GOLD AND PGE IN MASSIVE SULFIDE ORE OF THE UZELGINSK DEPOSIT, SOUTHERN URALS, RUSSIA

Abstract

The Uzelginsk deposit in the southern Urals, in Russia, is an example of a weakly altered classical massive sulfide deposit of the Uralian type. Pyritic Cu-Zn ore contains gold in two main modes: so-called "invisible" gold in sulfides, and mineral forms represented by native gold and tellurides. Gold and PGE concentrations in sulfides from the Uzelginsk deposit have been determined by instrumental neutron-activation analysis (INAA) and wet-chemical analysis with chromatographic and ICP-MS determinations in parallel. Most of the gold occurs invisibly in pyrite and in chalcopyrite, and ranges from 0.7 to 20 ppm and from 1 to 18 ppm, respectively. Sphalerite bearing fine-grained emulsion-like inclusions of chalcopyrite contains 1.8 to 10.6 ppm Au. The gold content of pure grains of sphalerite, as detected by INAA, is between 0.3 and 3 ppm. Tennantite is the main host of Ag; low-iron tennantite (0.1-0.6 wt.% Ag) is the dominant variety, whereas Ag-bearing tennantite (up to 8.4 wt.% Ag) is scarce. Considerable PGE enrichment (up to 354 ppb Pt, 1220 ppb Pd and 707 ppb Rh) was determined in heavy-mineral concentrates from Au-rich mineralization above the quite low levels of PGE background in the ore. Metamorphism of sulfide ores resulted in the redistribution of non-ferrous and precious metals and the formation of submicrometric inclusions of Au-Ag alloy and Au compounds with Te, Se, S, As, Sb and Hg. Some PGE are hosted by pyrite and may be sent to the tailings during processing; the remainder was deposited front a hydrothermal fluid during metamorphism and enriched the products of the latest paragenesis. Sulfide precipitation is shown to have occurred under conditions favorable for the incorporation of gold in solid solution in these minerals at high temperature (250-380degreesC). The paragenesis of native metals (gold, Silver, tellurium and rhenium), tellurides and sulfosalts was formed at the latest stage of the hydrothermal processes, at 165-190degreesC.