Abstract

The effects of weathering of platinum-group minerals (PGM) in the Limoeiro nickel-copper and platinum-group element [Ni-Cu-(PGE)] deposit reveal a progressive alteration of the abundant primary merenskyite, including neoformation of Pd-Cu±Bi rosettes and a rare I-rich PGM in the supergene reaction front of the deposit. Pyrrhotite with smaller equal proportions of chalcopyrite and pentlandite and minor PGM are typical of the fresh ore assemblage in the primary deposit. Weathering has produced a ∼3 m thick gossan cap over the base metal sulfide-rich parts of the Limoeiro primary ore. The mobility of Pt is very dependent on whether it is trapped in arsenide (sperrylite), which is very durable, or in bismuthotellurides (i.e., merenskyite–moncheite–melonite solid solution series), which break down more easily. The weathering over the Limoeiro deposit was not intense enough to overcome the pH- and redox-buffering capacity of the underling harzburgite and consequently Pd has not been leached, transported, or dispersed more than at a millimeter-scale. As a result, the Pt/Pd ratio of ∼0.4 remains roughly constant from the fresh ore at depth to the gossan on the surface. Weathering effects are apparent as deep as ∼50 m with the oxidation of pentlandite and pyrrhotite to produce violarite and pyrite, respectively. Chalcopyrite is more stable, beginning to alter at a depth of around 20 m. The PGM start to alter at a depth of ∼25 m where merenskyite becomes Pd-rich because much of the Te has been released into the adjacent goethite. With progressively more weathering towards the surface the merenskyite appears partially dissolved and pseudomorphed by goethite, which is often coated with newly formed Pd-Cu±Bi rosettes. A proportion of the remobilized Pd has precipitated as small (<4 μm diameter), euhedral to subhedral grains of a Pd-I-O compound. The formation of I-rich PGM is a consequence of the dominance of saline groundwaters in the Limoeiro area.

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