This study dealt with the potassic alteration that affected the mafic rocks associated with the Cu-Au mineralization of the Visconde deposit. These rocks have been considered as gabbros/(quartz)diorites, although their original composition could not have been determined. The pre-hydrothermal alteration mineral association consisted mostly of Mg-hornblende, plagioclase, magnetite ± quartz, which underwent sodic-calcic metasomatism and produced mainly Fe-pargasite and scapolite. Biotite is the typical mineral of the superimposed potassic alteration and, according to its abundance, could be recognized weakly (≤10%), moderately (10-40%) and strongly (>40%) biotitized rocks. This mica has not only replaced pre-existing minerals, notably Fe-pargasite, but also has precipitated from hydrothermal fluids. The weakly biotitized variety has locally preserved the subofitic texture and its is mainly composed of Mg-hornblende, Fe-pargasite, plagioclase and biotite. The moderately biotitized variety is, in general, foliated and, in addition to biotite, consists of Fe-pargasite, actinolite, plagioclase, scapolite, quartz and minor amounts of epidote, chlorite, magnetite and sulfides. In turn, the strongly biotitized variety commonly shows mylonitic foliation, being made up of biotite>>scapolite>Fe-pargasite, actinolite>quartz, plagioclase, tourmaline, epidote, chlorite, magnetite and sulfides. As the degree of biotitization increased, the rocks tended to be enriched in K2O, MgO, K and Ba, and empoverished in CaO, Na2O and Sr in response to the replacement of sodic-calcic minerals by biotite. The relationship between K and Rb is essentially linear, so that the Rb content can be used to distinguish weakly (Rb<125 ppm), moderately (125< Rb<225 ppm) and strongly (>225 ppm) biotitized varieties. On the other hand, K and Sr show an antipathetic relationship, meaning that Sr was removed along with Ca during the replacement of amphiboles and scapolite by biotite. The higher the degree of potassic metasomatism, the higher the amounts of Ni, Co, Cu and Au transferred to the rocks by the hydrothermal fluids, suggesting a direct link between the potassic alteration and the mineralization. These fluids were also capable of sscavaging REE from the rocks without modifying significantly their distribution patterns. The lowest values for REE are recorded in the strongly biotitized rocks.
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