Carbonate alteration associated with talc-chlorite mineralization in the eastern Pyrenees, with emphasis on the St. Barthelemy Massif

The eastern Pyrenees host a large number of talc-chlorite mineralizations of Albian age (112-97Ma), 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 degrees (V-SMOW). The delta O-18 values decrease down to values of 12 degrees towards the contact with dolostones. This O-18 depletion is accompanied by Mg enrichment, LREE fractionation and systematic shifts in the Sr isotope compositions, which vary from Sr-87/Sr-86 = 0.7087-0.7092 in unaltered calcite marbles to slightly more radiogenic compositions with Sr-87/Sr-86 = 0.7094 near dolomitization fronts. Dolostones have delta O-18 values (about 9 degrees) lower than calcitic marbles, higher REE content and more radiogenic Sr isotope composition (Sr-87/Sr-86 = 0.7109 to 0.7130). Hydrothermal calcites have delta O-18 values close to dolostones but substantially lower delta C-13 values, down to -6.5 degrees, 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 Sr-87/Sr-86 = 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 degrees C, which is in the delta O-18(H2O) = 2-4.5 parts per thousand range. Hydrogen isotope compositions of talc and chlorite indicate a delta D-H2O = 0 to -20 parts per thousand. This water probably derived from seawater, with minor contribution of evolved continental water.