Estudo do comportamento do ouro nas formações supergênicas da área Cachoeira, nordeste do Pará

Abstract

In the Cachoeira area, Gurupi region, northeast of Pará, there is a volcano-sedimentary sequence composed predominantly of strongly mylonitized dacites and rhyodacites, cut by bundles of quartz veins and venules from NS to NNE-SSW direction. In this sequence, gold is present in the form of fine “free” particles, disseminated both in the veins and in the heavily carbonated host rocks, as well as associated with a sulfide paragenesis consisting of pyrite, arsenopyrite, chalcopyrite and sphalerite. Over geological time, several surfaces marked by an old phosphate laterite, an immature ferruginous laterite and a level of ferruginous sandstone, respectively, developed. Furthermore, restricted-range gossans were formed early in this supergenic framework. In this work, only the most recent lateritic cover and the gossanic bodies were studied. Laterite comprises a thick saprolithic horizon, a mottled zone and a 3-meter-thick crust composed of hematite, goethite, kaolinite and generally subordinate quartz. In this alteration mantle, both gold and silver, although “invisible”, show an enrichment in the upper part of the profile and in particular in the crust. The trace elements Zn, Mn, Ni, Co, Cr, Cu and As, in turn, show higher contents in the mottled zone or at the top of the saprolite. The gossanic bodies, which mark the zones most enriched in sulphides, are composed essentially of goethite with microcrystalline quartz, kaolinite and sericite subordinated. In addition to fragments of veins. Gold, silver and arsenic show a tendency to enrich themselves in depth. In areas with higher levels of gold, it usually appears in fine visible particles or even tiny nuggets. On the other hand, the contents of Mn, Co, Zn and Pb suffer little variation in the studied profiles, while Ni and Cr show an enrichment at the top of the latter. It is assumed that during the formation of the lateritic cover, gold and silver suffered an incipient molybization, probably in the form of chlorides and/or organic complexes, whereas in the gossanic medium these elements migrated preferentially in the form of thiosulfates. In both cases the gold and silver would have been fixed by the iron hydroxide by adsorption and/or coprecipitation. Regarding the other trace elements, it is believed that their mobilization occurred mainly in the form of sulfates and according to the physical-chemical conditions of the medium. Its redistribution and fixation in the alteration products was probably controlled by its retention by poorly crystallized iron hydroxide.