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Geology; March 2009; v. 37; no. 3; p. 247-250; DOI: 10.1130/G25451A.1
© 2009 Geological Society of America
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Postsubduction porphyry Cu-Au and epithermal Au deposits: Products of remelting of subduction-modified lithosphere

Jeremy P. Richards1,*

1 Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta T6G 2E3, Canada

Correspondence: *E-mail: Jeremy.Richards{at}ualberta.ca.


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 ARC MAGMATISM AND CRUSTAL...
 POSTSUBDUCTION TECTONICS AND...
 POSTSUBDUCTION MAGMATIC...
 CONTROLS ON METAL ENDOWMENT
 CONCLUSIONS
 REFERENCES CITED
 
Porphyry Cu ± Mo ± Au and some epithermal Au deposits are formed from hydrothermal fluids exsolved from cooling, water-rich, calc-alkaline magmas emplaced in volcanoplutonic arcs above subduction zones. These magmas originate by partial melting of the metasomatized asthenospheric mantle wedge. However, there is increasing evidence for the existence of a suite of porphyry Cu-Au and epithermal Au deposits related to magmas generated after subduction beneath the arc has ceased. Associated magmas tend to be mildly alkaline, relatively sulfur poor, and emplaced as isolated complexes rather than in voluminous volcanoplutonic arcs. They are likely formed by remelting of previously subduction-modified arc lithosphere, triggered by postsubduction lithospheric thickening, lithospheric extension, or mantle lithosphere delamination. Metasomatized mantle lithosphere or hydrous lower crustal cumulates residual from first-stage arc magmatism contain small amounts of chalcophile and siderophile element–rich sulfides, and constitute a fertile source for hydrous, Au-rich, but relatively sulfur-poor magmas during later remelting. The recognition that porphyry Cu-Au and related epithermal Au systems can also form in postsubduction and collisional tectonic settings expands the range of geological environments and geographical terranes that are prospective for such deposits.


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
ARC MAGMATISM AND CRUSTAL...
 POSTSUBDUCTION TECTONICS AND...
 POSTSUBDUCTION MAGMATIC...
 CONTROLS ON METAL ENDOWMENT
 CONCLUSIONS
 REFERENCES CITED
 
Porphyry Cu ± Mo ± Au and related epithermal Au deposits are intrusion centered, and are formed at ≤4 and ≤1 km depth, respectively, by fluids exsolved from magmas emplaced at ~5–10 km depth in the upper crust (Sillitoe and Heden-quist, 2003). The majority of known deposits are genetically related to intermediate to felsic calc-alkaline magmas in volcanoplutonic arcs above active subduction zones (Fig. 1A). They are thus directly linked to the petrogenesis of arc magmas, and derive their fundamental characteristics (e.g., relatively high oxidation state and enrichments in alkalies, S, Cl, H2O, and some metals) from subduction processes. Arc magmas are predominantly formed by partial melting of the metasomatized wedge of asthenospheric mantle between the downgoing oceanic and overriding oceanic or continental plates (Ringwood, 1977). These basaltic magmas evolve and interact with the upper plate lithosphere as they ascend to form hybrid andesitic magmas characterized by relatively high oxidation states (typically ~2 log fO2 units higher than the fayalite-magnetite-quartz buffer) and high water contents (≥4 wt% H2O) (as reviewed in Richards, 2003). These two characteristics are critical to the formation of magmatic-hydrothermal ore deposits (Candela, 1992): the high oxidation state suppresses the formation of significant amounts of magmatic sulfide phases, which would strip the magma of chalcophile and siderophile metals (e.g., Fe, Cu, Au, Mo; Hamlyn et al., 1985; Richards, 2005); and the high water content results in saturation of the magma in an aqueous fluid phase upon ascent into the upper crust, into which these metals will efficiently partition (Candela and Holland, 1984).


Figure 01
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  		Figure 1. A: Porphyry Cu generation as a product of normal arc magmatism; continental arc is shown, but similar processes can occur in mature island arcs. MASH—melting, assimilation, storage, and homogenization. SCLM—subcontinental lithospheric mantle. B–D: Remelting of subduction-metasomatized SCLM or lower crustal hydrous cumulate zones (black layer) leading to potential porphyry Cu-Au and epithermal Au deposit formation. B: Collisional lithospheric thickening. C: Postcollisional lithospheric mantle delamination. D: Postsubduction lithospheric extension. High Sr/Y and La/Yb magmas may be generated in all cases by residual or fractionating hornblende (±garnet, titanite) in the lower crust.

 
Although magmas with these characteristics most commonly form directly in response to inputs from the subduction zone, it is also possible to extract compositionally and isotopically similar magmas from upper plate lithosphere that has been modified by prior subduction processes, but with melting caused by later postsubduction events (e.g., Johnson et al., 1978; Harris et al., 1986). Recently, a number of porphyry Cu-Au and epithermal Au deposits have been described in association with such magmas, with ages and from locations that are incompatible with a relationship to contemporaneous subduction, but consistent with postsubduction or collisional tectonic settings. It is proposed that these deposits were formed from hydrous magmas derived by partial remelting of the metasomatized lithospheric roots and hydrous cumulate zones of former arc systems, in response to a variety of mechanisms such as lithospheric thickening and thermal rebound, subcontinental lithospheric mantle (SCLM) delamination, or lithospheric extension (Figs. 1B–1D).


   ARC MAGMATISM AND CRUSTAL STRUCTURE
TOP
 ABSTRACT
 INTRODUCTION
 ARC MAGMATISM AND CRUSTAL...
POSTSUBDUCTION TECTONICS AND...
 POSTSUBDUCTION MAGMATIC...
 CONTROLS ON METAL ENDOWMENT
 CONCLUSIONS
 REFERENCES CITED
 
In mature island arcs or continental arcs, primitive basaltic magmas ascending from the mantle wedge typically stall at the base of the upper plate crust due to density contrasts (Hildreth and Moorbath, 1988). In a process described by Hildreth and Moorbath (1988) as melting, assimilation, storage, and homogenization (MASH), heat released from this mafic magma input causes partial melting of crustal rocks. Mixing and differentiation of these melts forms hybrid, intermediate-composition, calc-alkaline magmas, with low enough density that they can ascend into the upper crust. The cumulate residues from this process can be seen in exhumed lower crustal–upper mantle arc sections such as Talkeetna (Alaska; DeBari and Coleman, 1989) and Kohistan (Pakistan; Jagoutz et al., 2007). These cumulates contain large amounts of amphibole along with olivine, pyroxene, and plagioclase, and garnet in thicker arc sections. Davidson et al. (2007) proposed that these amphibole-rich cumulates act like a sponge, storing as much as 20% of the water in the original arc magma flux.

Magmas processed through these lower crustal MASH zones commonly display relatively high Sr/Y and La/Yb ratios due to the suppression of early plagioclase crystallization and the preferential partitioning of Y and middle and heavy rare earth elements into amphibole and garnet (Green and Pearson, 1985).


   POSTSUBDUCTION TECTONICS AND MAGMATISM
TOP
 ABSTRACT
 INTRODUCTION
 ARC MAGMATISM AND CRUSTAL...
 POSTSUBDUCTION TECTONICS AND...
POSTSUBDUCTION MAGMATIC...
 CONTROLS ON METAL ENDOWMENT
 CONCLUSIONS
 REFERENCES CITED
 
Magma generation beneath an arc may cease for several reasons, including migration of the locus of melting due to changes in the angle of subduction (e.g., North and South American Cordillera; Kay et al., 2005), subduction reversal or jumping (e.g., southwest Pacific; Gill and Whelan, 1989; Solomon, 1990), or collision that terminates subduction (e.g., Papua New Guinea and southwest Asia; Pearce et al., 1990; Richards et al., 1990; Cloos et al., 2005). Such processes are common and occur on relatively short time scales (~1 m.y.), with essentially instantaneous cessation of arc volcanism (Gill and Whelan, 1989). Depending on the cause, the former arc may undergo stress states ranging from tension (rifting) to compression (collision). Both states may lead to a brief resurgence in magmatism (Davies and von Blankenburg, 1995; Paquette et al., 2003).

In the case of postsubduction arc extension, decompression melting may occur in upwelling subduction-metasomatized asthenosphere and/or attenuated lithosphere, leading to the generation of mafic alkaline (shoshonitic or hawaiitic) magmas (Fig. 1D; Luhr, 1997; Paquette et al., 2003). Translithospheric extensional structures provide channelways for rapid ascent of mantle-derived magmas to upper crustal levels, with little crustal interaction (Richards et al., 1990).

In contrast, postsubduction arc contraction caused by collision may lead to crustal thickening and delamination of the SCLM, with partial melting occurring in depressed lower crustal rocks as isotherms rebound or hot asthenospheric melts invade (Figs. 1B and 1C). The resulting magmas are more felsic, commonly with calc-alkaline to mildly alkaline character, and have crustal radiogenic isotopic signatures (Harris et al., 1986; Davies and von Blankenburg, 1995). The presence of amphibole and/or garnet in the lower crustal former arc source rocks may accentuate the high Sr/Y and La/Yb signatures of these magmas, leading to their (mis-) identification as adakites in some cases. However, the lack of active subduction and their crustal isotopic compositions preclude a slab-melting origin (Hou et al., 2004; Wang et al., 2005).

In both cases (extension or contraction), the source region is previously subduction-modified lithosphere ± asthenosphere, and the postsubduction magmas therefore share many of the geochemical and isotopic characteristics of the preceding arc magmatism. However, because of the transience of these events (compared with steady-state subduction), the magmas will be formed in relatively small volumes and at relatively low degrees of partial melting (Davies and von Blankenburg, 1995; Jiang et al., 2006). Thus, postsubduction magmatism tends to be spatially isolated, and mildly (high-K ± Na calc-alkaline) to strongly alkaline in character.


   POSTSUBDUCTION MAGMATIC-HYDROTHERMAL ORE DEPOSITS
TOP
 ABSTRACT
 INTRODUCTION
 ARC MAGMATISM AND CRUSTAL...
 POSTSUBDUCTION TECTONICS AND...
 POSTSUBDUCTION MAGMATIC...
CONTROLS ON METAL ENDOWMENT
 CONCLUSIONS
 REFERENCES CITED
 
Extensional or Transtensional Environments and Alkalic-Type Epithermal Au Deposits
The most characteristic mineralization style in extensional postsubduction environments is alkalic-type epithermal Au, associated with mafic alkalic intrusive complexes (Richards, 1995). Examples include the Porgera and Ladolam (Papua New Guinea; Richards et al., 1990; Müller et al., 2002), Emperor (Fiji; Eaton and Setterfield, 1993), and Cripple Creek (Colorado; Kelley and Ludington, 2002) gold deposits. All of these systems formed in extensional or transtensional structural settings after subduction either ceased (Lihir, Porgera) or migrated away (Emperor, Cripple Creek).

Contractional Environments and Porphyry Cu-Au and Epithermal Au Deposits
Gold-rich porphyry and epithermal Au deposits associated with high-K calc-alkaline to shoshonitic magmas have recently been reported from arc collisional environments (including collision with continents, microcontinent fragments, or mature island arcs). Examples include: the Eocene Çöpler epithermal Au deposit in eastern central Turkey, which postdates Cretaceous–Paleocene Neo-Tethyan collision (Keskin et al., 2008); the late Miocene Sari Gunay epithermal Au deposit in northwest Iran, which postdates Paleogene–early Neogene Neo-Tethyan collision (Richards et al., 2006); Neogene porphyry Cu-Au deposits in the southwest Pacific, which followed collision or subduction reversal (Solomon, 1990); and mid-Miocene porphyry Cu-Au deposits in Tibet, which postdate Late Cretaceous collision between India and Asia (Hou et al., 2005).

The porphyry deposits in these settings closely resemble those from subduction-related arcs, except for a relationship to mildly alkaline intrusions, and a tendency to be relatively Au rich and Mo poor. These broad similarities, but also subtle differences, in both magma composition and ore deposit style suggest similar petrogenetic and metallogenic processes, differing only in detail.


   CONTROLS ON METAL ENDOWMENT
TOP
 ABSTRACT
 INTRODUCTION
 ARC MAGMATISM AND CRUSTAL...
 POSTSUBDUCTION TECTONICS AND...
 POSTSUBDUCTION MAGMATIC...
 CONTROLS ON METAL ENDOWMENT
CONCLUSIONS
 REFERENCES CITED
 
Postsubduction magmatism can be viewed as a second stage of melting of subduction-modified upper plate lithosphere, which may remobilize metals and other elements introduced during first-stage arc magmatism.

Arc magmatism leaves a large amount of hydrous residue at the base of the crust and in the lithospheric mantle (Fig. 1A), which, due to the high magmatic sulfur content (de Hoog et al., 2001), likely also contains some residual sulfide phases. Arc magmas, although sulfur rich, are also relatively oxidized, such that the bulk of the sulfur is present as SO2 dissolved in the magma (Carroll and Rutherford, 1985). Nevertheless, small amounts of sulfide (as melt or crystalline phases) can be expected to be present, which because of its high density will tend to settle out in cumulate zones. For example, Jagoutz et al. (2007) noted the presence of accessory Fe-Ni sulfides, along with Cr-spinel and Cr-magnetite, in lower crustal cumulates from Kohistan; McInnes et al. (1999) reported Fe-Ni sulfides with high concentrations of Au and platinum group elements (PGE) in meta-somatic veins in subarc mantle xenoliths from a submarine shoshonitic volcano near Lihir Island, Papua New Guinea; and Newberry et al. (1986) noted enrichments in Fe, Mn, Zn, and Cu in gabbroic cumulates from Talkeetna.

Chalcophile and siderophile elements such as Cu, Ni, Au, and PGE partition strongly into sulfide phases relative to silicate melts, with partition coefficients increasing from Cu < Ni < Au and PGE (Peach et al., 1990). Where moderate amounts of sulfide are present relative to the volume of silicate melt (R = 102–105; Fig. 2), the concentration of Cu in the magma will be minimally affected because of its greater overall abundance (tens to hundreds of parts per million) and lower partition coefficient (D {approx}103), whereas sparse (parts per million to parts per billion) Au, Ni, and other highly siderophile elements (D ≥ 105) will be depleted in the magma (Fig. 2; Campbell and Naldrett, 1979). In contrast, the complementary residual sulfides will be enriched in these highly siderophile elements. Thus, first-stage arc magmas tend to generate Cu-rich, relatively Au-poor porphyry systems, while leaving a relatively Au-rich residue in the lower crust and lithospheric mantle (Richards, 2005).


Figure 02
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  		Figure 2. Concentrations of Cu and Au in silicate magma as function of R = (mass of silicate melt)/(mass of sulfide melt) (Campbell and Naldrett, 1979). Cu-rich magmas can form at relatively low R factors (R = 102–105), but leave a relatively Au-rich sulfide residue. Remelting of this sulfide residue (R ≥ 105) during second-stage melting events can generate relatively Au-rich magmas.

 
If this fertile, hydrous residue is subjected to a second stage of melting, either by invasion of small volumes of hot, oxidized asthenospheric magma, or thermal rebound after lithospheric thickening (Figs. 1B–1D), then Au-rich residual sulfide phases may redissolve (R ≥ 105; Fig. 2) to generate magmas with relatively high Au/Cu ratios (Solomon, 1990; Richards, 2005) and perhaps also PGE enrichments (cf. Tarkian and Stribrny, 1999). Such magmas will have the potential to form porphyry Cu-Au and epithermal Au deposits.

A corollary of this process is that postsubduction magmas and associated hydrothermal ore deposits will be less sulfur rich than first-stage arc systems. Accordingly, arc magmas are commonly associated with high-sulfidation-style epithermal deposits, whereas postsubduction systems are more typically low sulfidation.


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 ARC MAGMATISM AND CRUSTAL...
 POSTSUBDUCTION TECTONICS AND...
 POSTSUBDUCTION MAGMATIC...
 CONTROLS ON METAL ENDOWMENT
 CONCLUSIONS
REFERENCES CITED
 
Postsubduction magmatism resulting from lithospheric thickening, thermal rebound, mantle lithosphere delamination, or lithospheric extension shares many geochemical and isotopic characteristics with subduction-related calc-alkaline magmatism, but tends to be more alkaline (shoshonitic, hawaiitic), less sulfur rich, and more isolated in distribution and timing. Such magmas are derived by remelting of the metasomatized roots of former arc magmatic systems, from which they inherit their arc geochemical signature and metal endowment.

Porphyry- and epithermal-style mineral deposits associated with postsubduction magmatism are Au rich relative to many arc-related deposits, a characteristic that may reflect remelting of small amounts of residual sulfide left in the deep lithosphere by arc magmatism. Because of their sparsity (under the relatively oxidizing but S-rich conditions of arc magmatism), these sulfide phases will be enriched in Au and other highly siderophile elements. During second-stage postsubduction magmatism, these small volumes of sulfide will remelt, releasing their metal contents to the alkaline silicate magma. Late-stage partitioning of these metals into hydrothermal fluids exsolved during cooling and crystallization of this magma at upper crustal levels generates magmatic-hydrothermal ore deposits superfi-cially similar to arc-related porphyry and epithermal deposits, but more enriched in Au and of generally lower sulfidation state.

Thus, regions of postsubduction magmatism, which may have been overlooked by mineral exploration strategies in the past, have the potential to host significant Au-rich porphyry and epithermal deposits.


   ACKNOWLEDGMENTS
 
This work was supported by a Discovery Grant from the Natural Sciences and Engineering Research Council of Canada. I thank S. DeBari, A. Greene, and R. Sillitoe for helpful advice, and J. Hedenquist, G. Yogodzinski, C. Macpherson, and A. Tomkins for constructive reviews.


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 POSTSUBDUCTION TECTONICS AND...
 POSTSUBDUCTION MAGMATIC...
 CONTROLS ON METAL ENDOWMENT
 CONCLUSIONS
 REFERENCES CITED
 

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Received for publication 18 August 2008

Revised manuscript received 28 October 2008

Manuscript accepted 30 October 2008




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