ABSTRACT
The Igarapé Bahia lateritic gold deposit in the Carajàs region of the Amazon has developed on a primary Cu(Au) mineralization zone hosted by chlorite schists. The parent rocks of the chlorite schists include basalts, pyroclastic and clastic sedimentary rocks formed in an Archaean rift basin. The intense weathering resulted in accumulation of Fe-, Mn-, Al-oxides/hydroxides, and kaolinite in laterite profiles. The laterite profile is as much as 180 m thick, and from base to top, consists of saprolite, pallid, mottled, ferruginous zones and top soil. Gold orebodies mainly occur in the ferruginous zone.
Manganese oxides are present as a major constituent in some particular samples (MnO up to 18.06 wt%), although contents of MnO generally range from 0.14 to 5.86 wt %. Similar to Fe-oxides, these Mn-oxides also act as “scavengers” to control the geochemical behaviours of many elements, especially some of the first row transition metals, even gold, silver and rare earth elements. Electron microprobe analysis, SEM and XRD showed that the Mn-oxides are cryptomelane (a cryptomelane-hollandite solid solution) and lithiophorite. The former occurs in both upper and lower levels of the profile whereas the latter only in the lower levels (lower part of the mottled zone and the pallid zone). The cryptomelane in the upper levels occurs as micro crystallites or amorphous materials, showing that it is undergone leaching. By contrast, most Mn-oxides from the lower levels are well crystallized, where lithiophorite occurs as typical hexagonal plate crystals, and cryptomelane as needles or fabrics. In other reported laterite profiles, however, cryptomelane occurs near the unweathered rock, followed upward by nsutite, pyrolusite, a second generation of cryptomelane and then lithiophorite. The difference of Mn-oxides occurrence in this study is probably attributable to a two-stage evolution of the profile. The dissolution and formation of Mn-oxides are largely related to the water table change where an active oxidation/reduction horizon occurs. After the first stage of lateritization, an incision of the landscape with a well-developed drainage system resulted in the second stage evolution of the profile to form a leached pallid zone, and dissolution and precipitation of the Mn-oxides in the upper and lower levels, respectively.
