The Chapada Cu-Au deposit, Mara Rosa magmatic arc, Central Brazil: Constraints on the metallogenesis of a Neoproterozoic large porphyry-type deposit

摘要

The large Chapada Cu-Au deposit is situated in Goias Magmatic Arc, Central Brazil, which is part of an accretionary orogen with tectonic evolution between ca. 900 and 600 Ma during the Brasiliano Orogeny. With a current reserve (proven and probable) of 420.94 Mt Cu (0.29%) and 479.84 Mt Au (0.25 g/t) or 3.83 Moz Au, the Chapada Cu-Au deposit is an important example of a large-tonnage, low-grade porphyry copper-gold deposit of Precam-brian age, in which the mineralization is disseminated and hosted mainly by biotite-rich schists and gneisses. In this work, we propose a two-stage genetic model for the Chapada deposit that includes an early porphyry-type Cu-Au event overprinted by a shear zone-hosted Cu-Au event. The first mineralization event represents a rare case of metamorphosed Neoproterozoic porphyry-type deposit which preserves distinct hydrothermal alteration-mineralization zoning developed at the interface between equigranular (867 8 Ma) to porphyritic (884 15 Ma) medium- to high-Kcalc-alkaline dioritic rocks and mafic to intermediate metavolcanic and metapyrodastic rocks. The cm values are positive (from + 2.17 to + 5.39), thus indicating the juvenile nature of the original rocks for both metavolcano-sedimentary and plutonic rocks. In this context, kyanite-rich rocks are interpreted as products of metamorphosed argillic hydrothermal alteration halo and occur as linear alignments oriented parallel to the Mara Rosa magmatic arc trend. The potassic hydrothermal alteration halo, which includes the main host rocks of the Chapada deposit, is characterized by biotite schist, muscovite-biotite schist, amphibole-biotite schist and biotite gneiss. The ore is represented predominantly by platy deformed chalcopyrite and pyrite disseminations along foliation plans (or axial surfaces of folds) and, to a lesser extent, by small massive concentrations in fold hinges. The ore shows a systematic oxide/sulfide zonation, including the peripheral pyritic shell and the central magnetite-rich zone. In general, the geological context of the deposit lacks preserved stocks, dykes and breccias, while deformed A-type quartz veins/veinlets are abundant.