Carbonate alteration associated with talc-chlorite mineralization in the eastern Pyrenees, with emphasis on the St. Barthelemy Massif |
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| Authors: | P Boulvais P de Parseval A D’Hulst P Paris |
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| Institution: | (1) Géosciences Rennes – UMR 6118, Université de Rennes I, Campus de Beaulieu, 35042 Rennes cedex, France;(2) Observatoire Midi-Pyrénées – LMTG – UMR5563, 14 avenue Edouard Belin, 31400 Toulouse, France;(3) Département de géologie et de génie géologique, Faculté des sciences et de génie, Université Laval, G1K 7P4 Sainte-Foy, Québec, Canada;(4) Talc de Luzenac France, BP 11, 09250 Luzenac cedex, France |
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| Abstract: | Summary The eastern Pyrenees host a large number of talc-chlorite mineralizations of Albian age (112–97 Ma), the largest of which
occur in the St. Barthelemy massif. There talc develops by hydrothermal replacement of dolostones, which were formed by alteration
of calcite marbles. This alteration is progressive. Unaltered calcite marbles have oxygen isotope composition of about 25‰
(V-SMOW). The δ18O values decrease down to values of 12‰ towards the contact with dolostones. This 18O depletion is accompanied by Mg enrichment, LREE fractionation and systematic shifts in the Sr isotope compositions, which
vary from 87Sr/86Sr = 0.7087–0.7092 in unaltered calcite marbles to slightly more radiogenic compositions with 87Sr/86Sr = 0.7094 near dolomitization fronts. Dolostones have δ18O values (about 9‰) lower than calcitic marbles, higher REE content and more radiogenic Sr isotope composition (87Sr/86Sr = 0.7109 to 0.7130). Hydrothermal calcites have δ18O values close to dolostones but substantially lower δ13C values, down to −6.5‰, which is indicative of the contribution of organic matter. The REE content of hydrothermal calcite
is one order of magnitude higher than that of calcitic marbles. Its highly radiogenic Sr composition with 87Sr/86Sr = 0.7091 to 0.7132 suggests that these elements were derived from silicate rocks, which experienced intense chlorite alteration
during mineralization. The chemical and isotopic compositions of the calcite marbles, the dolostones and the hydrothermal
calcites are interpreted as products of successive stages of fluid-rock interaction with increasing fluid-rock ratios. The
hydrothermal quartz, calcite, talc and chlorite are in global mutual isotopic equilibrium. This allows the calculation of
the O isotope composition of the infiltrating water at 300 °C, which is in the δ18O
= 2–4.5‰ range. Hydrogen isotope compositions of talc and chlorite indicate a δD
= 0 to −20‰. This water probably derived from seawater, with minor contribution of evolved continental water. |
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