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Separate lead isotope analyses of leachate and residue rock fractions: implications for metal source tracing in ore deposit studies

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Published in Mineralium Deposita. 2003, vol. 38, no. 2, p. 185-195
Abstract Separate lead isotope analyses of leachate and residue fractions are applied to a broad spectrum of rocks commonly investigated in metallogenic studies. Resulting data highlight a systematic behavior of leachate and res- idue fractions with respect to lead isotope compositions, which essentially depends on the mineralogical composi- tion of the rock. Granitoid and high-grade metamorphic rocks have residue compositions virtually identical to common lead. In contrast, low-grade metasedimentary rocks may have residue compositions swamped by radi- ogenic lead of leach-resistant zircons. Mafic magmatic rocks have residues that are often more radiogenic than leachates, depending on the ratio of leach-refractory zir- cons to common lead in the residual fraction of these rocks. Separate leachate and residue analyses of source rocks provide two lead isotope end members whose mix- ture may represent lead with the appropriate ore fluid composition. Our leaching experiments indicate that hot acid solutions (and by inference hydrothermal fluids) preferentially leach radiogenic lead from medium- to high-grade metamorphic and granitoid rocks, whereas they preferentially leach common lead from low-grade metasedimentary and mafic magmatic rocks. The method presented in this study provides a reliable alternative to other methods (i.e., age-correction of bulk-rock compo- sitions) for the determination of the common lead signa- ture of felsic to intermediate magmatic rocks. This may be preferable to age-corrected bulk-rock analyses, where ages to apply for corrections of bulk-rock data are notknown or where moderately to highly altered rocks must be used. Case studies of orogenic gold and MVT districts of Peru (Pataz and San Vicente, respectively) show that separate leachate and residue lead isotope analyses carried out systematically on whole rocks allow a more thorough evaluation of metal source reservoirs than does the stan- dard method of age-corrected or uncorrected bulk-rock analyses.
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Research groups Isotope Geochemistry, Geochronology and Thermochronology
Mineral Resources and Geofluids
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CHIARADIA, Massimo, FONTBOTÉ, Lluis. Separate lead isotope analyses of leachate and residue rock fractions: implications for metal source tracing in ore deposit studies. In: Mineralium Deposita, 2003, vol. 38, n° 2, p. 185-195. https://archive-ouverte.unige.ch/unige:18900

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Deposited on : 2012-03-19

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