The theory of lithosphere plate tectonics, embodying the concepts of sea-floor spreading, transform faulting, and underthrusting at continental margins and island arcs, is employed as a basis for an actualistic, though speculative, model for the origin and space-time distribution of porphyry copper and porphyry molybdenum deposits.
Porphyry ore deposits, occurring in the western Americas, southwest Pacific and Alpide orogenic belts, are thought to constitute a normal facet of calc-alkaline magmatism. Chemical and isotopic data cited are consistent with the generation of the components of calc-alkaline igneous rocks and porphyry ore deposits by partial melting of oceanic crustal rocks on underlying subduction zones at the elongate compressire junctures between lithospheric plates.
It is proposed that the metals contained in porphyry ore deposits were derived from the mantle at divergent plate junctures, the ocean rises, as associates of basic magmatism, and transported laterally to subduction zones as components of basaltic-gabbroic oceanic crust and small amounts of suprajacent pelagic sediments; evidence supporting the presence of significant amounts of metals in the oceanic crust is listed.
It is suggested that the temporal and spatial distribution of porphyry ore deposits is dependent on two principal factors, namely the erosion level of an intrusive-volcanic chain, and the time and location of magma generation, and the availability of metals, on an underlying subduction zone. The erosion factor is beli.eved to offer an explanation for the paucity of porphyry ore deposits in pre-Mesozoic orogenic belts, and for the relative abundance of exposed porphyry deposits of Upper Cretaceous-Paleogene age in post-Paleozoic orogens. Provinces with a high concentration of porphyry copper deposits, such as southern Peru-northern Chile and the southwest United States, may be interpreted as regions beneath which anomalously copper-rich oceanic crust was subducted at the time of porphyry copper emplacement; one possible explanation for the episodic formation of volumes of copper-rich oceanic crust is the presence of a heterogeneous distribution of metals in the low velocity zone of the upper mantle. Porphyry ore deposits seem to have formed during a series of relatively short, discrete pulses, perhaps correlable with changes in the relative rates and directions of motion of lithospheric plates. In some regions, such as Chile, porphyry ore deposits are arranged in parallel, linear belts, which may be explicable in terms of shifting loci of magma and included metal generation on a subduction zone, and which seem to be largely independent of control by tectonic lineament intersections. The time intervals during which the formation of porphyry deposits took place are shown to be broadly coincident with periods of lithosphere plate convergence, and porphyry deposits may still be forming above currently active subduction zones.
A number of potential regions for the discovery of porphyry ore deposits are suggested, and the importance to exploration of analyzing orogenic belts in terms of plate tectonics is emphasized.
RICHARD H. SILLITOE ร PโขdโขPHYRY COPPER