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Implications of Pb isotope signatures of rocks and iron oxide Cu-Au ores in the Candelaria-Punta del Cobre district, Chile

Published in Mineralium Deposita. 2003, vol. 38, no. 7, p. 900-912
Abstract Lead isotope ratios of ores of the Candelaria- Punta del Cobre iron oxide Cu-Au deposits and asso- ciated Early Cretaceous volcanic and batholithic rocks have been determined. For the igneous rocks, a whole- rock acid attack technique based on the separate anal- yses of a leachate and the residual fraction of a sample was used. The lead isotope systematics of leachate– residue pairs are significantly different for unaltered and altered igneous rocks of the Candelaria-Punta del Cobre district. Residues of unaltered igneous rocks likely rep- resent the common lead. In contrast, residues of all the altered igneous rocks except two samples have higher Pb isotope ratios than those of unaltered magmatic rocks and cannot represent common lead. We suggest that this is a result of the hydrothermal alteration suffered by these rocks and that the common lead composition of the altered igneous (volcanic and plutonic) rocks must have been similar to that of the unaltered batholith rocks. The conclusion that the altered volcanic rocks originally had a similar common lead isotope composi- tion as the batholith is consistent with geological and geochemical arguments (e.g., setting, regional geologic evolution, ages and relative distribution of volcanic andintrusive rocks, magmatic affinities), which indicate that these rocks were derived from similar Early Cretaceous parent magmas. The modification of the leachate– residue pair lead isotope systematics of most altered igneous rocks is consistent with a selective removal of lead and uranium from these rocks by an oxidized hydrothermal fluid. The result of the hydrothermal leaching has been to alter magmatic rocks in a way that (1) their leachable fraction is presently a mix of common lead similar to that of the ore event and of radiogenic lead evolved from a source with a consistently high Th/ U, and that (2) their residual fraction has less common lead than unaltered rocks. The outcrop area with altered volcanic rocks displaying anomalously high lead isotope ratios extends over 25 km along the eastern margin of the batholith. Since lead of the ores in the Candelaria- Punta del Cobre district has the same isotopic compo- sition as the common lead of unaltered magmatic rocks of the area, the lead isotope data are consistent with a derivation of the ore lead (and by inference of other metals like Cu) both directly from a magmatic fluid exsolved during crystallization of the batholith and/or from hydrothermal leaching of the volcanic rocks orig- inally having similar isotopic compositions as the bath- olith.
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Research groups Isotope Geochemistry, Geochronology and Thermochronology
Mineral Resources and Geofluids
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MARSCHIK, Robert, CHIARADIA, Massimo, FONTBOTÉ, Lluis. Implications of Pb isotope signatures of rocks and iron oxide Cu-Au ores in the Candelaria-Punta del Cobre district, Chile. In: Mineralium Deposita, 2003, vol. 38, n° 7, p. 900-912. doi: 10.1007/s00126-003-0385-9 https://archive-ouverte.unige.ch/unige:19395

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Deposited on : 2012-04-16

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