Evolution of ore-forming fluids associated with late Hercynian antimony deposits in Central/Western Spain: case study of Mari Rosa and El Juncalón

Abstract

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-boulangerite-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 H2O-NaCl-CO2-CH4-N2 system, but CO2 is absent in the El Juncalon mineralized rocks. The fluids cooled progressively, from initial circulation temperatures close to 400°C in the early stages to temperatures of approximately 150°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 N2 with respect to CH4 and CO2 in the volatile fraction. At Mari Rosa, massive stibnite deposition resulted from a boiling process at 300°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°-270°C and 50-300 bars at El Juncalon.