Geoquímica de los procesos de alteración hidrotermal en la mineralización de Sb de Mari Rosa (zona Centro Ibérica)

Abstract

The Central Iberian Zone of the Hesperian Massif hosts a series of late Hercynian veintype Sb deposits. One of them is the Mari Rosa mineralization, hosted by metagreywackes and slates belonging to the so-called Schist-Greywacke Complex (Upper Precambrian). The mineralization is characterized by a complex paragenesis comprising arsenopyrite-(pyrite), stibnite-gold and pyrite-pyrrhotite-galena-sphalerite-chalcopyrite-tetrahedrite-boulangeritestibnite. Of these only the second episode was of importance and gave rise to the main mineralized bodies of the deposito Two types of veins can be defined in Mari Rosa: Vp, showing pinch and swell structures parallel to the main foliation planes (S₂), and Vo, massive lensoid-shaped bodies of gold bearing stibnite postdating the Vp veins and cross-cutting S₂. Hydrothermal alteration consists oí a mild sericitization, chloritization and carbonatization oí the metasedimentary rocks around the veins. Due to the mineralogy oí the hostrocks (muscovite, chlorite, quartz), these alteration facies are not easily recognizable. However, the study of chemical changes around the veins allow a clear characterization of the hydrotermal processes. !hus, a remarkable increase in the ratio K₂O/Na₂O (from 1.2 in the regional rocks to 5.1 lfi the Vp contact rocks) and a decrease in the ratio SiO₂/volatiles (írom 27.4 in the regional rocks to 13.5 in the Vp contact rocks) can be used as clear-cut indicators of the extension and magnitude of the hydrothermal alteration. Other changes included increases lfi. Sb (between 5,000 and 10,000 times the regional background), Mo (up to 60 times the regional background) and Au (values oí up to 32 ppm) around the veins. Besides, a high positive correlation between Sb and Mo is observed (R = 1 in the veins and R = 0.9 in the host rocks). The isotopic signature of stibnite and pyrite (average δ³⁴S of 0.3‰ and -3.9‰, respectively) is close to the typical values of magmatic sulphur. This, together with the high correlations between Sb and Mo strongly suggest a genetic relationship between the Mari Rosa mineralization and the intrusion of the Alburquerque batholith.