CORRELATING CU-SULFIDE AND AU MINERALIZATION IN THE ERTSBERG-GRASBERG DISTRICT USING LA-ICP-MS AND HRXCT

The Ertsberg-Grasberg district in Papua, Indonesia, hosts to two of the largest intrusion-related Cu-Au deposits in the world: the Ertsberg East Skarn system and the Grasberg Intrusive Complex. Cu mineralization within the Grasberg porphyry and Ertsberg skarn systems primarily consists of bornite and chalcopyrite, with minor digenite and idiate. Native Au is commonly found in association with Cu mineralization where Au occurs as inclusions within or immediately proximal to primary Cu-sulfide minerals. At hydrothermal-ore forming temperatures, approximately 400° to 700° C, bornite and chalcopyrite can host up to 1800 ppm Au within the Cu-sulfide lattice. Upon retrograde cooling of the hydrothermal system, the ability of bornite and chalcopyrite to host Au decreases significantly to about 10 ppm, indicating that the Au could be expulsed from the sulfide lattice. Given the close association of native Au and Cu-sulfide concentrations, it is possible that native gold grains form as the Au emerges from the Cu-sulfides. Constraining the genetic and spatio-temporal relationship between Cu-sulfide and Au mineralization within these deposits is of significant interest with regard to the geometallurgical processing of the ore, and to future exploration. This study seeks to evaluate this relationship using High Resolution X-ray Computed Tomography (HRXCT) and Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS). Previous HRXCT studies on Ertsberg-Grasberg ore samples have identified numerous occurrences of native Au grains at the edges of Cu-sulfide masses. HRXCT data are used here to construct 3D Voronoi regions of potential Au “diffusional drainage” from within the Cu-sulfides, where the expectation is a positive correlation between Au grain size and modified Voronoi polyhedron volume, defined as the volume of sulfide closer to that grain than any other via a connected path through sulfide. LA-ICP-MS data are used to determine variations in Au contents of Cu-sulfide minerals using 2D transects away from Au inclusions in 3D contact with Cu-sulfide minerals.