Abstract
The weathering mantle of the Salobo deposit was developed upon metagraywackes with interbedded lens of iron formations. The profile is of the truncated type and constituted, from the base to the top, of a transition horizon, a saprolitic horizon and colluvial deposits. The two former horizons show lateral, textural and mineralogical variations which reflect the heterogeneous nature of the parent rock. Thus, alteration of the barren graywackes resulted essentially in the formation of a saprolite rich in clay minerais (hydrobiotite, smectite, kaolinite). On the other hand, weathering of the iron formations, host-rocks of copper sulfides (bornite, calcocite, chalcopyrite), originated a gossan type profile composed from the base to the top of a supergene sulfide zone, an oxidized minerais zone and a ferruginous saprolite. The supergene sulfides are digenite, covellite and bornite (?) while cuprite, malachite, azurite and native copper constitute the oxidized copper assemblage. Within the saprolite, smectite and goethite are the main copper enriched minerais. The evolution of such supergene setting involved oxidization and dissolution of primary sulfides with mobilization of Cu and Fe as soluble sulfatos. The downward and lateral transfer of these metais caused the formation of new mineral associations according to the Cu/S ratio in the solutions and the ambient pH and Eh conditions.In the reducing zone the ionic exchange between copper sulfate and primary sulfides yielded mainly digenite and covellite, while in the water-table fluctuation zone and immediately above, in more oxidizing conditions, copper precipitated as cuprite and native metal. Besides, reaction of copper sulfate with calcite from veins that cuts iron formations resulted in the fixation of Cu as malachite and azurite in neutral to alkaline conditions. In the Carajas region, the lateritic mantle was formed during the Lower Tertiary, but, during the Upper Tertiary and Quaternary, underwent dissection due to regional uplift. Thus, in the Salobo area, the duricrust and the upper saprolite were removed. Each phase of lowering of the hydrostatic levei resulted in the deepenning of the weathering profile and alteration, in oxidizing conditions, of previously formed supergene oxides and sulfides. One parte of the released copper then underwent downward leaching and reprecipitation in deeper reconstituted cementation and oxidation zones, while another part was fixed by neoformed goethite and clay-minerals.
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