A discussion on the lead isotope geochemistry of galenas from the Bambui group,Minas Gerais — Brazil

Lead isotope data on galenas from the Bambui group, Minas Gerais have been assembled and discussed. The isotope data, when plotted on the Single Stage Model III lead isotope development curve of Cumming and Richards (1975), yield 207/206 ages of 650±50 m.y and a linear array of data points with negative model ages, which intercept the growth curve at 650±60 m.y and 1850±150 m.y. The radiometric dates coincide with the two major orogenic episodes, the Brazilian and the Transamazonian. The data are interpreted in terms of a two stage model, where the source of lead for galenas, formed during Brazilian orogeny, is the basement rocks of Transamazonian age. The calculated Th/U value for the source rock is 2.9±0.3. The geographical distribution of lead isotope ratios shows a zonation in Vazante Paracatu region, an area containing large deposits of zinc. The implication of the distribution on the ore formation is discussed.