The Miocene Brahma porphyry Cu-Mo prospect in Central Chilean Andes (35°45′S): Geology, geochronology (U-Pb, Re-Os) and geochemistry

Abstract

The Brahma prospect is a Neogene porphyry copper deposit located in the Chilean Andes at ~ 35°45′S, beyond the southerly extension of the previously defined Mio-Pliocene metallogenic belt of Central Chile. We present the results of a comprehensive study of this prospect based on drill hole logging, petrography, geochemistry, and geochronology, with the aim of constraining its paragenesis and formation within the metallogenic background of the region. The prospect is hosted by dioritic and granodioritic equigranular units mineralized by fluids from at least two hydrothermal pulses in turn related to the emplacement of porphyritic intrusives and hydrothermal breccias. Existing exploration drill holes only intercepted the phyllic and argillic zone of the system, consistent with the paucity of potassic alteration and the dominance of pyrite, sphalerite, chalcopyrite and molybdenite mineralization. Zircon U-Pb determinations of the main igneous units present yielded ages of ~ 17 Ma, 14 Ma, and 9 Ma. Mineralization ages of ~ 14 Ma, 12 Ma, and 10 Ma were obtained by Re-Os determinations on molybdenite from samples of different paragenetic stages. These data indicate that (i) the prospect is composed of at least three main magmatic events, (ii) early mineralization is associated with the ~ 14 Ma magmatic pulse, and (iii) economic mineralization occurred at ~ 10 Ma and lasted until at least 9 Ma, as indicated by the presence of a younger mineralized porphyritic unit of this age. The obtained ages are comparable to those reported for the Los Pelambres deposit, a Cu-Mo porphyry located at 31°43′S and belonging to the Mio-Pliocene Metallogenic Belt of Central Chile. For the main intrusive units recognized in the Brahma prospect, whole rock geochemical analyses reveal arc affinities, juvenile signatures, and the involvement of amphibole in the fractionating assemblages. In a whole, our results support the existence of a complex deep magmatic reservoir that fed the magmatic-hydrothermal system for a long period, of at least 4 to 5 m.y., a common characteristic in other Miocene-Pliocene deposits in Chile and also in porphyry Cu deposits worldwide. In addition, our results highlight an interesting exploratory potential for the Brahma prospect, a long-lived Cu-Mo porphyry deposit of early Miocene age which southerly expands until ~ 36°S the Mio-Pliocene Metallogenic Belt of Central Chile.