Magmatic Controls on Porphyry Copper Genesis
Andreas Audétat, Adam C. Simon, 2012. "Magmatic Controls on Porphyry Copper Genesis", Geology and Genesis of Major Copper Deposits and Districts of the World: A Tribute to Richard H. Sillitoe, Jeffrey W. Hedenquist, Michael Harris, Francisco Camus
Download citation file:
This paper deals with the earliest stages of formation of porphyry Cu deposits, starting with the production of partial melts in the mantle and ending where upper crustal magmas reach their solidus and potentially have exsolved a metal-bearing hydrothermal fluid. During all these stages magmatic sulfides exert a major control on the budget of ore-forming metals in the magma. High metal concentrations in mafic arc magmas are favored by low degrees of partial melting in the mantle source region, and by limited removal (or effective redissolution) of magmatic sulfides in the lower crust. Ascending magmas accumulate in large, compositionally stratified magma chambers in the upper crust (5- to 15-km depth), which represent the exsolution source of the mineralizing fluids for the shallower porphyry Cu deposits. Interaction between mafic and felsic magmas in these magma chambers leads to partial mingling/mixing, volatile release, and the formation of magmatic sulfides that incorporate large amounts of Cu and Au, but only little Mo. For porphyry Cu mineralization, it is essential that these magmatic sulfides are subsequently destroyed and thereby release their contained metals to the mineralizing fluids. Evidence from experimental phase equilibria studies and melt inclusions hosted in phenocrysts from plutonic and volcanic rocks in arc environments, combined with fluid inclusion evidence from porphyry Cu deposits, suggest that silicate melts that ultimately give rise to porphyry Cu deposits are likely saturated first with a CO2-rich fluid and later give way to single-phase, low-salinity (typically 5–10 wt % NaCl equiv) aqueous fluids. At the typical fO2 conditions of porphyry Cu-forming magmas (ΔFMQ + 1 to ΔFMQ + 3), sulfur occurs mostly as SO2 in the fluid. Efficient Cu removal from the magma into the overlying porphyry environment is favored by the exsolution of an S-bearing volatile phase that has a low HCl/alkali chloride ratio. The ability of the ore fluid to scavenge and transport Cu increases with increasing fO2 and the concentration of K in the aqueous fluid, and may be maximized at high ratios of SO2/H2S of the fluid. Once formed, efficient focusing of the ore fluid into the upper portions of the magma chamber may be favored by the development of permeable melt channels that act as conduits for the ascent of ore fluid in a pressure gradient through the crystallizing magma. These conduits likely facilitate the contribution of S, Cu, and other metals from mafic silicate melt that ponds at deeper levels of the magma system.
Figures & Tables
Geology and Genesis of Major Copper Deposits and Districts of the World: A Tribute to Richard H. Sillitoe
It has been recognized for the past century that copper deposits, in common with those of many other metals, are heterogeneously concentrated 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. Various segments of circum-Pacific magmatic arcs, for example, have total contained copper contents that differ by two orders of magnitude. Each metallogenic epoch introduced its own deposit type(s), of which porphyry copper (and related skarn), followed by sediment-hosted stratiform copper and then iron oxide copper-gold (IOCG), are globally preeminent. Nonetheless, genesis of the copper provinces remains somewhat enigmatic and a topic of ongoing debate.
A variety of deposit-scale geometric and geologic features and factors strongly influence the size and/or grade of porphyry copper, sediment-hosted stratiform copper, and/or IOCG deposits. For example, development of major porphyry copper deposits/districts is favored by the presence of clustered alteration-mineralization centers, mafic or massive carbonate host rocks, voluminous magmatic-hydrothermal breccias, low sulfidation-state core zones conducive to copper deposition as bornite ± digenite, hypogene and supergene sulfide enrichment, and mineralized skarn formation, coupled with lack of serious dilution by late, low-grade porphyry intrusions and breccias. Furthermore, the copper endowment of all deposit types undoubtedly benefits from optimization of the ore-forming processes involved.
Tectonic setting also plays a fundamental role in copper metallogeny. Contractional tectonomagmatic belts, created by flat-slab subduction or, less commonly, 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 arc crust also undergoes mafic magma input during porphyry copper formation. The premier sediment-hosted stratiform copper provinces were formed in cratonic or hinterland extensional sedimentary basins that subsequently underwent tectonic inversion. The IOCG deposits were generated in association with extension/transtension and felsic intrusions, the latter apparently triggered by deep-seated mafic magmas in either intracratonic or subduction settings. The radically different exhumation rates characteristic of these various tectonic settings account well for the secular distribution of copper deposit types, in particular the youthfulness of most porphyry relative to sediment-hosted stratiform and IOCG deposits. Notwithstanding the importance of these deposit-scale geologic, regional tectonic, and erosion-rate criteria for effective copper deposit formation and preservation, they seem inadequate to explain the localization of premier copper provinces, such as the central Andes, southwestern North America, and Central African Copperbelt, in which different deposit types were generated during several discrete epochs. By the same token, the paucity of copper mineralization in some apparently similar geologic settings elsewhere also remains unexplained.
It is proposed here that major 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 downgoing slabs. As a result, superjacent lithospheric mantle and lowermost crust were metasomatized as well as gaining cupriferous sulfide-bearing cumulates during magmatic differentiation—processes that rendered them fertile for tapping during subsequent subduction-or, uncommonly, intraplate extension-related magmatic events to generate porphyry copper and IOCG districts or belts. The fertile lithosphere beneath some accretionary orogens became incorporated during earlier collisional events, commonly during Precambrian times. Relatively oxidized crustal profiles—as opposed to those dominated by reduced, sedimentary material—are also required for effective formation of all major copper deposits. Large sedimentary basins underlain by or adjoining oxidized and potentially copper-anomalous crust and filled initially by immature redbed strata containing magmatic arc-derived detritus provide optimal sites for large-scale, sediment-hosted stratiform copper mineralization. Translithospheric fault zones, acting as giant plumbing systems, commonly played a key role in localizing all types of major copper deposits, districts, and belts. These proposals address the long-debated concept of metal inheritance in terms of the fundamental role played by subduction-metasomatized mantle lithosphere and lowermost crust in global copper metallogeny.