Pebble, Alaska: A Super-Giant Porphyry Gold-Copper-Molybdenum Deposit
The Pebble porphyry deposit in southwestern Alaska hosts measured and indicated resources of 3 billion tonnes containing 31.3 million ounces of gold, 18.8 billion pounds of copper and 993 million pounds of molybdenum; plus an inferred resource of 1 billion tonnes containing 10.8 million ounces of gold, 5.8 million ounces pounds of copper and 361 million pounds of molybdenum.
The deposit is divided into the West, Central and East Zones. The West Zone occurs in small granodiorite stocks and in the wall rocks above the stocks and on their flanks. The wall rocks include Lower Cretaceous volcanosedimentary rocks intruded by early diorite sills and Cretaceous intrusion breccia. Central Zone host rocks are volcanosedimentary rocks, diorite sills and a series of Cretaceous granodiorite sills, which host higher-grade mineralization and become more abundant towards the East Zone stock. The East Zone, discovered in 2005 and not included in the resource estimate, occurs in a large granodiorite stock. Mineralization is continuous over vertical lengths exceeding 700 metres and grades are significantly higher in the East Zone which remains open.
The Pebble deposit contains strong potassic K-feldspar and hydrothermal brown biotite alteration, quartz-carbonate-sulphide vein stockworks, and abundant disseminated mineralization with moderate late sericite alteration.
Pebble, Molybdenum, sericite alteration, granodiorite, copper-gold, stockwork, Cretaceous, potassic alteration, Porphyry, Alaska