PGE MINERALIZATION IN ARCHEAN VOLCANIC SYSTEMS - GEOCHEMICAL EVIDENCEFROM THICK, DIFFERENTIATED MAFIC-ULTRAMAFIC FLOWS, ABITIBI GREENSTONE-BELT, ONTARIO, AND IMPLICATIONS FOR EXPLORATION

PGE-rich disseminated zones with discrete platinum-group minerals (Pd, Pt and Ph mineral phases) have been discovered in three thick (80-130 m), differentiated (peridotite-gabbro) mafic-ultramafic flows of the Archean Abitibi greenstone belt, Ontario. Three mineralization zones ( whole-rock Sigma PGE + Au = up to 1000 ppb) occur along four stratigra phic cross sections through a 2 km strike-length of the Boston Creek F low ferropicritic basalt. Their occurrence most strikingly correlates with lenticular-podiform concentrations of disseminated chalcopyrite ( 1%) and clinopyroxene + interstitial mag netite-ilmenite intergrowths (15-20% oxide), high concentrations of related metals (3000 ppm Cu, 30 00 ppm S, 1200 ppb Ag, and 1000 ppm V), strong PGE depletion in adjace nt rocks and along strike, and lithological and textural complexity in the margins of the central gabbro-diorite layer. The mineralization z one (whole-rock Ir + Pt + Pd + Au = 110 ppb) within Thee's Flow tholei itic basalt is somewhat similar in occurrence, style, and composition to those within the Boston Creek Flow. In contrast, the mineralization zone (whole-rock Ir + Pt + Pd + Au = 340 ppb) in Fred's Flow komatiit ic basalt most strikingly correlates with vesicle-filling intergrowths of pyrrhotite + pentlandite +/- chalcopyrite (2 modal %) and high who le-rock concentrations of Ni (2500 ppm), Cu (700 ppm), and S (1.1%) in the upper chilled margin of the flow. Although apparently uneconomic, these flow-hosted PGE mineralization zones are of interest in explora tion, because they an more similar in stratigraphic setting, style, an d composition to PGE-rich disseminated Fe-Cu sulfide mineralization zo nes within thick differentiated intrusions than to mineralization zone s in other Archean volcanic rocks. The characteristics of the minerali zation zones and their host rocks, especially high degrees of PGE enri chment, vertical and horizontal patterns of PGE depletion, and accumul ation of clinopyroxene + magnetite-ilmenite intergrowths, indicate a c ritical genetic role for variations in the regime of melt flowage. The mineralization zones in the Boston Creek and Thee's Flows are interpr eted to have formed by simultaneous in situ formation of PGE-rich Fe-C u sulfide and Fe-Ti oxide from flowing silicate liquid in the margins of internal lava channels. The mineralization zone in Fred's Flow is i nterpreted to have formed by pending and coalescence of PGE-enriched s ulfurous vapor bubbles in the upper chilled margin during olivine accu mulation on the base of a dynamic lava channel. The relative abundance of PGE mineralization zones and high degree of PGE enrichment in the Boston Creek Flow suggest that the most favorable exploration targets are rocks crystallized from late-stage, highly fractionated derivative liquids in large differentiated ferropicritic units.