PORPHYRY DEPOSITS - A REVIEW WITH EMPHASIS ON DEPOSITS IN CENTRAL EURASIA

Porphyry deposits are the world's most important source of Cu, Mo and Re, and are major sources of Au and Ag. They can also contain significant amounts of W and Sn. In Central Eurasia, porphyry Cu-Au deposits are represented by giant deposits such as Kal'makyr-Dalnee (Uzbekistan), with nearly 20 Mt Cu and more than 2200 t Au (production plus reserves), and Oyu Tolgoi (Mongolia), with reserves of 32 Mt Cu and 987 t Au. Both of these deposits also have significant Re contents. Other important deposits include the Kounrad (Kazakhstan) porphyry Cu-Au and Erdenet (Mongolia) porphyry Cu deposits. Verkhnee Kairakty (Kazakhstan) is a porphyry-type W deposit that is likely the largest W deposit in the world, with nearly 900,000 t of contained W.

Primary (hypogene) ore minerals in porphyry deposits are dominantly structurally controlled, with mineralization styles ranging from stockworks to veins, vein sets, fractures, and breccias. Ore minerals occupy large volumes of hydrothermally-altered rock and porphyry deposits range in size from tens of millions to billions of tonnes of ore. Ore grades for the different metals vary considerably but generally average less than one per cent. In porphyry Cu deposits, for example, Cu grades range from 0.2% to more than 1% Cu; in porphyry Au and Cu-Au deposits, Au grades range from 0.2 to 2 g/t Au. Associated igneous rocks vary in composition from diorite-granodiorite to high-silica granite; they are typically porphyritic epizonal and mesozonal intrusions, and commonly subvolcanic. A close temporal and genetic relationship between magmatic activity and hydrothermal mineralization in porphyry deposits is indicated by the presence of intermineral intrusions and breccias, and by unidirectional solidification textures (USTs) in genetically-related intrusions.

Porphyry deposits range in age from Archean to Recent, although most economic deposits are Jurassic or younger. In Central Eurasia, porphyry deposits are associated with Ordovician to early Mesozoic magmatic arcs that developed in Neoproterozoic-Paleozoic rocks of the complex Altaid tectonic collage.