ABSTRACT
The Grasberg porphyry Cu-Au-Ag deposit in Papua, Indonesia is primarily hosted by a funnel shaped body of Pliocene intrusive rocks known as the Grasberg Intrusive Complex (GIC). Cu-Au-Ag mineralization exhibits concentric grade contours in plan view and extends over a vertical depth of 1,500 m. Traditional geostatistical tools measure the spatial continuity between sample locations as a function of Euclidean distance and direction. Complex geometries often violate first- and second-order stationarity assumptions. The circular pattern of grade continuity at Grasberg challenges traditional linear estimation methods limited by a fixed anisotropy for a domain of interest. Locally Varying Anisotropy (LVA) kriging overcomes this limitation, removing anisotropy by multidimensional scaling according to a defined anisotropy grid. Implementation of LVA kriging to estimate block grades at the Grasberg porphyry Cu-Au-Ag deposit began in 2013 and continuous improvement efforts have focused on correctly modeling the anisotropy grid, known as the LVA field. The LVA field is the most important input to the LVA estimation process, as it defines the direction and magnitude of continuity of the attribute under consideration. The LVA field for Grasberg evolved from a perfect circle used for all three metals to a grid interpolated from guiding polylines on each level that are individualized for each metal. Progress is substantiated by cross validation techniques. The resulting models display grade distributions that better reflect field observations, and have improved the reliability of resource estimates for this complex domain.
