Los Skarns mineralizados en Sn-(Au) de L ’Alt Urgell (Pirineos Centrales)

Abstract

Two zones have been developed in the Sn-bearing skams of the Alt Urgell: silicate and ore zones. The silicate zone is made up of hedenbergite and minor amounts of Sn-bearing garnet, idocrase, K-feldspar, plagioclase, quartz, actinolite and epidote. The ore zone is constitued of pyrrhotite and arsenopyrite, followed by an association of chalcopyrite, cassiterite, native gold, native bismuth and bismuthinite. Quartz, grossularite and epidote are found together with the ore minerals. The main interest of this type of skams is their Sn content (< 1 wt%). The Au rate is lower than 1.5 g/tThe geochemical constraints of formation of these skams have been inferred from arsenopyrite geo thermometry, chlorite composition, and thermochemistry of the silicate and ore associations. The skams were formed from C 0 2-poor aqueous fluids, at pressures of about 2 kb and temperatures decreasing from T>480 °C for the minerals of the silicate zone, 475 ± 50 °C for the arsenopyrite + pyrrhotite association, to T<400 °C for the later ore minerals. The oxygen fugacity of the ore forming fluid was close to the quartz-fayalite-magnetite buffer, and the sulphur fugacity between the pyrrhotite +ldllingite +As and native bismuth +bismuthinite equilibria.