THE VIRORCO LAYERED INTRUSION, ARGENTINA

FERRACUTTI, G. R.; BJERG, E.A.; MOGESSIE, A.

Salzburgo

Congreso; Meeting of the German Crystallographic Society, the German Mineralogical Society and the Austrian Mineralogical Society.; 2011

Virorco is a layered mafic-ultramafic (M-UM) intrusion located in the Sierras de San Luis, Argentina (Fig. 1). This intrusion is part of a M-UM belt that extends over 100 km from NE to SW. The rocks constituting this belt carry a sulfide mineralization consisting of pyrrhotite, pentlandite and chalcopyrite, in veins and as disseminated to massive ore. Disseminated spinels are frequently associated with the sulfide minerals as well as platinum group minerals (PGM). Ferracutti et al. (2011) defined three types of layering in the Virorco body based on the petrology, geochemistry and textural features: a) colloform layering corresponding to gabbroidic rocks of the chilled magin (Fig. 2A), b) Compositional layering, which consist of mesoscale layering of pyroxenitic and hornblenditic alternating layers (Fig. 2B), and c) macroscale layering, which comprise pyroxenitic layers (Fig. 2C). All these layering show distinctive geochemical characteristics, with the more depleted rocks constituting the compositional layering and the more evolved the chilled margin, and three REE/Chodrite profile (Fig.2D, Sun and McDonough, 1989). The UM rocks of the belt, show a very clear relationship between the presence of spinels and the levels with mineralization of base metals (BM), PGM and content of PGE (Ferracutti et al, 2005). These features and the Cu/Pd vs. Pd and Cu/Pt vs. Ni/Pd relationships, allow the distinction of two groups of rocks, one with ratios that are typical of mantle rocks and the other depleted in PGE. The last one is interpreted as the result of an earlier sulphide segregation which removed Pt and Pd from the magma chamber. The parental magma of the M-UM rocks was a Mg-rich mafic magma with tholeiitic affinity. This conclusion is based on the evaluation of the whole rock chemistry, PGE and BM contents, textural evidences and comparisons with deposits from the world. The Virorco layered M-UM body and the remaining bodies in the area bear similar characteristics to well known stratified complexes developed in extensive tectonic regimes. Although more work is required, the available evidences suggest that BMS and PGM mineralization are very probably associated with deeper stratigraphic levels where ultramafic rocks are dominant.