THE MARI-ROSA LATE-HERCYNIAN SB-AU DEPOSIT, WESTERN SPAIN - GEOLOGY AND GEOCHEMISTRY OF THE MINERALIZING PROCESSES

The central Iberian zone of the Hesperian Massif hosts a series of lat e Hercynian vein-type Sb deposits. One of them is the Mari Rosa minera lization, hosted by metagreywackes and slates of the Schist-Greywacke Complex (Upper Precambrian). The mineralization is characterized by a complex paragenesis comprising three hydrothermal stages: stage H1 --> arsenopyrite-(pyrite); stage H2 --> stibnite-gold; and stage H3 --> e -chalcopyrite-tetrahedrite-boulangerite-stibnite. Of these only the se cond episode was of importance and gave rise to the main mineralized b odies of the deposit. Hydrothermal alteration consists of a mild seric itization, chloritization and carbonatization of the metasedimentary r ocks around the veins. Chemical changes in the hydrothermal halos incl ude a remarkable increase in the ratio K2O/Na2O, and a decrease in the ratio SiO2/volatiles, together with a sharp increase in Sb, Mo, Au an d N. Fluids associated with ore deposition lie in the H2O-NaCl-CO2-CH4 -N-2 compositional system. These fluids evolved, progressively cooling , from initial circulation temperatures close to 400 degrees C in the early stage (H1) to temperatures of approximately 150 degrees C in the late one (H3). Fluid composition evolution was characterized by a pro gressive increase in the bulk water content of the fluids and with an increase in the relative proportion of N-2 with respect to CH4 and CO2 in the volatile fraction. Massive stibnite deposition resulted from a boiling process developed at 300 degrees C and 0.9-1 Kb at a depth of 4-5 km. Geological, geochemical and fluid inclusion evidence suggest that the intrusion of the Alburquerque batholith (late Hercynian S-typ e granitoids) triggered hydrothermal activity leading to the transport and deposition of Sb and Au in Mari Rosa.