MINERALOGY AND GEOCHEMISTRY OF SOME BASE METAL OCCURRENCES IN THE CENTRAL TACONIC REGION (NEW YORK)

Our study focuses on the base metal occurrences (Pb-Zn; Cu-Pb-Zn) in the central part of the Taconic region. They form narrow veins and impregnations in the Ordovician dolostones and calc-schists. The optical mineralogical study revealed the following mineralogical record: galena, sphalerite, pyrite, tetrahedrite-tennantite, chalcopyrite and native gold with smithsonite, hemimorphite, aurichalcite, malachite, azurite, and anglesite as secondary minerals usually exhibiting replacement textures. The trace element composition of the sulfides showed high Ag content 257 ppm to 766 ppm (tetrahedrite-tennantite), Cd 4345 ppm to 5452 ppm (sphalerite), In 105 ppm (sphalerite), Ni 1844 ppm, Co 311 ppm, and As 122 ppm (pyrite). Se displays a homogeneous content for all the analyzed sulfides (2 ppm to 19 ppm). The Pb composition of galena is less radiogenic (206Pb/204Pb is 18.566-18.944) than in the Mississippi Valley type ores (J-type lead). The homogeneous Pb isotopic character, the similarity between Pb isotopic composition of these ores and Shawangunk Mountains base metal ores make us consider either a single source of the Pb or fluids from multiple sources that were mixed before galena precipitated. We could consider a possible common origin for the galenas hosted by the two units. A similarity with the other Appalachian ores is also indicated. The sulfur isotopes display a narrow range for individual mineral species (delta 34S is between 3.3 and –4.1). These values are close to those of the pyrite from the base of the Shawangunk Formation overlying the Taconic unconformity, but very different from the sulfur composition of the Shawangunk sulfides implying a different origin. A progressive decrease of the delta 34S for the sequence pyrite-sphalerite-galena is possibly related to the normal trend during the equilibrium fractionation. The low delta 34S could represent the result of the bacterial reduction of sulfur derived from an underlying unit.