Porphyry Cu-Au Systems
Porphyry copper deposits are copper orebodies which are associated with porphyritic intrusive rocks and the fluids that accompany them during the transition and cooling from magma to rock. Circulating surface water or underground fluids may interact with the plutonic fluids. Successive envelopes of hydrothermal alteration typically enclose a core of ore minerals disseminated in often stockwork-forming hairline fractures and veins. Porphyry orebodies typically contain between 0.4 and 1% copper with smaller amounts of other metals such as molybdenum, silver and gold.
The first mining of low-grade copper porphyry deposits from large open pits coincided roughly with the introduction of steam shovels, the construction of railroads, and a surge in market demand near the start of the 20th century. Some mines exploit porphyry deposits that contain sufficient gold or molybdenum, but little or no copper.
Porphyry copper deposits are currently the largest source of copper ore. Most of the known porphyrys are concentrated in: western South and North America and Southeast Asia and Oceania - along the Pacific Ring of Fire; the Caribbean; southern central Europe and the area around eastern Turkey; scattered areas in China, the Mideast, Russia, and the CIS states; and eastern Australia.Only a few are identified in Africa, in Namibia and Zambia; none are known in Antarctica. The greatest concentration of the largest copper porphyrys is in Chile. Almost all mines exploiting large porphyry deposits produce from open pits.
Pebble property is located in the Bristol Bay region of southwest Alaska, 320 km southwest of Anchorage
At a 0.30% copper equivalent (CuEQ) the Pebble deposit contains 5.1 billion tonnes of Measured and Indicated Mineral Resources grading 0.43% Cu, 0.35 g/t Au and 256 ppm Mo, an estimated 72 billion lb copper, 94 million oz gold and 4.8 billion lb molybdenum, as well as quantities of silver, palladium and rhenium.
Is a calc-alkalic copper-gold-molybdenum porphyry deposit which formed in association with granodiorite intrusions about 90 Ma. The deposit comprises the contiguous Pebble West and Pebble East Zones as several coalescing hydrothermal centres around small granodiorite stocks.
Mineralization formed during early K-silicate alteration and associated quartz-sulphide veins. Very high-grade mineralization was introduced to the Pebble East Zone during a late-stage of advanced argillic alteration centered on a major brittle-ductile fault zone.
Mineralization is dominated by hypogene pyrite, chalcopyrite and molybdenite; bornite is an important component in some parts of the East Zone. supergene mineralization forms a minor zone of oxide mineralization; the East Zone contains only hypogene mineralization.
The Cu-Au-Mo mineralization, as currently known, extends over an east elongated area of 4.9 by 3.3 km, and to a depth of 610 m (2000 ft) in the Pebble West Zone and to at least 1525 m (5000 ft) in the Pebble East Zone. Mineralization in the Pebble East Zone remains open to the east, northwest, south and southeast. A much larger zone of strong alteration and low-grade mineralization extends north, south and west of the known Pebble deposit