PHYSICOCHEMICAL CONDITIONS OF FLUID WALL ROCK INTERACTION AT AMPHIBOLITE-FACIES CONDITIONS IN 2 ARCHEAN HYDROTHERMAL GOLD DEPOSITS IN THE MT-YORK DISTRICT, PILBARA-CRATON, WESTERN-AUSTRALIA

Synamphibolite facies Archean gold mineralization in the Mt. York Dist rict, Pilbara Craton, Western Australia, is hosted in metamorphosed ba nded iron formation (Main Hill - Breccia Hill prospect), amphibolites, and ultramafic schists (Zakanaka prospect). Mineralization at Main Hi ll occurs in quartz breccias with sulfide matrices and in altered wall rock adjacent to quartz-biotite-amphibole+/-clinopyroxene veins. Alte ration associated with quartz veins is zoned, with biotite-pyrrhotite vein selvedges and a distal calcic-amphibole, arsenopyrite - lollingit e zone. Hydrothermal biotite and actinolite have highest Mg/(Mg + Fe) ratios where associated with abundant sulfarsenides in the distal alte ration zone. Whole-rock geochemical analyses and calculated metasomati c reactions indicate the addition of K, Al, S, As, Au, Ag, and Ni duri ng hydrothermal alteration. Mineralization at Zakanaka is characterize d by a broad wall rock alteration halo of biotite-amphibole, and zoned quartz-calc silicate veins proximal to ore. Wall rock adjacent to the veins contains pyrrhotite, pyrite, and gold. The alteration is explai ned by K-metasomatism distal to mineralization and K and Ca metasomati sm proximal to mineralization. Balanced metasomatic reactions and mass -balance calculations indicate addition of K and depletion of Na, Ca, Mg, and Fe in distal alteration zones and addition of K, Ca, Mg, Fe, a nd Ti in proximal zones. Gold precipitation at both prospects occurred through loss of S, and possibly As, from the ore fluid during sulfida tion reactions with Fe-rich amphiboles and biotites to form Mg-enriche d equivalents and sulfarsenides. Changes in the oxidation state of the ore fluid may have enhanced gold precipitation, though pH changes are unlikely to have been important. The controls on mineralization are t hus similar to those at many lower temperature, mesothermal deposits. The lack of consistently increasing Mg ratios of calc-silicate phases with increasing intensity of alteration and sulfidation at Main Hill m ay be the result of coupled substitutions in amphiboles and biotites d uring infiltration of a fluid with high-S, but low-As, activities.