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Porphyry Copper Provinces

Richard H. Sillitoe†

SEG International Exchange Lecturer and Plenary Speaker

27 West Hill Park, Highgate Village, London N6 6ND, England

†E-mail, aucu@compuserve.com

It has been recognized for the past century that copper deposits, in common with those of

many other metals, are heterogeneously distributed in Earth’s upper crust, resulting in areally

restricted copper provinces that were generated during several discrete metallogenic epochs

over time intervals of up to several hundred million years. Porphyry and any associated skarn

deposits dominate many of these provinces and account for approximately 70% of the global

copper inventory. Although porphyry copper deposits and/or prospects occur in most

volcanoplutonic arc terranes, comparably sized arc segments have total contained copper

contents that differ by more than two orders of magnitude.

A variety of deposit-scale geometric and geologic features and factors strongly

influence the size and/or grade of porphyry copper deposits and districts. These include the

presence of clustered alteration-mineralization centers, mafic or massive carbonate host

rocks, voluminous magmatic-hydrothermal breccias, bornite ± digenite-bearing core zones,

hypogene and supergene enrichment, and mineralized skarn development, coupled with lack

of serious dilution by late, low-grade porphyry intrusions and breccias. Furthermore, copper

endowment undoubtedly benefits from optimization of all aspects of the ore-forming process.

Tectonic setting also plays an important role in copper metallogeny. Compressional

tectonomagmatic belts, commonly created by flat-slab subduction or arc-continent collision

and characterized by crustal thickening and high rates of uplift and exhumation, appear to

host most large, high-grade hypogene porphyry copper deposits. Such mature arcs,

characterized by large-volume felsic magma chambers, must also undergo mafic magma

input during porphyry copper formation. Tectonically influenced differences in exhumation

rate also affect observed copper endowment, which is greatest in youthful arc terranes.

Notwithstanding the obvious importance of these deposit-scale geologic, regional tectonic,

and erosion rate criteria for effective copper deposit formation and preservation, they seem

inadequate, even collectively, to explain the localization of premier porphyry copper

provinces, such as the central Andes and southwestern North America, and, by the same

token, the paucity of appreciable copper mineralization in some apparently similar

tectonomagmatic settings elsewhere.

It is proposed here that premier porphyry copper provinces occur where restricted

segments of the lithosphere were predisposed to upper crustal copper concentration

throughout long intervals of Earth history. This predisposition was most likely gained during

oxidation and copper introduction by subduction-derived fluids, containing metals and

volatiles extracted from hydrated basalts and sediments in down-going slabs. As a result, the

suprajacent lithospheric mantle and underplated lower crust may have been metasomatized

and become fertile for tapping during subsequent subduction- or postsubduction-related

magmatic events to generate porphyry copper districts or belts. The fertile lithosphere

beneath some magmatic arc terranes was incorporated during earlier collisional tectonism,

commonly during Precambrian times. Translithospheric fault zones, including ancient

collisional sutures, commonly play a key role in localizing major copper deposits, districts,

and belts. These proposals address the long-debated concept of metal inheritance in terms of

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the fundamental role played by subduction-metasomatized mantle lithosphere in global

copper metallogeny.