THE MINERALOGICAL DISTRIBUTION OF GOLD AND RELATIVE TIMING OF GOLD MINERALIZATION IN 2 ARCHEAN SETTINGS OF HIGH METAMORPHIC GRADE IN AUSTRALIA

Gold in Archean lode-gold deposits at Mt. York (Pilbara Craton) and Gr iffin's Find (Yilgarn Craton), in Western Australia, located in amphib olite- and lower granulite-facies domains, respectively, is associated with composite arsenopyrite - lollingite grains. In combination, refl ected-light microscopy, scanning electron microscopy (SEM), and second ary-ion mass spectrometry (SIMS) reveal the mineralogical distribution of gold and the timing of gold mineralization relative to the peak of metamorphism. In both deposits, composite arsenopyrite - lollingite g rains are distinctly zoned, with arsenopyrite generally rimming a core of lollingite. Native gold, where present, is preferentially located at grain boundaries between arsenopyrite and lollingite. SIMS analysis demonstrates that the majority of the gold occurs as ''invisible'' go ld in lollingite, whereas arsenopyrite is essentially free of such gol d. Textural relationships confirm that gold was precipitated together with lollingite, broadly synchronously with the peak of metamorphism i n both lode-gold deposits. The gold was concentrated preferentially at the arsenopyrite lollingite grain boundaries during replacement of lo llingite by arsenopyrite, under slightly retrograde metamorphic condit ions, apparently due to the inability of the arsenopyrite structure to accommodate the gold at these still relatively high temperatures. Thi s study demonstrates that gold was initially coprecipitated with lolli ngite under high P-T conditions and that the particulate (visible) gol d, alone, does not indicate the true paragenetic position of initial d eposition of gold in these deposits.