Physicochemical model for the genesis of Cu-Ag-Au-Hg solid solutions and intermetallics in the rodingites of the Zolotaya Gora gold deposit (Urals, Russia)

Valery V. Murzin, Konstantin V. Chudnenko, Galina A. Palyanova, Dmitry A. Varlamov, Evgeny A. Naumov, Franco Pirajno

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Abstract

In this contribution we examine the compositions of solid solutions and intermetallics of the system Cu-Ag-Au-Hg and the physicochemical conditions of their formation in rodingites from the Zolotaya Gora gold deposit (Southern Urals, Russia). Thermodynamic calculations, modeling the formation of mineral assemblages of rodingite and Cu-Ag-Au-Hg mineralization, were carried out using a “Selektor-C” software package. Two probable models for the genesis of Au-Ag-Cu-Hg solid solutions and Au-Cu intermetallics in rodingites are: 1) hydrothermal; the result of single-stage discharge in open space of deep-sourced gold-bearing fluid with the composition corresponding to rodingite, taking into account its interaction with host serpentinites. 2) metasomatic; deep-seated gold-bearing fluid (W) rising to the surface interacts with early formed rodingite (R) at different ratios (W/R). T and P-conditions of modeling: 450 °C, 3 kbar; 350 °C, 2 kbar; 250 °C, 1 kbar. Results of the calculations on the “hydrothermal” and “metasomatic” models showed different degrees of similarity of natural and theoretical model associations of rodingites. The metasomatic model is better for corresponding to real mineral compositions and mineral paragenesis in the natural Cu-Ag-Au-Hg system observed at the deposit. In this model the chlorite-garnet-pyroxene rodingite is replaced by a chlorite-rich rock with increasing W/R. In this case all gold minerals of Zolotaya Gora deposit (Au-Cu intermetallics and Au-Ag solid solutions) are formed at 250–450 °C. Gold-copper solid solution formed at a temperature of 450 °C (W/R >10). Au-Ag-Hg solid solutions and native copper are formed only at 250 °C. According to the hydrothermal model native copper and AuCu3 were absent phases, but other Au-Cu intermetallics (AuCu, Au3Cu) precipitate if gold concentration in the solution is higher than 0.5 ppm. Thermodynamic calculations proved the possibility of formation of equilibrium assemblages of rodingite minerals and gold-bearing minerals with the participation of water-chloride complexing and low CO2 fluids. At a temperature <350 °C the main status of gold in solution are Au(HS)2 and AuHS0, while at higher temperatures it occurs as AuOH0. Formation of Au-Cu intermetallics occurred under the effect of weak-acid hydrothermal solutions (pH = 3.5 ÷ 5) with low fugacity of O2 and S2: log fO2 = −26 ÷ −47, log fS2 = −8 ÷ −20. Both models (hydrothermal and metasomatic) explain the formation of Au-bearing rodingites and can be used for predicting potential gold-bearing rodingite targets.

Original languageEnglish
Pages (from-to)81-97
Number of pages17
JournalOre Geology Reviews
Volume93
DOIs
Publication statusPublished - 1 Feb 2018

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