Hostname: page-component-78c5997874-m6dg7 Total loading time: 0 Render date: 2024-11-13T01:55:41.524Z Has data issue: false hasContentIssue false
  • English
  • Français

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

Published online by Cambridge University Press:  16 April 2018

John F. W. Bowles ,
Saioa Suárez ,
Hazel M. Prichard  and
Peter C. Fisher
John F. W. Bowles*
Affiliation:
School of Earth and Environmental Sciences, University of Manchester, Manchester M13 9PL, UK
Saioa Suárez
Affiliation:
School of Earth and Ocean Sciences, Main College. Park Place, Cardiff University, Cardiff, Wales CF10 3AT, UK Department of Mineralogy and Petrology, UPV/EHU, 48940 Leioa and Ikerbasque, 48011 Bilbao, Spain
Hazel M. Prichard
Affiliation:
School of Earth and Ocean Sciences, Main College. Park Place, Cardiff University, Cardiff, Wales CF10 3AT, UK
Peter C. Fisher
Affiliation:
School of Earth and Ocean Sciences, Main College. Park Place, Cardiff University, Cardiff, Wales CF10 3AT, UK
*
*E-mail: john.bowles@manchester.ac.uk
Get access Rights & Permissions [Opens in a new window]

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.

Keywords

platinum-group mineralsFreetown IntrusionSierra Leonealluvialinclusionsneoform
Type
Article
Information
Mineralogical Magazine , Volume 82 , Special Issue 3: Special Issue dedicated to the work and memory of Professor Hazel M. Prichard (1954–2017) , June 2018 , pp. 577 - 592
Copyright
Copyright © Mineralogical Society of Great Britain and Ireland 2018 

Access options

Get access to the full version of this content by using one of the access options below. (Log in options will check for institutional or personal access. Content may require purchase if you do not have access.)

Footnotes

Associate Editor: Iain McDonald

†

Deceased Jan 2017

This paper is published as part of a thematic set in memory of Professor Hazel M. Prichar

References

Augé, T. and Legendre, O. (1992) Pt-Fe Nuggets from alluvial deposits in eastern Madagascar. The Canadian Mineralogist, 30, 983–1004.Google Scholar
Bowles, J.F.W. (1986) The development of platinum-group minerals in laterites. Economic Geology, 81, 1278–1285.CrossRefGoogle Scholar
Bowles, J.F.W. (2000 a) A primary platinum occurrence in the Freetown Layered Intrusions, Sierra Leone. Mineralium Deposita, 35, 583–586.Google Scholar
Bowles, J.F.W. (2000 b) Prassoite, vysotskite and keithconnite from the Freetown Layered Complex, Sierra Leone. Mineralogy and Petrology, 68, 75–84.Google Scholar
Bowles, J.F.W., Giże, A.P., Vaughan, D.J., Norris, S.J. (1995) Organic controls on platinum-group element (PGE) solubility in soils: initial data. Chronique de la Recherche Minière, 520, 65–73.Google Scholar
Bowles, J.F.W., Prichard, H.M., Suárez, S. and Fisher, P.C. (2013) The first report of platinum-group minerals in magnetite-bearing gabbro, Freetown Layered Complex, Sierra Leone: occurrences and genesis. The Canadian Mineralogist, 51, 455–473.Google Scholar
Bowles, J.F.W., Suárez, S., Prichard, H.M. and Fisher, P.C. (2017) Weathering of PGE-sulfides and Pt-Fe alloys, in the Freetown Layered Complex, Sierra Leone. Mineralium Deposita, 52, 1127–1144.CrossRefGoogle Scholar
Bowles, J.F.W., Suárez, S., Prichard, H.M. and Fisher, P.C. (2018) The mineralogy, geochemistry and genesis of the alluvial platinum-group minerals of the Freetown Layered Complex, Sierra Leone. Mineralogical Magazine, 82(S1), S223–S246.Google Scholar
Campbell, G., MacLean, L., Reith, F., Brewe, D., Gordon, R.A. and Southam, G. (2018) Immobilisation of platinum by Cupriavidus metallidurans. Minerals, 8, 10, 1–18.Google Scholar
Cook, N.J., Ciobanu, C.L., Merkle, R.K.W. and Bernhardt, H.-J. (2002) Sobolevskite, Taimyrite, and Pt2CuFe (Tulameenite?) in complex massive Talnakhite Ore, Noril'sk Orefield, Russia. The Canadian Mineralogist, 40, 329–340.CrossRefGoogle Scholar
Hattori, K.H., Cabri, L.J. and Hart, S.R. (1991) Osmium isotope ratios of PGM grains associated with the Freetown Layered Complex, Sierra Leone, and their origin. Contributions to Mineralogy and Petrology, 109, 10–18.CrossRefGoogle Scholar
Kim, W.-S. and Chao, G.Y. (1991) Phase relations in the system Pd-Sb-Te. Canadian Mineralogist, 29, 401–409.Google Scholar
McCallum, M.E., Loucks, R.R., Carlson, R.R., Cooley, E.F. and Doerge, T.A. (1976) Platinum metals associated with hydrothermal copper ores of the New Rambler Mine, Medicine Bow Mountains, Wyoming. Economic Geology, 71, 1429–1450.CrossRefGoogle Scholar
Pollett, J.D. (1931) Platinum mining in Sierra Leone. Engineering and Mining World, 2, 747–748.Google Scholar
Pollett, J.D. (1951) The geology and mineral resources of Sierra Leone. Colonial Geology and Mineral Resources, 2, 3–28.Google Scholar
Reith, F., Zammit, C.M., Shar, S.S., Etschmann, B., Bottrill, R., Southam, G., Ta, C., Kilburn, M., Oberthür, T., Bail, A.S. and Brugger, J (2016) Biological role in the transformation of platinum-group mineral grains. Nature Geoscience, 9, 294–298.CrossRefGoogle Scholar
Stumpfl, E.F. and Tarkian, M. (1974) Vincentite, a new palladium mineral from south-east Borneo. Mineralogical Magazine, 39, 525–527.Google Scholar
Tarkian, M., Klaska, K.-H. and Stumpfl, E.F. (2002) New data on vincentite. The Canadian Mineralogist, 40, 457–461.Google Scholar
Zaccarini, F., Tredoux, M., Miller, D.E., Garuti, G., Aiglsperger, T. and Proenza, J.A. (2014) The occurrence of platinum-group element and gold minerals in the Bon Accord Ni-oxide body, South Africa. American Mineralogist, 99, 1774–1782.CrossRefGoogle Scholar