Depósito Cu-Au Visconde, Carajás (PA): geologia e alteração hidrotermal das rochas encaixantes

Abstract

Visconde Cu-Au deposit, Carajás (PA): geology and hydrothermal alteration of the host rocks. The Cu-Au Visconde deposit is located 15 km east of the Sossego mine (Canaã de Carajás, in the State of Pará) and it lies in the contact zone between the Itacaiúnas Supergroup (2.76 Ga) and the basement (> 3.0 Ga). The main rocks present in the deposit are the Serra Dourada granite, gabbros/quartzdiorites and rhyodacites, all variably altered and deformed. The Planalto Intrusive Suite has also been identified in the neighborhood, but it shows distinct alteration features. Mafic and felsic dikes crosscut the whole rock pile. The hydrothermal alteration has evolved, under rheological conditions that changed from ductile-brittle to brittle, from an early and pervasive calcic-sodic type (albite, scapolite or amphiboles) to a potassic one (K-feldspar and Cl-biotite), restoring, though locally, the sodic-calcic characteristics (albite, epidote, apatite, tourmaline and fluorite) to finally acquire a calcium-magnesium signature (clinochlore, actinolite, carbonates, and subordinate talc). In the Serra Dourada granite, albitization, epidotization, and tourmalinization were dominant in contrast with scapolitization, biotitization, amphibolitization and magnetitization in gabbros/quartzdiorites or K-feldspatization in rhyodacites. The main ore bodies are represented by chalcopyrite- and bornite-rich breccias and veins, besides chalcopyrite + pyrite ± molybdenite ± pentlandite disseminations. The basic metal suite is Fe-Cu-Au ± REE. Abundant sulfide was precipitated in the transition from potassic to calcic-magnesian alteration, along with different proportions of apatite, scapolite, actinolite, epidote, magnetite, tourmaline, calcite, gypsum, and fluorite. Metals were transported by hydrothermal fluids enriched in Na, Ca, K, Fe, and Mg, in addition to P, B, F, and S species. The similarities overcome the differences, so that the Visconde and Sossego deposits can be considered co-genetic.