No abstract

Integrated studies carried out on the Proterozoic sediment-hosted Pb-Zn sulfide deposits of the São Francisco Craton, Brazil, allowed the estimation of (1) ages of the host sequences, (2) timing of mineralization, (3) possible sources of metal and sulfur, (4) temperature and salinity range of mineralizing solutions, (5) sources of fluids and (6) possible mechanisms of fluid flow. Contrary to the deposits from other Proterozoic basins, the Brazilian deposits do not contain world class metal reserves. This can probably be attributed to the lack of sufficient investments in exploration and mining development research activities. However, the sulfide deposits of the São Francisco Craton do share several important geological, structural, isotopic and fluid characteristics with some of the giant deposit types. Using these data we develop a more realistic metallogenic model for the Proterozoic sediment-hosted Pb-Zn sulfide deposits.

The Proterozoic sediment-hosted Zn-(Pb) sulfide and non-sulfide deposits of the São Francisco Craton, Brazil, are partially syn-diagenetic and epigenetic and were probably formed during extensional events. The majority of the deposits occur within shallow water dolomites. The Pb isotopic data of sulfides are relatively homogeneous for individual deposits and plot above the upper crust evolution curve of the Plumbotectonic model. Some of the deposits are characterized by highly radiogenic lead (206 Pb/ 204 Pb z 21) originating from the highly radioactive crust of the São Francisco Craton. Pb and S isotopic data suggest the sources of metal and sulfur for the deposits to be the basement rocks and seawater sulfates in the sediments, respectively. The relatively high temperatures of formation (100 to 250 8C) and moderate salinity (3% to 20% NaCl equiv.) of the primary fluid inclusions in the sphalerite crystals suggest the participation of basinal mineralizing fluids in ore formation. The steep paleo

The Morro Agudo is the only Zn-Pb mine in Brazil, with an ore reserve of 9,470,000 tonnes, at average grades of 6.13% Zn and 2.00% Pb. The host rocks are dolomites from the Morro do Calcário facies of the Vazante Formation, Bambuí Group. The ore is composed of sphalerite and galena. Gangue minerals include dolomite, quartz, barite and calcite. The Morro Agudo mineralization is closely associated with a N-S trending fault zone, which dips from 20 to 70 degrees towards the west. The orebodies are stratabound and sometimes displaced by late faults. Shallow-water sedimentary structures such as teepee, laminated stromatolites and microcrystalline quartz nodules (length-slow type) suggest an evaporitic paleoenvironment. The structural and textural nature of the mineralization, as well as some replacement features of the ore, indicate that the main mineralization stage occurred in the early stages of diagenesis. Fluid inclusion (FI) studies of primary and pseudosecondary FI in sphalerite crystals indicated moderate saline solutions (around 14%wt Eq. NaCl). The wide distribution of salinity values within the orebodies could be related to the higher salt concentration in the fluid near the fault zone. Total homogenization temperature and salinity data indicate a zoning pattern closely related to the main fault zone. Paleotemperatures are relatively higher in the 'A' Block, closer to the main fault zone. Away from the fault, temperatures reduced gradually. The stratiform ore of the 'N' Body exhibits the lowest temperature interval.