EVOLUTION OF ORE-FORMING FLUIDS ASSOCIATED WITH LATE-HERCYNIAN ANTIMONY DEPOSITS IN CENTRAL-WESTERN SPAIN - CASE-STUDY OF MARI-ROSA AND EL-JUNCALON

Located in Central-Western Spain, the late Hercynian Mari Rosa and El Juncalon vein-type antimony deposits are hosted by Late Precambrian me tasedimentary rocks and are spatially related to Late Carboniferous/Ea rly Permian granitoids. At Mari Rosa, the following hydrothermal stage s are recognized: 1) arsenopyrite-(pyrite), 2) stibnite-gold and 3) te -galena-sphalerite-chalcopyrite-tetrahedrite-??? gerite-berthierite-st ibnite-ullmannite-cobaltite). Only the second stage is of economic imp ortance, containing massive stibnite. El Juncalon is characterized by a simple mineral paragenesis composed of stibnite with very minor pyri te. Gold is associated with stibnite, which contains up to 30 ppm Au a t Mari Rosa and up to 0.4 ppm Au at El Juncalon. Fluids associated wit h ore deposition lie in the H2O-NaCl-CO2-CH4-N2 system, but CO2 is abs ent in the El Juncalon mineralized rocks. The fluids cooled progressiv ely, from initial circulation temperatures close to 400-degrees-C in t he early stages to temperatures of approximately 150-degrees-C in the late episodes. The fluid composition evolution is also characterized b y a progressive increase in the overall water content of the fluids an d by an increase in the relative proportions of N2 with respect to CH4 and CO2 in the volatile fraction. At Mari Rosa, massive stibnite depo sition resulted from a boiling process at 300-degrees-C and 0.9-1 kbar . Unmixing of the fluid was induced by sudden pressure drops associate d with dilational jogs during low-angle fault movements. Cooling of th e fluid is recognized as the driving mechanism for stibnite deposition at 250-degrees-270-degrees-C and 50-300 bars at El Juncalon.