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Exploration and Mining Geology; July-October 2007; v. 16; no. 3-4; p. 145-158; DOI: 10.2113/gsemg.16.3-4.145
© 2007 Canadian Institute of Mining, Metallurgy & Petroleum
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Geology and Chemistry of the El Abuelo Calcic Fe-skarn and Related Cu-(Ag)-Bearing Hydrothermal Veins, Chubut Province, Southern Argentina

M.E. Lanfranchini1,{dagger}, R.E. De Barrio2 and R .O. Etcheverry1

1 Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), Instituto de Recursos Minerales (INREMI), FCN y M, UNLP, Calle 64 N° 3 entre 119 y 120, (1900) La Plata, Argentina.
2 Instituto de Recursos Minerales (INREMI), FCN y M, UNLP, Calle 64 N° 3 entre 119 y 120, (1900) La Plata, Argentina.

Correspondence: {dagger} Corresponding Author: E-mail: mlanfranchini{at}inremi.unlp.edu.ar

The El Abuelo Ca-Fe-magnetite skarn and related hydrothermal quartz veins are located at Cerro Pepita Hill, in southwestern Chubut province of southern Argentina, 1700 km southwest of Buenos Aires. These deposits are developed in a continental magmatic arc environment linked to the Andean orogeny. Iron skarn mineralization is mainly hosted by an Upper Jurassic to Early Cretaceous sedimentary sequence interbedded with Upper Jurassic basaltic andesite, and is spatially associated with Early Cretaceous calc-alkaline dikes. Ore grades vary between 40 and 63 wt.% Fe. In addition, anomalous metal contents (>10 000 ppm Cu and up to 81 g/t Ag) are present in the hydrothermal quartz veins.

At least three paragenetic mineral assemblages have been identified in the El Abuelo exoskarn: (1) amphibole > epidote ± chlorite ± quartz, formed by isochemical contact metamorphism; (2) Fe-clinopyroxene ± Ca-garnet, formed during prograde metasomatic anhydrous exoskarn formation; and (3) actinolite ± epidote ± chlorite ± quartz ± magnetite > titanite, resulting from hydrous retrograde alteration of exoskarn. Endoskarn alteration involved an early Mg-Fe-clinopyroxene ± Fe-garnet assemblage followed by retrograde actinolite replacement of pyroxene. Lower temperature hydrothermal alteration related to quartz veins was superimposed on the skarn assemblages, showing an innermost sericite ± adularia ± pyrite ± chalcopyrite assemblage and an external propylitic halo.

Some metallogenic characteristics of the mineralization and the geological setting resemble those in several iron oxide copper-gold deposits elsewhere in the world.

Key Words: Calcic Fe-skarn • hydrothermal veins • Lower Cretaceous • Argentina






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