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.

LEAD ISO TOPIC DA 1í\ A ND TIIE META LOGENETI C EVOLU 7YON O FTII E ZIN C AND U i\D DEPOSrrS OF 'IH E NEO PROTEROZ OICCO VERS OF TlIE SÃ O FRA NCIASCO CRATON Pb isoto pe nnnlyscs lmvc bceu carried o ut on sulfidcs Irem diffcrcnt Ncoprotcro zoic scdim cn tnry rocks hosti ng Z n-Pb dcp osit s of lhe Sã o Fran cis co C raton, Brazil. Th e isotope data 01' 80 sulfide sam plcs fro m ti ve minernli zcd arcas yield high radio genic values (mea n 1lK'PbP J.l pb ratio >2 1) a nel show a rcm arkablc hom ogen eit y for so me de posit s. ln thc Pb-Pb isoto pe plol ali data points full ubovc lhe plumbotectonic mod el le ud isot o pc grow th c urve fo r un uppcr crus taI so urcc. Th e age valucs calc ukue d for lhe sourcc rock s in so me a rca s from sec ondary isochrons (1.7 to I.X Ga), are concordam with the radiometricall y dat ed age values fo r the highly rad ioac tivo bascm cn t roc ks of lhe São Fra ncisco Crat on. T his wo uld imply that lhe meta is for man y 01' the dcp osits uppear to have bcc n dcrived from thc bascmem rock s thcmsel vcs. Th e sp read in the iso tope da ta for the sulfides Irom ali de posit s cuu bc exp laiuc d o n thc basi s o f: (I) di verse na ture and com pos ition of bascmcm rock so urces in eac h deposi t. (2) nature 01" lhe rcnc tivity (ionic strength. pi !, e tc.) o f the m inera liz ing so lutions a nel (3) thc Icngth s of time or periods o f interuc tion o f the mineralizing fluid with the so urcc roc ks. Th e ore fonn ing episode dated by Pb iso topic mcrhod (-0.65 Ga) is probabl y associuted with ex tens ional evcnrs that took plucc during rhc c vo lutio n o f the Neoproterozoic bas ins in rhc Silo Fran c isc o Craton .

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