Osmium Isotope Heterogeneity in Os-Ir-Ru Inclusions within Platinum-Iron Alloy Placer Grains from the Freetown Layered Complex; Sierra Leone

Abstract

The measurement of osmium isotope ratios has been used in previous studies to evaluate the origin and mode of formation of platinum-group element mineralisation. In particular, where platinum-group mineral (PGM) grains occur as placer deposits in areas of the Cordilleran mountain chains of the USA and Canada, in Kalimantan, and in Sierra Leone, osmium isotopes have provided useful insights into their possible origins. (Bowles et al., submitted., Lyon et al., 1997; Hattori and Cabri, 1992) In the present work, the genesis of PGM grains in placers associated with the Freetown igneous complex, Sierra Leone, is being investigated. The Freetown intrusion is a roughly circular body approximately 30 km in diameter. It consists of five main cycles (zones) of troctolite, olivine-gabbro, norite, gabbro and anorthosite, four of which are exposed on the Freetown Peninsula and the fifth seen only in a borehole. Approximately one third of the intrusion is exposed on land with the four outcropping zones forming four arcuate ranges of hills on the Freetown Peninsula. The PGM grains are found in soils, and in sediments in, or near, the rivers and streams draining the area. The grains are mostly 1-2 mm in diameter, with larger grains known to occur. The grains have been reported as exhibiting both euhedral and dendritic growth habits. (Bowles, 1995). The main platinum-group minerals found in this locality occur as: (1) pyritohedra of erlichmanite (OsS2) and (2) platinum-iron alloy grains (Pt3Fe) with inclusions of Os-Ir-Ru alloy, erlichmanite (OsS2), laurite (RuS2), tulameenite (CuFePt2) and cuprorhodsite (CuRh2S4). The previously reported osmium isotope ratios measured on the Os-lr-Ru phases within Pt3Fe grains from Sierra Leone are within the range 0.15 0.25, when converted to equivalent 187Os/188Os ratios (Hattori, et al., 1991). This variation was determined between inclusions from separate grains, not between inclusions within a single grain, and has been variously interpreted as resulting from magrnatic, metasomatic or supergene processes. The present study also concentrates on the Os-IrRu alloys within Pt3Fe grains, the emphasis being on collection of data from Pt3Fe grains containing more than one Os-Ir-Ru inclusion to establish whether or not there is any detectable variation of the 187Os/188Os ratios within and between individual inclusions.