Bulletin of the Geological Society of Malaysia, Volume 9, Nov. 1977, pp. 51 – 66
U.S. Geological Survey, Menlo Park, California 94025, USA
Abstract: Porphyry copper deposits and their associated intrusives are widespread throughout the world and particularly abundant in the circum-Pacific region, although not all parts contain porphyry copper deposits. The available data indicate that some plutons related to porphyry copper deposits originated along subduction zones and some in continental crust so far removed from subduction zones that any relation is improbable. Of those deposits related to subduction zones, some, like those in the Caribbean and in places in the Philippines, formed at junctions between oceanic plates, and others, like those in the Andes of South Amerika, formed at the junctions between oceanic and continental plates. Plutons unrelated to subduction zones include those in the interior of the Soviet Union and most, if not all, of those in the western United States.
Potassium-argon ages of porphyry copper deposits indicate that they formed throughout the Phanerozoic Eon. However, because they form in tectonically active zones at depths of 3 km or less, erosion seems to have destroyed most of the older ones. The youngest porphyries are near active subduction zones. Here uplift and consequent erosion are the most active, exposing young deposits.
The composition of the plutons associated with Caribbean porphyry copper deposits shows that most are low in K-feldspar and quartz. With differentiation, the K-feldspar and quartz content increases. The array of compositions can be defined by a differentiation trend line that seems characteristic of igneous rocks derived from oceanic crust.
PI-Qz-Or ternary diagrams for southern Arizona are similar for (1) all Laramide plutons. (2) Laramide plutons associated with porphyry copper deposits in space and time, and (3) all Laramide igneous rocks in the Ray district, which contains a large porphyry copper deposit. The differentiation trend line for all the southern Arizona igneous rocks is strikingly different in slope and direction from that for the Caribbean intrusive rocks. The ternary diagram for all igneous rocks in the Bingham district, which contains the most productive porphyry copper deposit in the United States, is distinct from that for southern Arizona. Other geographic provinces in the western United States in which no porphyry copper deposits occur contain suites of igneous rocks similar to other suites in the western United States associated with porphyry copper deposits, so that it appears that porphyry copper deposits are not related to any unique composition or suite of igneous rocks in continental crust.
A common feature of porphyry copper deposits from all environments is their spatial and temporal relation to an intrusive body that is among the most differentiated (quartz-and K-feldspar-rich) and among the youngest. This implies that differentiation plays a part in the origin of the porphyry copper deposits and in the characteristics of the igneous rocks with which they are associated.
The role of assimilation and contamination in igneous rocks associated with porphyry copper deposits is not clear. The initial strontium ratio of the El Salvedor deposit, South America, indicates no assimilation. In general, the few data obtained are contradictory on the contribution of wallrocks to ore deposits and magmas.