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Pebble is a giant, classic calc-alkalic porphyry copper-gold-molybdenum deposit. It is composed of the East and West zones which represent two coeval hydrothermal centers within a single system. The West zone extends from surface to ~500-m depth and is centered on four small granodiorite plutons emplaced into flysch, diorite, and granodiorite sills, and alkalic intrusions and breccias. The much higher grade East zone extends to at least 1,700-m depth and is hosted by a larger granodiorite pluton and adjacent granodiorite sills and flysch. The granodiorite plutons merge with depth. On the east side of the deposit, high-grade mineralization has been dropped 600 to 900 m into the NE-trending East graben, where the deposit remains undelineated to the east and to depth.The large size and high hypogene grades of the Pebble deposit may reflect a combination of multiple stages of metal introduction with vertically restricted, lateral fluid flow induced by hornfels aquitards in flysch (Lang and others, 2013). Pebble contains the largest gold resource of any known porphyry. Molybdenite at Pebble is enriched in rhenium (averaging 906 ppm); pyrite in pyrophyllite alteration zones contains up to 3 ppm palladium. The mineralization was accompanied by several types of alteration (prehydrothermal hornfels, deep sodic-calcic, potassic, sodic-potassic, illite, advanced argillic, qsp halo, propylitic). As of November, 2010, the resources of the Pebble prospect had been calculated at 4 levels of copper-equivalent cut-off (CuEQ) (Ghaffari and others, 2011; Pebble Partnership, 2010). The measured and indicated mineral resources at the lowest report cutoff grade (0.3% CuEQ) are reported as 5,942 Mt at 0.42% Cu, 0.35 g/t Au, and 250 ppm Mo. In addition, inferred mineral resources (same cutoff) are reported as 4,835 Mt at 0.24% Cu, 0. 26 g/t Au, and 215 ppm Mo. |
