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 (S 2 ), and Vo, massive lensoid-shaped bodies of gold bearing stibnite postdating the Vp veins and cross-cutting S 2 . Hydrothermal alteration consists oi a mild sericitization, chloritization and carbonatization oi the metasedimentary rocks around the veins. Due to the mineralogy oi 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 K 2 O/Na 2 O (from 1.2 in the regional rocks to 5.1 lfi the Vp contact rocks) and a decrease in the ratio SiO 2 /volatiles (irom 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 oi 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 δ 34 S 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.
Highlights
The Central Iberian Zone of the Hesperian Massif hosts a series of late Hercynian veintype Sb deposits
Hydrothermal alteration consists oí a mild sericitization, chloritization and carbonatization oí the metasedimentary rocks around the veins
The study of chemical changes around the veins allow a clear characterization of the hydrot~ermal processes. !hus, a remarkable increase in the ratio KzO/NazO and a decrease in the ratio SiOz/volatiles can be used as clear-cut ~ndicato~s of the ~xtension and magnitude of the hydrothermal alteration
Summary
El medio encajante de la mineralización de antimonio de Mari Rosa está constituido por los materiales detríticos del Alcudiense Inferior (Tamain, 1972) o Serie Domo Extremeño (ITGE, 1993), aunque en esta zona son más comúnmente conocidos como Complejo Esquisto Grauváquico. Las pizarras negras son rocas de grano muy fino compuestas por bandas lutíticas con materia orgánica que alternan ocasionalmente con otras más ricas en cuarzo en un bandeado composicional que probablemente corresponde a la estratificación original So de la serie. En el sector de la mineralización, los materiales aflorantes no muestran evidencias de estar afectados por este metamorfismo térmico, si bien es cierto que las paragénesis de menor temperatura del mismo pueden coincidir con las de metamorfismo regional de bajo grado presentes en estas rocas. En algunos puntos se observa el desarrollo de una tercera esquistosidad, S3' que crenula a la esquistosidad principal anterior y que generalmente está restringida a las litologías de pizarras negras. Con posterioridad se produce una fracturación aproximadamente ONO-ESE y un sistema conjugado de fracturas subverticales de dirección NE-SO y NO-SE, correspondiendo este último al episodio de fracturación tardihercínica definido por Arthaud y Matte (1975) en el macizo Hespérico
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