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Abstract

The central Iberian zone of the Hesperian Massif hosts a series of late Hercynian vein-type Sb deposits. One of them is the Mari Rosa mineralization hosted by metagreywackes and slates of the Schist-Grey􀂷 wacke Complex (Upper Precambrian). The mineralization is characterized by a complex paragenesis comprising three hydrothermal stages: stage HI 􀀂 arsenopyrite-(pyrite); stage H2 􀀂 stibnite-gold; and stage H3 􀀃 pyrite-pyrrho t i te-galena -s pha leri te-ch alco py ri te-tetrahed ri te-bo u 1- angerite-stibnite. Of these only the second episode was of importance and gave rise to the main mineralized bodies of the deposit. Hydrothermal alteration consists of a mild sericitization, chloritization and carbonatization of the metasedimentary rocks aroLtnd the veins. Chemical changes in the hydrothermal halos include a remarkable increase in the ratio K20/Na20, and a decrease in the ratio Si02/volatiles, together with a sharp increase in Sb, Mo, Au and N. Fluids associated with ore deposition lie in the H20-NaCI-C02-CH4-N2 compositional system. These fluids evolved, progressively cooling, from initial circulaion temperatures close to 400 QC in the early stage (Hi) to temperatures of approximately 150°C in the late one (H3). Fluid composition evolution was characterized by a progressive increase in the bulk water content of the fluids and with an increase in the relative proportion of N2 with respect to CH4 and CO2 in the volatile fraction. Massive stibnite deposition resulted from a boiling process developed at 300°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-type granitoids) triggered hydrothermal activity leading to the transport and deposition of Sb and Au in Mari Rosa. Antimony veins, some of them gold bearing, represent part of the de􀂸osits formed during this episode. These antimony depos'its crop out within the cores of Hercynian anticlines and are hosted by Upper Precambrian metasedimentary rocks. They all have stibnite as the main ore mineral. Genetic relationships have been suggested between some of these mineralizations and the granitoids (GumieI1983). However, preliminary fluid inclusion studies in the area of Mari Rosa (Fig. 1) (Ortega et al. 1991a, b) showed fluid compositions within the system H20-NaCICO2- CH4-N2, thus suggesting a possible metamorphic signature (e.g. Swanenberg 1980; Kreulen and Schuiling 1982; van den Kerkhof 1988; Guillot 1989; Boiron et al. 1990). In this sense, the aim of the work was to develop a better understanding of the ore forming process from one previously selected case i.e. the Mari Rosa veins. Mari Rosa was elected as a study site because this deposit has already received some attention from a fluid inclusion studies viewpoint and moreover was considered to be representative enough of a wide number of Sb veins in western central Spain. The work required a comprehensive study, involving a reappraisal of the regional geological background, the structural control of the mineralization, the geochemistry of altered rocks, a characterization of fluid composition and evolution and an estimation of conditions for ore deposition. An integrated interpretation of these data has allowed definition of a complex metallogenical process involving fluid-rock interactions under medium PT conditions for the Sb precipitation (Ortega 1993).