AU/AG RATIO VARIATIONS AT MINA III, MINA NOVA AND MINA INGLESA GOLD DEPOSITS, CRIXÁS GREENSTONE BELT, BRAZIL

Abstract

The Mina HI and Mina Nova gold deposits occur at the transition between metabasic volcanic rocks and chemical and detrital metasedimentary rocks, while Mina Inglesa occurs within metakomatiites, all in the Archean Crixas Greenstone Belt. Mina III contains three main mineralized zones: the Upper (UOZ-ms), Lower (LOZ-qv and LOZ-cs) and Garnet (GOZ) Ore Zones. The UOZ-ms is composed of massive sulfide lenses within Fe-dolomitic marbles and Fe and Al-rich rocks. A concordant quartz vein within carbonaceous schists represents the LOZ- qv, while the LOZ-cs consists of an arsenopyrite-pyrrhotite-bearing carbonaceous schist close to the quartz-vein contact. The GOZ comprises concordant quartz veins within quartz-chlorite-muscovite-garnet schist. At Mina Nova, two main ore zones occur: the Orebody 1 (OB1-cs), consisting of an arsenopyrite-pyrrhotite-bearing carbonaceous schist and the Orebody 2 (OB2-cm), represented by a carbonate-muscovite schist lenses within carbonaceous schist). Mina Inglesa has one mineralized concordant quartz vein (MOB) within talc schists. Mineralogical characterization of the orebodies indicates gold association with arsenopyrite, pyrrhotite, chalcopyrite, carbonate, quartz, micas and oxides at the UOZ; with quartz, carbonaceous matter and arsenopyrite, and less frequently with carbonate, pyrrhotite and micas at the (LOZ-qv and LOZ- cs); mainly with quartz, micas and arsenopyrite, and more rarely with chalcopyrite and oxides at the GOZ; with arsenopyrite, pyrrhotite and carbonate at the OB1 ; with carbonate, muscovite and arsenopyrite at the OB2; and with quartz and galena at the MOB. Microprobe analysisin individual gold grains of all ore zones indicate very low values of other elements (less than 0.5 wt. %). The identified Au/Ag ratio variations are intimately associated with host minerals or mineral associations of each ore zone. Initial Au/Ag ratios in source-rocks, transport (hydrosulfide and chloride complexes) and deposition mechanisms (fluid-rock interactions and fluid immiscibility) may have had great importance in the observed chemical variations.