Geology Controls Grade Variance at Grasberg Cu-Au Porphyry Deposit, Papua-Indonesia

The Grasberg mining district is located in the core of the highlands of West New Guinea island, in the central ranges of the Papua Province of Indonesia. The collision between Australia Plate and Pacific Plate on Late Miocene generate NW-SE compressional structures orientation that produced reactivation of NE striking dilation to control the multiple series intrusion of host to Cu-Au porphyry deposit (Sapiie & Cloos, 2004). The oldest of Dalam Intrusion characterized by andesitic rock texture, phyllic to potassic alteration, low-moderate Cu-Au grade (0.25 1). The latest Kali Intrusion characterized by propylitic to potassic alteration, quartz monzodioritic, dominant barren with EqCu is below the cut-of-grade (EqCu <0.25). The forecast block model is built in quarterly basis based on technical sampling assay method combination between diamond drill core and blasthole cutting samples. The actual grade model develop in daily basis following incremental blasthole cutting samples and be used for the ore reconciliation in monthly basis. The Cu-Au grade variance can be a loss and gain comparing to the forecast block model up to +/-30%. Focused on the negative variance or grade loss within pit radial sector 300°-330° at Dalam Intrusion and Southeast Kali host to phyllic-potassic alteration, and positive variance or grade gain observed dominantly along contact intrusion host to propylitic-potassic alteration. The veinlet relationships observed early vein bearing Cu-sulfide in NE direction intensively cut by SE and NS veins that filled by barren quartz-anhydrite±gypsum and quartz-sericite-pyrite as the latest hydrothermal fluid and affected on grade variance results. The spatial data distribution also influenced on grade variance model related to the alteration and mineralization style within the Grasberg Cu-Au porphyry deposit.