Mineralogia e geoquímica da zona de alteração do depósito de Salobo, Serra dos Carajás

Abstract

The Salobo 3A copper deposit, located in the Mineral Province of Carajás, State of Pará, is part of a set of sulphide copper occurrences, in a sequence of metasedimentary and matavolcanic rocks, which is more than 70 km long. The product resulting from the alteration, in an equatorial climate, of these mineralized rocks reaches a thickness of 70 to 100 m and presents as a particularity the absence of copper mineralization typical of oxidation zones, preserving, however, approximately, the copper contents of the primary ore. For this research, three profiles were selected for mineralogical and geochemical studies, aiming to identify the Cu support phases, as well as the behavior of some important elements (Ni, Co, Au, Ag. Mo) along the profiles. Mineralogical studies allowed us to conclude that the profiles studied have a low degree of maturity. As a result, it was only possible to characterize horizons in the profiles from the identification of the nature of the clay-minerals present, which were identified as: clay-minerals 15 Å (smectite, vermiculite), 10 Å (illite), 7 Å ( kaolinite) and interstratified (hydrobiotite). The data obtained allow us to conclude that: a) the most important copper support phases are the alteration products of biotites: hydrobiotite and vermiculite. b) Ni, after its release from primary minerals, seems to remain bound to smectite and in some cases to vermiculite, when at depth. Close to the surface or in more mature profiles, Ni seems to be associated with Fe and Mn oxyhydroxides. c) in the destabilization of ferromagnesian minerals, the released Co seems to remain in the profiles, absorbed on the Fe and Mn oxy-hydroxides. d) Au, probably originated from the most magnetic shales, shows to be associated with secondary Fe minerals, and may also be adsorbed in clays, close to the surface. e) Ag may be originally replacing part of the Cu in the chalcopyrite and, in the destabilization of this, it is associated mainly with Mn oxides and Fe hydroxides. f) Mo, originating from molybdenite, must be adsorbed on Fe oxyhydroxides or on clay minerals, requiring more detailed studies for more precise conclusions. Although lateritic enrichment in Cu in Salobo soils was expected, what was observed is that there is only temporary conservation of this element in the intermediate stages of alteration of biotites which, once altered to the point of phyllosilicates 1:1, with the maturation of the profile , will tend to lose this Cu, leached by the percolation waters.