Fluid inclusion investigation of the neoproterozoic lead-zinc sulfide deposit of nova redenção, Bahia, Brazil

Abstract

The Pb-Zn (Ag) mineralizations of Nova Redenção, Una-Utinga Basin, are hosted by dolarenites, dolomicrites and siliceousferruginous breccias of the Salitre Formation, Una Group. An ore reserve of about 2.5 Mt, with average grade of 6,1% Pb, 0,5% Zn, 32 ppm Ag and 10 ppm of Cd has been estimated by Companhia de Pesquisa de Recursos Minerais (CPRM) in the main sector of the deposit. The mineralization is composed mainly of galena and sphalerite, which occur in different ways (disseminated, stratabound, oolith and brecciacement, filling stilolytes/pockets, and veins). The orebodies are directly associated with N50°W trending regional fractures and faults, which are thought to represent deep basement structures reactivated during and after the Neoproterozoic carbonate basin sedimentation. Relationships between sulfides and the shallow sedimentation facies are striking. Several evidences of early-formed sulfates have been observed, including length-slow or lutecite-type microcrystalline quartz, aggregates of quartz with gypsum structure, microcrystalline pseudo-fibrous silica nodules, indicating both a lithostratigraphic and a sedimentary facies control. The study of primary fluid inclusions in sphalerites contemporaneous to galena showed the presence of moderate to high (around 24.3wt% eq. NaCl) aqueous solutions, containing Ca, K and Mg salts. The homogenization temperatures (TH) have a mean value of 185°C. Previous sulfur isotopic studies (d34S) (Misi et al. 1999) in cogenetic galenasphalerite pairs also indicated temperatures in the range 140°-180°C. Sulfur isotopic data in sulfides and sulfates suggested that sulfur was derived from seawater, possibly by thermochemical reduction (Misi et al. 1999). The characteristics of the mineralization along with the obtained fluid inclusion and isotopic data permit to suggest the following met allogenic model for the Nova Redenfao deposit: high concentration of met al brines have migrated throughout deep reactivated structures of the basement, forming sulfides by thermochemical reduction of sulfur present in the basin. The stratigraphic and sedimentary facies control is indicated by the presence of sulfates formed probably by sea-water evaporation in shallow zones of sedimentation.