Mineralogical and textural evolution of the Alvo 118 copper-bearing gossan: Implications for supergene metallogenesis in Carajás Mineral Province, Brazil

Abstract

The Carajás Mineral Province is home to one of the most extensive copper-gold belts globally, where hypogene orebodies were partially transformed into gossans. The Carajás Mountains host mature gossan profiles modified by lateritization. Conversely, the surrounding denuded landscape holds deep immature gossans characterized by generation mechanisms that remain poorly understood. Alvo 118 is a typical deposit from this area, with numerous drill cores available for evaluation. Mineralogical, textural, and mineral chemistry investigations showed that the hypogene mineralization, consisting of massive and disseminated chalcopyrite with minor nukundamite, was partially oxidized. The resulting gossan comprises a range of copper-bearing minerals, including malachite, pseudomalachite, cuprite, tenorite, native copper, ramsbeckite, chrysocolla, and libethenite, with relics of a secondary sulfide zone represented by chalcocite. The host rocks are relatively well-preserved from supergene alteration near the gossan due to the chalcopyrite's low acidifying potential and the deep occurrence of the gossan orebody, resulting in an immature profile. The gossan orebody is sectioned into four zones; the mineral succession of each zone indicates independent evolution, with a general transition from slightly acidic to alkaline conditions and increasing oxidation potential, with local oscillations related to water table fluctuations. This environment was established by the interaction of acid solutions derived from sulfide dissolution with gangue minerals (calcite and apatite) and secondarily with host rocks (chloritites and granodiorites), leading to system buffering. Ti, Mn, Zn, Ni, V, and Co were incorporated into the gossan mainly by goethite, while malachite contains Ba, Ca, Cr, Mn, and Zn. Gold and Pb were detected in goethite, cuprite, tenorite, and native copper. This set of mineralogical zones and mineral successions record an important and unique stage of the supergene evolution of the Carajás Mineral Province, with prominent copper-gold mineralization showing no effects from lateritization.