2, and could have been produced by about 11% melting of an enriched MORB source (E-DMM) at a temperature of about 1270°C, and a pressure of about 1.5 GPa. A distinctive feature of Gorely volcano, compared with frontal volcanoes of Kamchatka, is the unusually high temperature (925 ± 20°C) of formation of the subduction component corresponding to the region of existence of water-bearing melts. [ABSTRACT FROM AUTHOR]

The article presents a study which aims to determine the conditions of formation of Os-bearing platinum group minerals (PGM) in oceanic and subcontinental mantle at the Middle Urals, Russia. Methods used to examine the intrinsic oxygen fugacity of Ru-Os-Ir alloys from the Verkh-Neivinsky Dunite and morphology and chemical composition of PGE mineralization are offered. Results reveal the similarity of redox conditions of the formation of Ru-Os-Ir alloys formed in oceanic and continental mantle.

Abstract: Oxygen fugacity (fO2) and temperature variations in a complete lithospheric mantle section (70–220km) of the central Siberian craton are estimated based on 42 peridotite xenoliths in the Udachnaya kimberlite. Pressure and temperature (P–T) estimates for the 70–140km depth range closely follow the 40mW/m2 model conductive geotherm but show a bimodal distribution at greater depths. A subset of coarse garnet peridotites at 145–180km plots near the “cold” 35mW/m2 geotherm whereas the majority of coarse and sheared rocks at ≥145km scatter between the 40 and 45mW/m2 geotherms. This P–T profile may reflect a perturbation of an initially “cold” lithospheric mantle through a combination of (1) magmatic under-plating close to the crust–mantle boundary and (2) intrusion of melts/fluids in the lower lithosphere accompanied by shearing. fO2 values estimated from Fe3+/∑Fe in spinel and/or garnet obtained by Mössbauer spectroscopy decrease from +1 to −4 Δlog fO2 (FMQ) from the top to the bottom of the lithospheric mantle (∼0.25log units per 10km) due to pressure effects on Fe2+–Fe3+ equilibria in garnet. Garnet peridotites from Udachnaya appear to be more oxidized than those from the Kaapvaal craton but show fO2 distribution with depth similar to those in the Slave craton. Published fO2 estimates for Udachnaya xenoliths based on C–O–H fluid speciation in inclusions in minerals from gas chromatography are similar to our results at ≤120km, but are 1–2 orders of magnitude higher for the deeper mantle, possibly due to uncertainties of fO2 estimates based on experimental calibrations at ≤3.5GPa. Sheared peridotites containing garnets with u-shaped, sinusoidal and humped REE patterns are usually more oxidized than Yb, Lu-rich, melt-equilibrated garnets, which show a continuous decrease from heavy to light REE. This further indicates that mantle redox state may be related to sources and modes of metasomatism. [Copyright &y& Elsevier]

The article focuses on a study which examined the redox conditions of osmium-rich minerals formation from the Guli Massif, Russia. It notes on the intrinsic oxygen fugacity of native osmium and iridium osmium from dunite and chromatite. The factors that influenced the regular difference in a chemical composition of platinum group element (PGE) are noted.

The Zaonega Formation in northwest Russia (~2.0 billion years old) is amongst the most complete successions that record the middle of the Palaeoproterozoic era. As such, geochemical data from the formation have played a central role in framing the debate over redox dynamics in the aftermath of the Great Oxidation Event (GOE). However, uncertainty over local redox conditions and the degree of hydrographic restriction in the formation has led to contradictory interpretations regarding global oxygen (O2) fugacity. Here, we provide new iron (Fe) isotope data together with major and trace element concentrations to constrain the local physiochemical conditions. The Zaonega Formation sediments show authigenic Fe accumulation (Fe/Al ≫ 1 wt.%/wt.%) and δ56Fe ranging from −0.58‰ to +0.60‰. Many of the data fall on a negative Fe/Al versus δ56Fe trend, diagnostic of a benthic Fe shuttle, which implies that Zaonega Formation rocks formed in a redox-stratified and semi-restricted basin. However, basin restriction did not coincide with diminished trace metal enrichment, likely due to episodes of deep-water exchange with metal-rich oxygenated seawater, as evidenced by simultaneous authigenic Fe(III) precipitation. If so, the Onega Basin maintained a connection that allowed its sediments to record signals of global ocean chemistry despite significant basinal effects. [ABSTRACT FROM AUTHOR]