Abstract

Two gossans developed from separate ore shoots at the Perseverance nickel sulfide deposit are notable for their different textures and compositions. The gossan developed from matrix sulfides in serpentinized ultramafic rock has high Ni and Cu contents and exhibits good retention of sulfide textures. The second gossan, developed from massive sulfides enclosed in metasedimentary rock, has high Cu but low Ni contents, and sulfide textures have been obliterated.Gossan-forming processes were simulated in electrochemical reaction cells using electrodes of sulfide ore assemblages, both with and without gangue minerals. Results of these experiments, together with results from studies of the adsorption of Ni and Cu onto goethite, were used to develop a model for the chemical processes involved in the weathering of sulfide ore to gossan. This model explains the different characteristics of the gossans in the two shoots. In the first gossan, the pH has apparently been buffered at a value in excess of 7 by carbonates and mafic gangue minerals, resulting in greater retention of Ni than in the second gossan, which lacks appreciable amounts of carbonates and mafic minerals. Cu, however, is catalytically involved in the goethite-forming process and is adsorbed onto goethite at a lower pH than is Ni, with the result that the Cu content is high in both gossans. Retention of sulfide pseudomorphs in the first gossan is attributed to silicification which, in turn, appears to be related to the low Al content of the ore from which the gossan was developed.

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