Porphyry style and related Hydrothermal Deposits

Hydrothermal veins , Skarn, Porphyry copper ore deposit

Porphyry Copper Deposits: Characteristics Porphyry copper deposits are huge, low-grade deposits. A typical deposit has a grade of 0.3 to 1% Cu and 50-1000 million tonnes of ore. The largest contains 5000 million tonnes of ore -a small mountain. They provide 50% of the world's copper. The largest deposits include Chuquicamata (Chile: see Practical Booklet),Bingham Canyon (USA) and Bougainville (New Guinea). They only came on stream with the advent of large-scale mining at the turn of the century . The form of the ore is a stockwork, where the ore is disseminated in the bulk of the rock and within a network of small fractures The principal primary ore minerals are iron and .copper-iron sulphides, typically pyrite, chalcopyrite and bornite. Secondary ores include chalcocite (Cu2S), covellite (CuS) as well as a range of other copper minerals. There are many by- products, the most important of which are gold (as native gold) in oceanic arcs and molybdenum (as molybdenite) in continental arcs. Some deposits are copper-gold and copper-molybdenum deposits and some are molybdenum deposits. Porphyry tin deposits also exist. The host rocks are porphyritic granodiorite intrusions and their country rocks. The setting is almost always a volcanic arc (i.e. subduction zone). The fluids that deposit the metals also alter the granodiorite and any country rocks to give an alteration halo. Formation Porphyry copper deposits are derived from volcanic arc magmas, which have high water contents. During fall in temperature and pressure, and the crystallization of anhydrous minerals, water can separate from magma and accumulate at the top of the magma chamber (see below). Build up of water pressure (vapour pressure if it boils) can cause the lithostatic pressure plus tensile strength of the rocks to be exceeded catastrophically to form the stockwork. Escaping fluids can then deposit ore minerals in the fractures. Typically, the escape of magmatic fluids is just the first event. Hydrothermal processes can then redissolve and redeposit the metals.

Porphyry copper ore mine

Porphyry copper ore types Disseminations Veinlets

Supergene sulfides ore deposits S + CuS covellite Cu2S chalcocite CuO22-aq Cu2S Cu native copper Primary Sulfide ore Water table Leached zone Ground surface Supergene sulfides ore deposits Cu2+aq Cu2O Cuprite Cu2CO3(OH)2 Malachite CuS