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 metasedimentary rocks and are spatially related to Late Carboniferous/Early Permian granitoids. At Mari Rosa, the following hydrothermal stages are recognized: 1) arsenopyrite-(pyrite), 2) stibnite-gold and 3) pyrite-(pyrrhotite-galena-sphalerite-chalcopyrite-tetrahedrite-boulange rite-berthierite-stibniteullmannite- cobaltite). Only the second stage is of economic importance, containing massive stibnite. El Juncalon is characterized by a simple mineral paragenesis composed of stibnite with very minor pyrite. Gold is associated with stibnite, which contains up to 30 ppm Au at Mari Rosa and up to 0.4 ppm Au at El Juncalon. Fluids associated with ore deposition lie in the H (sub 2) O-NaCl-CO (sub 2) -CH (sub 4) -N (sub 2) system, but CO (sub 2) is absent in the El Juncalon mineralized rocks. The fluids cooled progressively, from initial circulation temperatures close to 400 degrees C in the early stages to temperatures of approximately 150 degrees C in the late episodes. The fluid composition evolution is also characterized by a progressive increase in the overall water content of the fluids and by an increase in the relative proportions of N (sub 2) with respect to CH (sub 4) and CO (sub 2) in the volatile fraction. At Mari Rosa, massive stibnite deposition resulted from a boiling process at 300 degrees C and 0.9-1 kbar. Unmixing of the fluid was induced by sudden pressure drops associated with dilational jogs during low-angle fault movements. Cooling of the fluid is recognized as the driving mechanism for stibnite deposition at 250 degrees -270 degrees C and 50-300 bars at El Juncalon.