Porphyry Copper Deposits
Authors: Hunt John P. 
Published: November 2019

Fifty to seventy-five years ago, geologists such as Emmons, Locke, Lindgren, Sales, and Perry developed genetic concepts about porphyry copper deposits and their associated high-grade veins, breccias, and skarns which are still valid, such as their relations to granitic plutons and the evolutionary nature of their development. Forty years ago, Charles Meyer’s pioneering studies of vein formation and wall-rock alteration at Butte laid the groundwork and provided methods by which many advances in subsequent decades were achieved in describing and interpreting patterns of mineralization and alteration in these magmatically affiliated hydro-thermal deposits. Thirty years ago, Hemley’s hydrothermal experiments, carefully patterned after actual field assemblages, revealed the fundamental chemistry of hydrogen metasomatism in processes of wall-rock alteration.

During the 1940s through the 1970s, much new knowledge of porphyry coppers was gained through the work of several competent industrial organizations and many university and U.S.G.S. researchers. At El Salvador, Chile, systematic field mapping and coordinated on-site laboratory observations documented in detail the evolutionary nature of mineralization and the transition from magmatic to hydrothermal environments. At about the same time, D. Lowell’s discovery of the Kalamazoo orebody in Arizona focused attention on the economic importance of accurate descriptions of patterns of mineralization and the interpretation of their postmineral histories.

The discovery and description of new disseminated orebodies at Butte, and skarn orebodies at Bingham and at several southwestern U.S. districts, provided new insight into the evolution and correlation of hydrothermal processes in both carbonate and aluminous wall-rock environments. District mapping, especially at Yerington, produced accurate estimates of the original depth of emplacement of porphyry copper orebodies, as well as quantitative measurement of postmineral basin-and-range deformation.

These and other field-oriented studies, together with complementary theoretical and laboratory work, have revealed a central genetic theme common to many porphyry copper deposits. Essential and sequential elements of this theme are the generation of calc-alkaline to alkaline magmas deep in the crust or upper mantle; the rapid rise of these magmas through the crust without loss of volatiles; the shallow emplacement of magmas as batholiths and cupolas; fractional crystallization; the intense fracturing of cupolas and release of magmatic fluids and associated mineralization; and the interaction of cooling batholith and cupola(s) with inrushing meteoric water and late hydrothermal mineralization.

Differences among porphyry copper deposits may in many cases reflect differences in timing and extent of interaction with meteoric water and the deposits’ postmineral histories. There is probably more than one tectonic mechanism capable of generating porphyry copper magmas of variable compositions. The sulfur content and oxidation state of porphyry copper magmas are more fundamental problems than is the source of copper.

During the 1980s, mapping, exploration, and other field studies of porphyry copper deposits have slowed. Fortunately, several valuable government-sponsored research programs are under way which likely will provide some further advances in our understanding of the origins of porphyry copper deposits.

During the 1990s we should apply both our present knowledge and the valuable new laboratory tools now available toward formulating more general and more unified genetic concepts, applicable not only to porphyry coppers but also to many other classes of hydro-thermal deposits associated with igneous processes. Such concepts also would stimulate the search for more profitable and higher grade types of copper orebodies in porphyry copper districts, such as high-grade veins and horsetail zones, high-grade breccias, and high-grade limestone replacement and skarn ores. Whether such progress in fact will be achieved depends a great deal on whether government agencies, universities, and industry can work together and sponsor critical field-oriented work, such as regional, district, and mine-scale mapping, supported by a broad spectrum of modern theoretical and laboratory investigations.