Hydrothermal Deposits - Metals and Society: An Introduction to Economic Geology - page 88

4.3.2 Porphyry Deposits
Introduction : Porphyry deposits are the world's most important source of Cu and
Mo, and also produce significant amounts of Au, Ag, W, and Sn (Sinclair 2007 ;
Sillitoe 2010 ). They account for about 50-60% of world Cu production and more
than 95% of world Mo production. In contrast to VMS deposits, which normally are
small (1-5 mt) but of high grade (3-10% ore metals), porphyry deposits are
enormous but of low grade. The best-known deposits are in the cordillera of
North and South America, the location of the Bingham ore body in the USA
(2,733 million tons of ore grading 0.7% Cu and 0.05% Mo) and the Chuquicata
ore body in Chile (10,837 mt of 0.56% Cu and 0.06% Mo) (Box 4.3). The latter
deposit is the site of what is said to be the world's biggest open-pit mine and its
neighbour, the El Teniente deposit, is exploited in the biggest underground mine.
Sinclair's ( 2007 ) compilation lists 44 deposits with reserves greater than one billion
tons of Cu, Mo, or Au ore.
Another large deposit is the Grasberg ore body in Irian Jaya, the Indonesian
(western) portion of New Guinea, which contains about 2,100 mt of ore grading
1.2% Cu and 1.2 g/t of Au, making it, on one hand, the biggest gold mine and the
third biggest copper mine in the world, and on the other, the site what has been
described as the “world's worst eyesore”. The conflict created by the environmental
damage engendered by an enormous mining operation in a region of fragile, high-
altitude rain forest and the immense economic benefit of the operation, which
contributes 2% of the entire gross domestic product of a very poor country, starkly
illustrates the dilemma associated with the exploitation of the Earth's natural
resources.
Structure and Mineralization : Returning to geological issues, porphyry deposits
derive their name from the phenocryst-bearing felsic to intermediate shallow-level
intrusions with which they are associated. The form of porphyry deposits is highly
varied and includes irregular, oval, solid, or “hollow” cylindrical and inverted cup
shapes. As shown in Fig. 4.8 , the ore bodies are superimposed on the upper parts of
relatively small granitic plutons which represent offshoots from larger batholiths at
greater depths. The shallow-level plutons are located in the lower portions of
volcanoes and no doubt are parts of conduits that supplied magma to overlying
volcanism. Both the ore bodies and the plutons are composite structures built up
a many individual pulses of magma and hydrothermal fluid. The ores are not
confined to the plutons but extend outwards into the surrounding rocks (Fig. 4.7 ).
Closely associated with the mineralization is moderate to intense alteration which
displays a zoning concentric about the pluton. This alteration also extends well
outside the zone of mineralization and is used as a guide during the exploration of
this class of deposits. The zoning in the alteration in the deposits from western
USA, which are considered a classic type of porphyry deposit, is illustrated in
Fig. 4.8 .
The mineralization consists of small concentrations of sulfide minerals, disse-
minated or dispersed in small veins and replacement patches in the highly altered