ABSTRACT

 

Platinum placer deposits associated with zoned ultramafic intrusions (Uralian-/Alaskan-type complexes with dunitic core and clinopyroxenite rim) are historically of great economic importance and are currently regaining in interest due to new discoveries in the Russian Far East. Heavy-mineral concentrates from five PGE-Au placers were studied for their magmatic (primary) platinum-group minerals (PGM) and their alteration (secondary) products. These placers are: Zolotaya, Fadeevka, and Kedrovka in the Paleozoic foldbelt of southern Primorye, and Darya and Kondyor in the Archean Aldan Shield, Siberian Platform, Khabarovskiy Krai. The placers are mostly derived from Paleozoic/Mesozoic multiple intrusion complexes of Uralian/Alaskan-type with late alkaline felsic intrusions.

The detailed PGM mineralogy of the Zolotaya and Fadeevka river placers is based on more than 150 hand-picked mineral grains from alluvial heavy-mineral concentrates. The dominant PGM phase is Pt-Fe alloy, with the remainder consisting of Os-Ir (+Pt+Ru) alloys. A broad spectrum of PGE-bearing sulfides, sulfarsenides, arsenides, tellurides, bismuthides, antimonides, and Au-bearing alloy occurs as inclusions or rims within/on the PGE alloy phases. Multiphase inclusion assemblages (predominantly sulfides) in the alloy matrix are of likely primary origin; whereas rims and replacement aggregates are formed by hydrothermal overprint. This hydrothermal overprint assemblage is represented by cooperite [PtS], sperrylite [PtAs2], kashinite [(Ir,Rh)2S3], (Rh,Pt)3S4 phase, cuprorhodsite-cuproiridsite-malanite group [CuRh2S4-CuIr2S4-CuPt2S4], minerals of the platarsite-hollingworthite-irarsite solid solution [Pt(As,S)2-Rh(As,S)2-Ir(As,S)2)], stumpflite [Pt(Sb,Bi)], mertieite-II [Pd8Sb3], Sb-bearing irarsite [IrAs(Sb)S], Ir(Pt)Te(As,Sb) phase, tolovkite [IrSbS], maslovite [PtTeBi], (Ir,Os)7As3, (Ir,Os)2As3, IrAsSb phase, and Au-Ag alloy. Geological, mineralogical and chemical observations point to a connection of the Zolotaya and Fadeevka placers to Late Permian dunite-hornblendite-gabbro intrusions of Uralian/Alaskan-type. Particularly, the composition of chromite from heavy-mineral concentrates and from inclusions within PGM from the Zolotaya placer is typical of Uralian/Alaskan-type intrusions, i.e., high chromium and iron content at relatively low titanium content.

First mineralogical and chemical data on a new PGM discovery at the Kedrovka river placer are based on samples from a field expedition in summer 2000. The preliminary examination of the PGM association shows the absence of Pt-Fe alloys and refractory PGE as major PGM phases, and a spinel composition which excludes an origin from Uralian-/Alaskan-type intrusions. The alluvial PGM of the Kedrovka River are probably derived from Paleozoic ophiolites.

The study of PGM occurrences in the Aldan Shield is focused on a particular mineralogical feature, i.e. the occurrence of exotic euhedral coarse-grained Pt-Fe alloy (up to several cm large) and other PGM, together with gold mineralization. The best documented example is the alluvial Kondyor PGE-Au deposit, located within a Mesozoic alkaline ultramafic ring complex. The Pt-Fe alloy macrocrystals have unusual trace-element composition, i.e. very low in PGE other than the main mineral component, and elevated contents of Sb and Sn, as derived from electron microprobe and reconnaissance proton microprobe analysis. The Pt-Fe alloy phases occur in association with a complex reaction mineral assemblage composed of Pd- and Pt-bearing antimonides, arsenides, bismuthides, tellurides, sulfides and intermetallic compounds with Au, Cu, Pb, Sn, Hg.

A heavy-mineral concentrate from the Darya river, 75 km SW of Kondyor, yielded a similar PGM assemblage, with the first occurrence of a macrocrystal assemblage composed of ferroan platinum, plus idiomorphic crystals (mm size) of cooperite and mertieite-II. The origin of the unique macrocrystals from the Kondyor and Darya deposits is little understood, and may be related to late-magmatic hydrous overprint.