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LEAD ISOTOP E EVIDENCE REGARDING AGE OF GOLD DEPOSITS IN THE NOVA LIMA DISTRICT, MINAS GERAIS, BRAZll

1984; Sociedade Brasileira de Geologia; Volume: 14; Issue: 3 Linguagem: Inglês

10.25249/0375-7536.1984147152

2177-4382

R. I. Thorpe, G. L. Cumming, D. Krstic,

Geochemistry and Elemental Analysis

Lead isotope data for trace galenas associated with five generally stratíform gold deposíts in the vicinity of the Morro Velho mine confirm an Archean age for the host Nova Lima Group. The least radíogenic lead, from the Bela Fama deposit, yields a model age of 2 710 Ma, possibly near the true age for the host rocks, including the various facies of iron formation and related chemical sediments that constítute and are associated with the primary ore zones. Galenas from the Esperança III and Bicalho deposits give model ages of about 2,550 Ma and 2,575 Ma, another from Bicalho a model age of about 2,495 Ma, and those from the Cuiabá and Faria deposits model ages of 2,410 to 2,425 Ma. A more radiogenic specimen from the Bela Fama deposit has a model age of 1,935 Ma. Alternatively, two-stage calculations can be applíed to some of these data. The galenes in any case are interpreted to reflect deformation of the gold deposits and host rocks, and accompanying relatively local remobilization of constítuents, including silica and lead, during a number of Proterozoic orogenic events. Correlations are suggested between these events and isotopic resetting events that have been identified for basement rocks in the region. These are the most primitive lead isotope compositions that have been obtained for Brazilían galerias.