GOLD OCCURRENCES IN THE STOCKWORK MINERALIZATION IN THE ERTSBERG DIORITE, WEST PAPUA, INDONESIA

SEM/EDS analysis of samples from the gold-rich (>1 ppm Au) Ertsberg porphyry Cu-Au deposit demonstrate that important amounts gold were deposited in the stockwork ore zone by late stage sericitic fluids in addition to that deposited by early fluids associated with potassic alteration. Gold occurred in all three vein types studied: 1) early quartz-bornite ± chalcopyrite ± anhydrite veins, with potassic alteration (secondary biotite and K-feldspar), 2) quartz-bornite ± chalcopyrite veins with green sericite halos, and 3) late-stage pyrite ± chalcopyrite veins with broad, white sericite halos. Of the eight gold grains observed in the samples of the early-stage quartz-biotite-biotite veins (vein type 1), four grains were Au-Ag tellurides, one was a Au telluride, and three were electrum. Most gold mineral grains were between 1-3 microns, but one Au-Ag telluride grain was 20 microns. All of the electrum grains occurred on the margins of bornite grains, whereas the tellurides were associated with either bornite or chalcopyrite. Seven of the nine gold mineral grains observed in the green sericite veins (vein type 2), were electrum. Only one Au-Ag telluride grain and one native gold grain were observed in the green sericite vein samples. Gold mineral grains in the green sericite veins ranged in size from 2 to 6 microns. High fineness electrum grains (Au:Ag 20:1 to 40:1) were restricted on the margins of bornite grains associated with quartz, whereas lower-fineness electrum grains (Au:Ag 1:10 to 4:1) occurred exclusively as subequant blebs on and within chalcopyrite associated with orthoclase. Gold in the late-stage pyrite ± chalcopyrite-white sericite vein (vein type 3) occurred as electrum (2 grains) and one gold telluride. Electrum occurred with pyrite or chalcopyrite overgrowing pyrite, but the gold telluride grain was associated with the chalcopyrite overgrowths. All three gold mineral grains were located on the margins of sulfide grains, ranging from 5-20 microns.