Platinum-group minerals from five placer deposits in British Columbia, Canada
Platinum-group minerals from five placer deposits in British Columbia, Canada (in S (super 3) ; sulfides, structures, and synchrotron light; a tribute to Michael E. Fleet, Grant S. Henderson (editor), Yuanming Pan (editor) and Robert F. Martin (editor))
The Canadian Mineralogist (October 2005) 43, Part 5: 1687-1710
- alloys
- arsenides
- British Columbia
- Canada
- chemical composition
- cooperite
- dunite
- electron probe data
- formula
- heavy mineral deposits
- heavy minerals
- igneous rocks
- intrusions
- iridium
- iron
- metal ores
- metals
- osmium
- peridotites
- placers
- platinum
- platinum group
- platinum minerals
- platinum ores
- plutonic rocks
- ring complexes
- ruthenium
- SEM data
- sperrylite
- sulfides
- ultramafics
- Western Canada
- zoning
- iridosmine
- tulameenite
- platarsite
- cuproiridsite
- alaskan-type intrusions
The PGM from a selected suite of Au- and PGE placer deposits in British Columbia have been analysed by EPMA. The PGM placer grains (0.1-1.5 mm) mainly comprise various Pt-Fe-(Cu) alloys, 'Pt (sub 3) Fe'-type alloys, Fe-rich Pt with an atomic Sigma (PGE):(Fe + Cu + Ni) ratio of 3.6-5.6, '(Pt,Ir) (sub 2) Fe'-type alloy, members of the tulameenite-tetraferroplatinum solid-solution series, less common Ir-dominant Ir-Os-(Ru-Pt) alloys, subordinate Os-dominant alloys and minor Ru-rich alloys and rutheniridosmine, the latter with an atomic Ir:Os:Ru proportion close to 1:1:1. Trace amounts of PGE sulphides and sulpharsenides: cooperite, Ni-rich cuproiridsite, unusual sperrylite-type and platarsite-type phases crystallized at a late stage in low-S environments. Some PGM grains contain micro-inclusions of diopside, augite, ferro-edenite, a potassian sodic-calcic amphibole, talc, clinochlore and euhedral quartz; high mg values in the ferromagnesian minerals of these inclusions suggest highly magnesian source rocks. Textural and compositional data, in particular the the zoned intergrowths of Pt-Fe-(Cu) alloy grains, which broadly resemble the the zoned Pt-Pd-Cu stannides from the Noril'sk complex, indicate a sequence of crystallization: (Pt,Ir,Rh) (sub 3) Fe --> (Pt,Ir,Rh) (sub 1+x) (Fe,Cu) (sub 1-x) or Pt (sub 3) Fe(Cu,Ni). The zoned Pt-Fe-Cu alloys probably formed by fractional crystallization of primary solid solutions under closed-system conditions of the Cu-poor core. The Cu-rich Pt-Fe alloys formed around these core zones and at their peripheries during a significant drop in temperature at a late stage of crystallization of the composite alloy grains. The compositions of micro-inclusions and exsolution lamellae of Os- and Ir-dominant alloys in Pt-Fe alloys imply uniform temperatures of equilibration within the range 750-800 degrees C. The preservation of faceted morphologies on many of the placer PGM grains implies a relatively short distance of transport from their source.
