ORIGIN OF Au-Ag MINERALIZATION IN SPHALERITE ORES FROM THE DATA ON CO-CRYSTALLIZATION OF SPHALERITE WITH Ag AND Au IN MODEL HYDROTHERMAL SYSTEMS
V.L.Tauson, S.V.Lipko, D.N.Babkin, N.V.Smagunov, O.Yu.Belozerova, E.V.Kaneva
A.P. Vinogradov Institute of Geochemistry, Siberian Branch of the Russian Academy of Sciences, Irkutsk,В Russia
Keywords: experiment, silver, gold, tin, sphalerite, hydrothermal systems
Abstract
Au-Ag mineralization occurrences in sphalerite ores of hydrothermal genesis are paradoxical in view of the incompatibility of these elements in sphalerite. The formation of sphalerite with Au and Ag impurities under hydrothermal crystallization of ZnS at 450 oC and 1kbar pressure was studied experimentally. Sn impurity was taken as a source of point defects in crystals modelling the interaction of Au and Ag with vacancies. The Ag solubility in low-Fe sphalerite is estimated as 3.8±0.7 µg/g, Au ̶ ≤0.6 µg/g. The main forms of Ag and Au occurrence in sphalerite are the inclusions of (Ag,Au)xS phases with x varies mainly from 1.8 to 2, and Au varies from 0.01 to 0.75 a.p.f.u. The primary forms of the elements in ores might be the microinclusions (Ag,Au)1.8-2.1S or close to (Ag,Au)S at higher fS2. In presence of Sn, the solubilities of Au and Ag become higher. The Au behavior corresponds to the substitution reaction Sn4+ + Au+ + v‒ ↔ 2 Zn2+ under the presence of two types of vacancy defects (v-) ‒ the “inherent” vacancies dependent on the crystallization conditions and the vacancies accompanying Sn4+
incorporation. Ag entrance is seemingly more dependent on fS2 conditions and does not correlate with Sn. The extra vacancies arise because of metastable crystallization under the conditions of oversaturation of growth medium. This is supported by the spherulite morphology of growth products and the admixture of wurtzite ZnS form. The elevation of distribution and
cocrystallization coefficients are observed for both noble metals (NM), upon that Au changes from incompatible to the category of highly compatible elements in sphalerite. The geochemical environments favorable for the formation of imperfect mineral crystals are considered. Such crystals are capable to uptake NM and other incompatible in “ideal” crystal elements because of their interaction with vacancies, both constitutional (inherent to the substance) and non-equilibrium defects, and surficial nano-sized formations (nonautonomous
phases). The evolution of these initially “invisible” forms of NM under metamorphic processes and remobilization of ore substance may result in Au and Ag escape and aggregation into microparticles.
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