Abstract

Lead isotope analyses of galena from five ore deposits and six prospects in the Aysén region of southern Chile are reported. Most of the deposits are either low sulfidation epithermal gold-silver veins or skarn and manto deposits; the majority are either suspected to be, or dated as, Late Jurassic to mid-Cretaceous. Galena lead isotope data for most of the deposits from southern Chile cluster near the "orogene" within a "plumbotectonic" model framework. Average values (206Pb/204Pb = 18.53, 207Pb/204Pb = 15.63, and 208Pb/204Pb = 38.50) are near Jurassic to Cretaceous model ages on the "orogene" curve of Zartman and Doe (1981) and the second-stage curve of Stacey and Kramers (1975) on a 206Pb/204Pb versus 207Pb/204Pb plot. These model ages are compatible with absolute ages as currently known. The elongate trends in the general cluster indicate mainly an orogenic model fit, suggesting variable mixing of lead from different sources, mainly model upper crust and lesser model mantle and lower crust reservoirs. Galena lead associated with one deposit (El Faldeo) is relatively radiogenic, and lies near a Jurassic age on the "upper crustal" curve of Zartman and Doe (1981), which is compatible with the Ar/Ar age of the deposit. Galena lead isotope clusters define three main groups of deposits. These three groups appear to be related to three mineralizing events, dated by K-Ar and Ar/Ar, in the Late Jurassic (group 3), and in the Early and mid-Cretaceous (groups 1 and 2 respectively). Averages for group 1, the northern group including El Toqui and Katerfeld, are 206Pb/204Pb = 18.51, 207Pb/204Pb = 15.62, 208Pb/204Pb = 38.48. Averages for group 2, the southern group with Fachinal and Mina Silva, are 206Pb/204Pb = 18.56, 207Pb/204Pb = 15.63, 208Pb/204Pb = 38.52. Averages for group 3, the southernmost group with the El Faldeo, Lago Chacabuco and Lago Cochrane prospects, are 206Pb/204Pb = 18.83, 207Pb/204Pb = 15.65, 208Pb/204Pb = 38.63. The Cretaceous deposits (groups 1 and 2) contain orogene-type lead that becomes increasingly radiogenic southward. Lead from the Late Jurassic deposits (group 3) appears to reflect mixing of orogene lead with highly radiogenic lead. The observed linear array of lead in group 3 probably reflects mixing of orogene lead with highly radiogenic lead, which was likely extracted by selective leaching of mineralizing hydrothermal solutions from the metamorphic basement.