APPLICATION OF VEIN ABUNDANCE STUDIES IN EXPLORATION IN THE ERTSBERG PORPHYRY DEPOSIT, WEST PAPUA, INDONESIA

Quantitative vein abundance measurements of drill core can delineate exploration vectors in the diorite of the Ertsberg porphyry deposit and are most useful in bodies of rock that have marginally anomalous copper and gold assays. Within the Ertsberg porphyry deposit, copper and gold grades correlated with the abundance of: 1) bornite-anhydrite veins with biotite alteration, and 2) bornite-quartz ± chalcopyrite veins with green sericite alteration, but do not correlate with 3) pyrite ± chalcopyrite veins with white sericite alteration, 4) magnetite-actinolite veins with propylitic alteration, or 5) sulfide-poor quartz veins associated with biotite-K-feldspar alteration. Although copper and gold grades had erratic spatial distribution outside the ore zone, overall total vein abundance (n) and the abundances of vein types 1 and 5 and total vein abundance decrease systematically laterally away from the ore zone. Vein abundance is highest both within the ore zone and in low-grade rocks underlying the ore zone, making vein abundance values a measure of proximity to the center of mineralization, irrespective of grade. The region with high vein abundance also contains the most intensely biotite-altered rock and the most dikes of magmatic biotite-rich diorite in the study area. Very high vein abundance values in low-grade rocks beneath the ore zone are due primarily to the presence of barren quartz and K-feldspar veins. High vein abundances laterally adjacent to the ore zone are due to the presence of barren hairline biotite veins. Late stage veins (types 2 and 3) characterized by sericitic alteration styles have a more erratic distribution due to clustering of such veins. Late stage veins occur both within and peripheral to the ore zone. Clustering of sericitic veins, however, makes vein abundance data for late-stage veins a weaker indicator of exploration vectors. Regions cut by swarms of late-stage veins, however, may suggest mineralization at depth. Changes in the nature and distribution of veins through time have important implications on the way porphyry systems cool, the extents of fluid flow paths, and the distribution and redistribution of metals.