Inclusions in an isoferroplatinum nugget from the Freetown Layered Complex, Sierra Leone

ABSTRACTInclusions of platinum-group minerals (PGM) within alluvial isoferroplatinum nuggets from the Freetown Peninsula, Sierra Leone, are aligned with their shape determined by the structure of their host. The edges of the majority of the inclusions lie at 0°, 45° or 90° to external crystal edges of the nugget which shows that the inclusions are not randomly oriented earlier minerals incorporated within their host. The inclusions are later infills, probably formed at the surface of the nugget during growth and subsequently enclosed by the growing nugget. PGM on the present surface of the nugget represent the last stage of this partnership. A single nugget containing abundant inclusions is described here but similar features are observed in other nuggets from the same area. The inclusions contain laurite (RuS2), irarsite–hollingworthite (IrAsS–RhAsS), Pd–Te–Bi–Sb phases, Ir-alloy, Os-alloy, Pd-bearing Au, an Rh–Te phase, Pd–Au alloy and Pd–Pt–Cu alloy. PGM found on the nugget surface include laurite, irarsite and cuprorhodsite (CuRh2S4). The Pd–Te–Bi–Sb phases may include Sb-rich keithconnite (Pd20S7) and compositions close to the kotulskite–sobolevskite solid-solution series (PdTe–BiTe). Textural evidence suggests that formation of the nuggets began with the isoferroplatinum host and the voids were filled starting intergrowths of laurite and irarsite–hollingworthite with both laurite and irarsite–hollingworthite often showing compositional zonation and each of them replacing the other. Filling of the voids probably continued with Pd-Cu-bearing gold, Sb-rich keithconnite (Pd,Pt)20.06(Te,Sb,Bi)6.94, keithconnite, telluropalladinite Pd9(Te,Bi)4, RhTe and finally Ir-alloy and then Os-alloy. The nuggets are thought to be neoform growths in the organic- and bacterial-rich soils of the tropical rain forest cover of the Freetown intrusion. The mineralogical assemblage in the layered gabbros of the intrusion has been previously shown to differ from the alluvial assemblage in the rivers and these inclusions, not seen in Pt3Fe in the unaltered rocks, add a further item to the catalogue of differences.