MINERALIZAÇÃO E ALTERAÇÃO HIDROTERMAL NO DEPÓSITO CUPRO-AURÍFERO DE SERRA VERDE, PROVÍNCIA MINERAL DE CARAJÁS

Abstract

Among known cupro-auriferous ore deposits of the Itacaiunas shear belt (Carajás Mineral Province), the Serra Verde deposit is the only one that is iron oxide-poor. Located halfway between the cities of Curionópolis and Parauapebas, it is hosted by the Archean Rio Novo Group and consists mainly of two massive sulfide lenses that are concordant to subconcordant with the stratigraphy of the metavolcano-sedimentary host pile. These metric lenses have been thermally metamorphosed by the Estrela granitic complex (EGC), dated at 2.763 Ga, and locally deformed by shear zones. The ore is composed of chalcopyrite with lesser amounts of pyrrhotite, pyrite, cubanite, molybdenite, sphalerite and mackinawite, in this order of abundance. Gold occurs mostly within sulfide grains but its contents are generally below 1 ppm. Metals with more expressive concentrations are silver (8-11 ppm), cobalt (243-935 ppm) and nickel (773-2623 ppm). Rare-earth element totals are up to 42 ppm. Quartz, Mg-hornblende, fluorapatite and ilmenite are the most common gangue minerals. In the deposit area and surroundings three major hydrothermal events have been recorded. The oldest event (I) was contemporaneous with the formation of the host rocks in a submarine environment. The other two are related respectively to the emplacement of the EGC (event II) and the development of shear zones (event III). Successive fluid circulation caused the host rocks to be hydrothermally altered to mineral assemblages that characterize propylitization, silicification, tourmalinization and amphibolitization, in addition to local potassification and albitization. Mineralogical and chemical data allow to estimate at 480-300o C and 270- 200o C, respectively, the prevalent temperature intervals of the events II and III. The massive nature of the sulfide ore, the concordant to subconcordant attitude of the lenses in relation to the host rocks, the temporal relationships between the lenses and the EGC, the occurrence of stratiform tourmalinite beds associated with the lenses, and the types of hydrothermal alteration are consistent with a volcanogenic massive sulfide typology. Geochronological data, albeit preliminary, corroborate this interpretation since they point to similar ages for both the mineralization and host rocks. On the other hand, the overwhelming dominance of chalcopyrite over iron sulfides and the uncommon occurrence of fluorapatite and ilmenite do not fit well the proposed model. A compromising hypothesis is to envisage a hydrothermal system in which fluids rich in Cu, P and Ti, derived from a hidden magmatic source, mixed with sea-water and debouched on to the seafloor where sulfides + Au were deposited.