Platinun-group element dispersion associated with mafic and ultramafic rocks in Alaska

Abstract

Surficial dispersion of platinum-group elements (PGE) from 2 mafic and ultramahc complexes in Alaska has been investigated. Work comprised study of PGE occurrence in primary rock exposures and drainage channel media; results allow a model of baseline predictions of the effect of PGE primary dispersion and climate on PGE surficial dispersion to be tested. One outcrop area of the Tonsina complex (Bernard Mountain) contains typical ophiolite PGE mineralisation, dominated by discrete Ru-Os alloy and laurite (RuS:) inclusions in Cr spinel grains, associated with chromitite horizons in mantle sequence harzburgite and dunite. More rarely Pt-Pd-Cu alloys are found associated with pyrrhotite and chalcopyrite in websterite of the cumulate sequence. Another studied outcrop area (Dust Mountain) differs in having undergone a lesser degree of partial melting in the upper mantle, precluding podiform chromitite formation. Sulphide-associated Pt-Pd mineralisation in olivine clinopyroxenite cumulates has been magmatically upgraded by interaction with late-stage partial melts, forming high grade Pt-Pd mineralisation in association with thin chromitite bands. Serpentinisation has reduced most primary platinum-group mineral (PGM) sulphides to alloys. The alloy dominated PGM assemblage at Tonsina gives rise to coherent Pt-Pd dispersion haloes in overlying drainage. PGE surficial dispersion is controlled by primary PGE mineralogy and the outcrop extent of PGE mineralisation, greatest for the low grade sulphide-associated Pt-Pd mineralisation in Bernard Mountain websterite. Analyses of -63 /xm stream sediment fractions suggest that both Pt and Pd are removed from primary rock sources by simple mechanical derivation; the apparent dominance of mechanical processes in surficial dispersion, even where primary PGE mineralisation is sulphide-associated, is a function of the cold temperate climate. Stream profile plays a large part in Pt and Pd accumulation in sediments; mechanical accretion of Pt grains appears to proceed downstream from an unknown primary source of malleable Pt-bearing PGM. PGE primary dispersion in the Goodnews Bay zoned ultramafic complex is less well established; nevertheless isoferroplatinum nuggets from overlying drainage exhibit intergrowths with PGM sulpharsenides that are repeated in rare, much finer-grained tetraferroplatinum grains from chromitite chip samples. Intimate PGM intergrowths (e.g. Widmanstatten textures) in placer nuggets provide supporting evidence for simple mechanical derivation of PGM from primary source rocks. The most likely primary PGE source is metasomatically-altered chromitite-rich dunite, representing a totally eroded vertical extension of chromitite-rich dunite currently in exposure. Mechanical accretion of PGM grains in stream bedload seems unlikely, given the extremely durable nature of the most common PGM; isoferroplatinum (Pt3Fe), Ir-Os alloys and hollingworthite-irarsite (RhAsS-IrAsS). PGM nugget evolution is restricted to mechanical abrasion, fracturing along cleavages and crystal faces and plucking of individual PGM grains from coarser composite nuggets. Drainage geochemistry has definite potential in exploration for primary PGE deposits associated with mafic and ultramafic rocks in Alaska. Sampling procedures should be modified so as to maximise fines retention; Pt/Pd ratios in -63 /im sediment fractions can be used not only to differentiate sulphide- and chromitite- associated primary PGE mineralisation in ophiolite complexes, but also to distinguish between ophiolites and zoned ultramafic complexes.