LATERAL ZONATION OF GOLD MINERALOGY IN THE GRASBERG PORPHYRY CU-AU DEPOSIT, INDONESIA

Gold grades in the Grasberg porphyry deposit (West Papua) decline sharply outward from a central high grade zone, in contrast to a more gradual outward decrease in Cu grades. Using an ESEM in gaseous mode, we examined uncoated, polished thin sections of veined samples from core drilled horizontally from the center of the deposit. We analyzed the compositions of 109 Au-bearing grains with EDS and examined all samples with a reflected and transmitted light petrographic microscope.

In the central high grade zone (78-110 m from the deposit center), Au only occurred as gold (x̅ = 8.9 μm) in bn-cpy-mt-qtz-Kfsp-bio veins. Coexisting hypersaline fluid inclusions with multiple daughter crystals and vapor-dominated inclusions characterized this quartz. Gold occurred most commonly in bornite (62%), but also in mt, qtz, Kfsp (33%), and chalcopyrite (5%).

In the intermediate zone (123-205 m), Au occurred as both gold and electrum in bn-cpy-anhy-mt-qtz-Kfsp-bio veins. Gold (x̅ = 3.5 μm) occurred in bornite (45%), Kfsp, anhy, qtz (36%), and chalcopyrite (19%). Electrum accounted for 10-18% of Au-bearing grains (x̅ = 2.5 μm, 33 wt% Ag) and occurred more frequently in chalcopyrite (57%) than bornite (43%).

In the distal zone (229 m), where pyrite and 30% sericitic alteration of feldspars overprinted early cpy>>bn-qtz-anhy-plag-bio veins, Au occurred as gold, electrum, and telluride. Gold (x̅ = 4.0 μm) accounted for 95% of Au in this zone, primarily with bornite (69%), but also in chalcopyrite (25%), and Kfsp, anhy, qtz (6%). This sample contained one electrum grain (1.4 μm, 26 wt% Ag) and one Au telluride (2.4 μm), both in bornite.

Overall, 54% of gold grains occurred in bornite. Calculated relative masses of gold and enclosing bornite ranged from 3,000-18,000 ppm. Eight of the 45 grain pairs exceeded the experiment solubility of Au in bornite (Simon et al., 2000), suggesting at least some bornite-associated gold precipitated from gold-saturated solutions rather than in a bornite solid solution.

The steep decline in Au grade probably reflects a limit of fractures mineralized by hypersaline brines, whereas the gradual decrease in Cu grades reflects a lateral transition to abundant pyrite-chalcopyrite veins with sericitic alteration. Gold in the distal, low grade zone was mineralogically similar to the high grade zone, but veins were sparser.