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

Abstract

Inclusions 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.

The Fate of Platinum-Group Minerals in the Exogenic Environment—From Sulfide Ores via Oxidized Ores into Placers: Case Studies Bushveld Complex, South Africa, and Great Dyke, Zimbabwe

“…In evidence to the contrary, previous work [27,57] and the present review (e.g., Figure 3E,F) have shown that the (Pt,Pd)-bismuthotellurides and -tellurides are the PGM species most susceptible to oxidation/weathering and are unstable in the supergene environment. PGM of this mineral group and the equally unstable sulfarsenide inclusions have only survived oxidation as they are well-shielded by the enclosing Pt-Fe alloy grain, and the described multi-PGM assemblage [98] as well as their Pt-Fe alloy host are definitely not products of supergene formation. Concentration of rare and stray elements should follow a chemical gradient which is not in evidence in this environment.…”

“…In contrast, a secondary origin of PGM in placers and soils is anticipated by several authors [81,[91][92][93][94][95][96][97][98]. As size, shape, composition and micro texture of many eluvial and alluvial PGM differ from those observed in bedrocks and ores, it was proposed that secondary PGM come into being in a simplified process: (1) Serpentinization or weathering leads to the decomposition of base-metal sulfides carrying PGE in solid solution; (2) PGE are removed and transported as colloidal particles; (3) The colloids may coalesce or accrete to form larger particles and aggregates of PGE alloys.…”

“…Bowles and coworkers [93][94][95][96][97][98] studied alluvial PGM grains (mostly Pt-Fe alloys, laurite-erlichmanite, Os-Ir alloys) from Sierra Leone and reported contrasting features of the primary and placer PGM [97], a main reason for the proposal that the latter developed as a result of breakdown of the primary PGM during weathering, movement of the PGE in solution, and growth of new PGM in placers with a different mineral assemblage, mineralogy and mineral chemistry [97]. The processes postulated for PGM "neoformation" include [97]: (1) long term weathering in an area of warm climate and high rainfall;…”

“…Recently, a Pt-Fe alloy grain from the Freetown Layered Complex, Sierra Leone, containing numerous and diverse inclusions of laurite, irarsite-hollingworthite (IrAsS-RhAsS), Pd-Te-Bi-Sb phases, Ir-alloy, Os-alloy, Pd-bearing Au, a Rh-Te phase, Pd-Au alloy, and Pd-Pt-Cu alloy was presented [98]. This grain was interpreted to be "neoform" growths in the organic-and bacterial-rich soils of the tropical rain forest cover of the Freetown intrusion [98]. However, such an assemblage of PGM, typical of magmatic, high temperature ores, is highly improbable to form from dispersed PGE, As, S, Te, Bi, etc.…”

Introduction to the Special Issue dedicated to the work and memory of Professor Hazel Margaret Prichard (1954–2017)

“…One of these (McCreesh et al, 2018) concerns the 'Waterberg Project', currently under exploration by Platinum Group Metals Ltd in the northern limb of the ∼2.05 Ga Bushveld Complex (South Africa). The second is a detailed study of placer PGM nuggets derived from the ∼193 Ma Freetown Layered Complex, Sierra Leone (Bowles et al, 2018). Thirdly, Augé et al (2018) describe the mineralogy and geochemistry of PGM from mineralized horizons in the ∼200 Ma Conakry Igneous Complex (Guinea).…”