Supergene neoformation of Pt-Ir-Fe-Ni alloys: multistage grains explain nugget formation in Ni-laterites

Ni-laterites from the Dominican Republic host rare but extremely platinum-group element (PGE)-rich chromitites (up to 17.5 ppm) without economic significance. These chromitites occur either included in saprolite (beneath the Mg discontinuity) or as ‘floating chromitites’ within limonite (above the M...

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Bibliographic Details
Published in:Mineralium deposita 2017-10, Vol.52 (7), p.1069-1083
Main Authors: Aiglsperger, Thomas, Proenza, Joaquín A., Font-Bardia, Mercè, Baurier-Aymat, Sandra, Galí, Salvador, Lewis, John F., Longo, Francisco
Format: Article
Language:English
Subjects:
Catalysts
Desulfurization
Discontinuity
Earth and Environmental Science
Earth Sciences
Floating
Geology
Grains
Growth
Iridium
Iron
Isometric
Laterites
Limonite
Mineral Resources
Mineralogy
Minerals
Multistage
Nanoparticles
Platinum
Platinum base alloys
Serpentinization
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Summary:Ni-laterites from the Dominican Republic host rare but extremely platinum-group element (PGE)-rich chromitites (up to 17.5 ppm) without economic significance. These chromitites occur either included in saprolite (beneath the Mg discontinuity) or as ‘floating chromitites’ within limonite (above the Mg discontinuity). Both chromitite types have similar iridium-group PGE (IPGE)-enriched chondrite normalized patterns; however, chromitites included in limonite show a pronounced positive Pt anomaly. Investigation of heavy mineral concentrates, obtained via hydroseparation techniques, led to the discovery of multistage PGE grains: (i) Os-Ru-Fe-(Ir) grains of porous appearance are overgrown by (ii) Ni-Fe-Ir and Ir-Fe-Ni-(Pt) phases which are overgrown by (iii) Pt-Ir-Fe-Ni mineral phases. Whereas Ir-dominated overgrowths prevail in chromitites from the saprolite, Pt-dominated overgrowths are observed within floating chromitites. The following formation model for multistage PGE grains is discussed: (i) hypogene platinum-group minerals (PGM) (e.g. laurite) are transformed to secondary PGM by desulphurization during serpentinization; (ii) at the stages of serpentinization and/or at the early stages of lateritization, Ir is mobilized and recrystallizes on porous surfaces of secondary PGM (serving as a natural catalyst) and (iii) at the late stages of lateritization, biogenic mediated neoformation (and accumulation) of Pt-Ir-Fe-Ni nanoparticles occurs. The evidence presented in this work demonstrates that in situ growth of Pt-Ir-Fe-Ni alloy nuggets of isometric symmetry is possible within Ni-laterites from the Dominican Republic.
ISSN:0026-4598
1432-1866
1432-1866
DOI:10.1007/s00126-016-0692-6