Weathering of PGE sulfides and Pt–Fe alloys in the Freetown Layered Complex, Sierra Leone

Abstract

Fresh and weathered rocks and saprolite from Horizon B of the Freetown Layered Complex contain platinum-group minerals (PGM). The PGM in the fresh rocks are 1-7 μm across, including cooperite (PtS), isoferroplatinum (Pt 3 Fe), minor tetraferroplatinum (PtFe), tulameenite (Pt 2 FeCu), Os-bearing laurite (RuS 2 ), and other base metalsulfide (BMS)-bearing PGM. The weathered rocks contain fewer of those PGM but a high proportion of disordered Cu-(±Pd)-bearing Pt-Fe alloys. The saprolite hosts scarce, smaller (1-3 μm) ordered PtFe and disordered PtFe 3 . The PtFe alloys became increasingly Fe rich as weathering proceeded. Pt-Fe oxides appeared during weathering. Copper sulfides associated with the primary PGM and cooperite (with <3% Pd) were destroyed to provide the minor Cu and Pd found in some of the disordered Pt-Fe alloys. Platinum-and Pd-bearing saprolites have retained the original rock fabric and, to a depth of about 2 m, surround residual rocks that show progressive weathering (corestones). Ground water passing through the saprolite has transported Pt and Pd (and probably Au) in solution down slope into saprolite over unmineralized rocks. Transport is marked by changes in the Pt/Pd ratio indicating that the metals have moved independently. Palladium is present in marginally higher concentrations in the deeper saprolite than in the corestones suggesting some retention of Pd in the deeper saprolite. Platinum and Pd are less concentrated in the upper saprolite than the deeper saprolite indicating surface leaching. Alteration occurred over a long period in an organic and microbial rich environment that may have contributed to the leaching and transport of PGE.

C14–22 n-Alkanes in Soil from the Freetown Layered Intrusion, Sierra Leone: Products of Pt Catalytic Breakdown of Natural Longer Chain n-Alkanes?

“…The rocks beneath the sample site consist of a layered gabbroic sequence within which there is a PGE-enriched horizon containing small, weathered PGM [1, 2,26]. The PGM are disseminated with rare minute Cu-sulfides.…”

“…The humic and fulvic acid contents of a sample from that soil have been examined previously [12,13]. A comparison of the geochemistry of the rocks, saprolite, and soil has indicated that Pt, Pd and Au are mobile within the saprolite and soils [2]. The saprolite that covers the PGE-enriched horizon contains Pt, Pd, and Au.…”

“…Platinum-group minerals (PGM) have been located in these rocks and have shown to be altered by superficial weathering [1,2]. Alluvial PGM have been recovered from the streams draining the intrusion and they are notably different in mineral assemblage, composition, and size from the PGM in the unaltered host rocks [3,4].…”

<strong>C₁₄-₂₂ n-alkanes in Soil from the Freetown Layered Intrusion, Sierra Leone: Products of Pt Catalytic Breakdown of Natural Longer Chain n-alkanes?</strong>

“…Weathering of these rocks does not produce a gossan. Comparison of the geochemistry of the rocks, saprolite and soil has indicated that Pt, Pd and Au are mobile within the saprolite and soils [2].There is a shallow soil, normally about 10 cm in thickness and a sample from that soil has been examined previously [12,13]. The thin soil overlies saprolite derived from weathering of the layered rocks.…”

“…Platinum-group minerals (PGM) have been located in these rocks and shown to have been altered by superficial weathering [1,2]. Alluvial PGM have been recovered from the streams draining the intrusion and they are notably different in mineral assemblage, composition, and size from the PGM in the unaltered host rocks [3,4].…”