Isotope characterization of lead in galena from ore deposits of the Aysén Region, southern Chile

• Published in: Mineralium deposita 2001-01, Vol.36 (1), p.45-57
• Main Authors: Townley, Brian K., Godwin, Collin I.
• Format: Article
• Language: English
• Subjects: Cretaceous; Geophysics; Isotopes; Jurassic; Leaching; Lead; Mineralogy; Silver
• ISSN: 0026-4598; 1432-1866
• DOI: 10.1007/s001260050285

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 (^sup 206^Pb/^sup 204^Pb=18.53, ^sup 207^Pb/^sup 204^Pb=15.63, and ^sup 208^Pb/^sup 204^Pb=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 ^sup 206^Pb/^sup 204^Pb versus ^sup 207^Pb/^sup 204^Pb 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 (group3), and in the Early and mid-Cretaceous (groups1 and 2 respectively). Averages for group1, the northern group including El Toqui and Katerfeld, are ^sup 206^Pb/^sup 204^Pb=18.51, ^sup 207^Pb/^sup 204^Pb=15.62, ^sup 208^Pb/^sup 204^Pb=38.48. Averages for group2, the southern group with Fachinal and Mina Silva, are ^sup 206^Pb/^sup 204^Pb = 18.56, ^sup 207^Pb/^sup 204^Pb=15.63, ^sup 208^Pb/^sup 204^Pb=38.52. Averages for group3, the southernmost group with the El Faldeo, Lago Chacabuco and Lago Cochrane prospects, are ^sup 206^Pb/^sup 204^Pb=18.83, ^sup 207^Pb/^sup 204^Pb=15.65, ^sup 208^Pb/^sup 204^Pb=38.63. The Cretaceous deposits (groups1 and 2) contain orogene-type lead that becomes increasingly radiogenic southward. Lead from the Late Jurassic deposits (group3) appears to reflect mixing of orogene lead with highly radiogenic lead. The observed linear array of lead in group3 probably reflects mixing of orogene lead with highly radiogenic lead, which was likely extracted by selective leaching of mineral