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 (Pt3Fe), minor tetraferroplatinum (PtFe), tulameenite (Pt2FeCu), Os-bearing laurite (RuS2), and other base metal-sulfide (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 PtFe3. The Pt–Fe 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.
Highlights
The present study investigates the sequence of mineralogical modifications undergone by the primary PGEmineralization during weathering and erosion with a systematic survey of the Platinum-group minerals (PGM) in: (i) fresh host rocks from the most PGE-enriched and best exposed igneous horizon, Horizon B, enlarging on the study by Bowles et al (2013), (ii) adjacent weathered rock, and (iii) saprolite resulting from the in situ weathering of the horizons
The present study focuses on fresh and weathered rock and saprolite samples all taken from a trench (Fig. 2a) dug across Horizon B to examine how the PGM alter during weathering
This study has extended our understanding of the occurrence of PGM in fresh rocks from the PGE-enriched layer known as Horizon B in the Freetown Igneous Complex and the alteration of the PGM assemblage during the weathering of the rock to form saprolite
Summary
Platinum-group minerals (PGM) have been known in eluvial and alluvial deposits from the Freetown Layered Intrusion in Sierra Leone, West Africa, since 1926 and have been extracted from streams draining westwards into the Atlantic (Junner 1929, 1930; Junner and Harwood 1930; Pollett 1931, 1951). Bowles (2000b) has demonstrated the existence of PGE-bearing layers (up to 0.69 g/t Pt; Fig. 1) describing, for the first time, stratiform primary sources for the PGM These layers consist of magnetite-bearing gabbros that contain PGE mineralization associated with minor Cu-rich and Ni-poor sulfides (Bowles et al 2013). The PGE-bearing Horizon B occurs on a ridge formed by resistant pyroxenites that runs south from Mateki Hill which is inland from the village of York (Fig. 1) This horizon has been traced for over 5 km, with a central section hidden by faulting, and it has consistent PGE assays along strike (Bowles 2000b). There is a fresh to partially altered core with an outer weathered zone that is surrounded by
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