Giant gold deposits

Historically, there has been scientific debate on the source of metals in the gold-copper mines of the Pacific Rim. Were the metals derived from the crust or the mantle? New work has shown that the metals are coming from the mantle, and that the crust does not play a significant role in contributing metals during ore formation.

Gold deposits are associated with volcanoes formed where portions of oceanic crust are being pulled deep into the Earth (subducted). Operating like a giant conveyor belt, these plates transport water and oxygen from the surface down into the mantle.

Water-bearing minerals in the subducted crust dehydrate, generating a plume of fluid that rises through the mantle. The fluid scavenges gold and copper from the deep mantle, transporting metals to the upper mantle where they are deposited in sulfide-rich veins. Melting of these vein-rich portions of the upper mantle creates batches of magma enriched in copper and gold, and when these fertile magmas reach the crust they are prone to produce gold-copper deposits. [ CSIRO Exploration, Australia]

Representing a new style of mineralization on the modern sea floor, large gold-rich veins have been recovered – a minimum of 600mt Au – from the top of Conical seamount, a shallow (1,050-m water depth) submarine volcano located south of Lihir island, Papua New Guinea. Papua New Guinea hosts several giant gold ore bodies.

Over half of the world’s known gold deposits have been formed in hydrothermal areas. Gold dissolves in water heated in the earth to temperatures over 200°C. As the water moves through channels in the rock and cools, the gold is precipitated out to form the deposit. A University of Auckland study show it only takes a blink of a geological eye for giant gold deposits to form; about 55,000 years, the same as the life expectancy of an active volcano. [sampling active gold deposition areas]