In addition, the process of storing magma in lower-crustal MASH zones serves to build up the volume of magma available for upper-crustal plutonism, and therefore for shallow porphyry ore formation. In normally dipping subduction zones, partial melting in the high-temperature core of the metasomatized asthenospheric mantle wedge will generate hydrous, moderately oxidized (FMQ +1 to +2, where FMQ is the difference from the fayalite-magnetite-quartz oxygen buffer in log fO2 units), incompatible element-rich basaltic magmas. Shallow or flat subduction will eliminate the asthenospheric mantle wedge, with the result that mantle-derived magmatism will cease or will migrate inland to where the slab eventually steepens (Fig.
Thus, although the initial, explosive fluid flow event may be marked by a breccia pipe or diatreme (Fig. 4B), this permeable structure will subsequently be intruded by more slowly flowing bubbly magma (forming dikes and stocks) interleaved with multiple sets of veins and stock works, each potentially depositing metals to build up an ore deposit (Figs. However, such deposits represent end members of a spectrum of deposits that range through marginally subeconomic (at today's metal prices, using current technology, and under current geopolitical conditions) to background concentrations. The sample in (B) is from drill hole OTD 976 at ~1227.0 m, and contains 3.18% Cu and 1.18 g/t Au. 
Because these conditions characterize most subduction zones in the Phanerozoic, it is estimated that 90% of arc systems are fertile for porphyry ore formation (Fig. In so doing, it also highlights which steps might deserve closer scrutiny to assess how they control or limit various processes, leading to more accurate quantification. Any magma that escapes into the upper crust is likely to do so under high pressure, leading to volcanism (e.g., Tibaldi, 2008). 2, and in the text). A large porphyry Cu deposit will not form in the absence of a large volume of source magma in the midupper crust, so these basic arc geodynamic conditions seem to be a first-order prerequisite for ore formation from fertile arc magmas. 1; Ringwood, 1977; Tatsumi, 1986; Peacock, 1993; Wallace, 2005). Similarly, pressure reduction on the magma chamber by sudden unroofing or mass wasting (e.g., volcanic sector collapse; Voight et al., 2006) could have the same effect. Typical hypogene grades (of primary mineralization, unaffected by supergene enrichment processes that occur during weathering) are in the range 0.5%1.5% Cu, <0.01%0.04% Mo, and 0.01.5 g/t (ppm) Au, with tonnages of ore commonly measured in the hundreds of millions to billions of tonnes (Sillitoe, 2010). Two recent examples illustrate the point. Normally this will result in the destruction of the orebody and dispersion of the metals. Porphyry Cu deposits form on geologically very short timescales of 100,000 yr or less (Arribas et al., 1995; Marsh et al., 1997; Shinohara and Hedenquist, 1997; Barnes, 2000; von Quadt et al., 2011; Weis et al., 2012; Chiaradia et al., 2013; Chelle-Michou et al., 2015, 2017; Mercer et al., 2015; Buret et al., 2016; Correa et al., 2016; Li et al., 2017; Cernuschi et al., 2018).
Only a few Phanerozoic arc systems are known to be largely barren, including the Paleo-Tethyan arcs of Eurasia and Japan, where it has been proposed that subduction of reduced oceanic crust (Richards and engr, 2017) or the presence of reduced lithologies in the upper plate (Tomkins et al., 2012; Sillitoe, 2018) may have locally degraded magma fertility by causing early sulfide saturation. 
In contrast, porphyry Cu deposits form where the flow of large volumes of magmatic-hydrothermal fluid has been focused into narrow (typically 1-km-wide) cylindrical or elliptical zones rising above the batholith, termed cupolas (Fig. The relatively high probability of successfully negotiating step 3 reflects the global uniformity of the MASH process in Phanerozoic arcs. Only the rarest high-grade deposits, with 1.2% Cu (or Cu equivalent, including the value of byproduct Mo and/or Au) in hypogene ores, can currently be mined from more expensive underground operations. In fact, if sulfide separation was voluminous and its redissolution was not almost complete, then this process would be expected to decrease the efficiency of ore formation. In situations where hydraulic connectivity between the source region and surface is established, such as in extensional terrains, magma density is matched against the density of the entire rock column, and relatively dense basaltic magmas can be erupted through lithosphere that includes dense mantle and lower crust (Walker, 1989; Takada, 1994). If the flow stops or slows, the cupola zone will rapidly cool down, ore deposition will cease, and any new pulse of magmatic-hydrothermal fluid flow will need to establish a new pathway, effectively restarting the process of ore formation; in such cases, the same total tonnage of metal might be transported by the combined hydrothermal systems as by a single flow event, but it will likely be deposited in different places, and therefore at lower grades. As noted above, a batholith containing 100200 km3 of typical andesitic magma (4 wt% H2O) could in theory source all the fluid and Cu in a giant porphyry Cu deposit (10 Mt Cu), but this would require almost complete degassing of the magma chamber, highly efficient extraction of Cu from the melt, and channeling of the entire fluid volume up a single cupola zone. (1996) anticipated this in a conceptual model (their Fig. Copper is mainly deposited over a narrow temperature interval between ~550350 C along this pathway, at depths of 1.54 km (Fig.
Hydrothermal alteration (potassic, phyllic, and argillic) overprint all intrusions except late dikes. This paper seeks to identify the main steps along the path from magma genesis to hydrothermal mineral precipitation that affect the chances of forming an ore deposit (defined as an economically mineable resource) and attempts to estimate the probability of achieving each step. 4A). The value of a commodity is controlled by its availability and usefulness: a commodity in high demand but low availability will command a high price, with the result that smaller and/or lower-grade (more common) deposits will be economic. A further effect observed at trace-element levels is the enrichment of hydrous andesitic magmas in Sr and depletion in middle and heavy rare-earth elements (MREEs and HREEs, including Y), again resulting from delayed plagioclase crystallization (Sr enrichment) and abundant early crystallization of amphibole (which preferentially partitions MREEs and Y; Castillo et al., 1999; Macpherson et al., 2006; Richards and Kerrich, 2007; Nandedkar et al., 2016). MASH stands for melting, assimilation, storage, and homogenization (Hildreth and Moorbath, 1988). Above normally dipping subduction zones (30), this metasomatic flux will interact with a wedge of hot asthenospheric mantle between the downgoing slab and the upper plate at depths of ~100 km, whereas during shallower subduction, this flux may directly impinge on the base of the overlying lithosphere (discussed below). Large high-grade skarn and porphyry deposits nevertheless remain rare, and most porphyry Cu deposits are characterized by pervasive but lower-grade stock-work zones in coeval porphyritic intrusions and their immediate volcanosedimentary host rocks (Fig. The probability that such fluids will be exsolved from hydrous arc magmas emplaced in the midupper crust is 100% (i.e., it is inevitable). Prior to the turn of the past century, porphyry Cu deposits were not considered economic under any circumstances because the grades were too low. Similar deep magmatic-hydrothermal fluids were reported from the Bingham Canyon porphyry Cu-Au-Mo deposit, Utah (Redmond et al., 2004). This work was supported by a Natural Sciences and Engineering Research Council of Canada Discovery Grant (RGPIN-2017-05082), which is gratefully acknowledged. Arc magmas globally are H2O-Cl-Srich and moderately oxidized (FMQ = +1 to +2) relative to most other mantle-derived magmas (FMQ 0). In contrast, the slightly higher oxidation state of arc magmas means that a significant proportion of the dissolved sulfur is present as sulfate rather than sulfide, reducing the tendency of the magma to saturate early in large volumes of sulfide phases, and therefore to become depleted in metals (Carroll and Rutherford, 1985; Richards, 2003, 2011a, 2015; Jugo et al., 2005). Long-lived, static arcs with normal subduction angles (~3045) and moderately compressional stress states will generate large volumes of basaltic arc magma that become trapped by density contrasts at the base of the upper-plate crust (MASH zones). Transpressional conditions are therefore ideally suited to the formation of mid- to upper-crustal batholiths and associated porphyry deposits (Fig. (2010), and Sillitoe (2010). The approach uses estimates of probability of various processes that affect arc magmas as they ascend from the mantle wedge source to the shallow crust. As these magmas rise into the crust and begin to crystallize, they will reach volatile saturation, and a hydrous, saline, S-rich, moderately oxidized fluid is released, into which chalcophile and any remaining siderophile metals (as well as many other water-soluble elements) will strongly partition. This state needs to be maintained for several million years to allow a sufficient thermal and mass flux to reach the top of the upper plate, a necessary condition for forming large upper-crustal volcanoplutonic systems and associated ore deposits (Richards, 2003; McCuaig and Hronsky, 2014; Whattam and Stern, 2016; Chiaradia and Caricchi, 2017; Schpa et al., 2017; Rees Jones et al., 2018). However, these tectonic conditions, and especially the transition from contractional to transpressional strain, are relatively uncommon during the history of arcs, reducing the overall probability to perhaps 1% (steps 14 in Fig. During flat subduction, the downgoing plate will dehydrate as before, but instead of the released fluids interacting with a hot asthenospheric mantle wedge, they will directly encounter the relatively cool base of the upper-plate lithosphere, causing hydration and metasomatism (Fig.
However, although all arc volcanoplutonic complexes are accompanied by some hydrothermal and fumarolic alteration, mineralization is not always present, and economic deposits (ore deposits) are rare (perhaps 1 in 1000 prospected systems actually prove to be economic for mining). Porphyry Cu deposits typically form 1.54 km below the surface (rarely down to 9 km), whereas high-sulfidation type epithermal Cu-Au deposits may form coevally in the shallower parts of the system (Arribas et al., 1995; Einaudi et al., 2003; Sillitoe and Hedenquist, 2003; Simmons et al., 2005). Jeremy P. Richards; Porphyry copper deposit formation in arcs: What are the odds?. The MASH model considers mantle-derived basaltic magmas to be too dense to pass through the upper-plate crust in the absence of a hydraulic head (Walker, 1989), restricting their eruption at surface to extensional tectonic settings (backarc rifts; Fig. (B) Gray dacite porphyry dike injected along the centerline of a quartz-(molybdenite) vein (white), cutting and cut by earlier and later quartz-(molybdenite) veins in metasedimentary country rocks (MAX porphyry Mo deposit, British Columbia). This process may not lead to any particular increase in metal endowment in the magmatic system (perhaps even the opposite; Chiaradia 2014), but it may act to enrich derivative melts in volatiles (H2O, S, and Cl) and further increase oxidation state (Burnham, 1979; Candela, 1992; Streck and Dilles, 1998; Rohrlach and Loucks, 2005; Chambefort et al., 2008; Richards, 2015; Hutchinson and Dilles, 2019) and forming relatively viscous, intermediate composition magmas. 3B) or areas of thin or mafic crust (nascent island arcs). Most porphyry Cu deposits in this region of Chile have undergone some degree of supergene Cu enrichment, but the Escondida orebody, discovered below gravel cover in 1981, was the most spectacular, when drill hole DDH61 intersected 250 m of chalcocite-enriched material averaging 3% Cu (pre-production reserves estimated to be 1.76 Mt at 1.59% Cu; Lowell, 1991; Ortiz, 1995; current measured sulfide resource is 5350 Mt at 0.63% Cu; BHP Annual Report, 2017). Consequently, after only a few million years, these systems may have been eroded down to levels where the mineralization can be accessed from surface. However, during subduction, the wedge of mantle asthenosphere above the downgoing oceanic lithosphere becomes metasomatized by fluids and melts derived from prograde metamorphism of the slab (Ringwood, 1977; Tatsumi, 1986; Peacock, 1993). Such intense focusing of fluid flow, especially in sheeted (parallel) vein sets, implies a structural control, either along preexisting structures that offer high-permeability fluid pathways or along hydraulically fractured veins and breccias that are aligned by a regional stress field (Tosdal and Dilles, 2020). doi: https://doi.org/10.1130/GES02086.1. However, there is a feedback loop, because if the largest and richest deposits become too rare, then availability declines, and prices will go up, making smaller or lower-grade deposits economic.
Exsolution of volatiles from magma during depressurization and/or cooling is an inevitable consequence of decreasing solubility in silicate melts with decreasing pressure and temperature, as well as the low concentration of volatiles in most crystallizing phases relative to silicate melt (which increases the volatile content of the residual melt; Burnham, 1979; Candela, 1989). (C) Disseminated chalcopyrite (Cp; yellow) in mafic andesitic wall rock, El Teniente, Chile (reflected light photomicrograph). Edmonds and Woods, 2018). These components then become incorporated into primary arc magmas, which are enriched relative to MORB in H2O (15 wt%, locally up to 8 wt%), Cl (5002000 ppm), CO2 (~35007600 ppm), and S (9002600 ppm) (Sobolev and Chaussidon, 1996; de Hoog et al., 2001; Fischer and Marty, 2005; Wallace, 2005; Kimura and Ariskin, 2014; Kamenetsky et al., 2017). Building on ideas relating to the triggering of large explosive volcanic eruptions (Christopher et al., 2015; Cashman et al., 2017; Sparks and Cashman, 2017), Richards (2018) suggested that an external trigger, such as sudden magma chamber depressurization or seismic shaking, might be required to prompt the onset of wholesale devolatilization in a batholithic-sized magma chamber (cf. For example, it will not be sufficient simply to progressively devolatilize a 200 km3 magma chamber over its typical million year history, because it will not be possible to sustain a single hot fluid pathway toward the surface over this long period, with the result that the exsolved fluids will cool quickly at depth before their flow can be focused, leading to reprecipitation of metals at background levels. The transition from a protracted period of compression, leading to the build-up of a large lower-crustal MASH zone, to transpression, would facilitate the rapid ascent and emplacement of a large volume of evolved magma (i.e., a batholith). However, the degree to which this accumulation occurs, and the proportion of the total fluid volume that becomes focused here, will depend on the number and prominence of such apices. These fluids will generate subsurface hydrothermal systems that have the potential to transport and precipitate metals to form ore deposits. This means that, at the time of formation, most porphyry Cu deposits will be too deep to mine from open pits at surface, and only the richest deposits could be mined as underground operations. Here the magma undergoes fractional crystallization and interaction with lower-crustal lithologies, evolving to intermediate calc-alkaline compositions. The low water contents and reduced nature of MORB (and tholeiitic nascent arc) magmas render them unprospective for porphyry-type magmatic-hydrothermal ore formation because chalcophile and siderophile metals (Cu, Au, and platinum-group elements [PGEs]) will tend to be lost to early precipitating sulfide phases (e.g., Mitchell and Keays, 1981; Hamlyn et al., 1985; Peach et al., 1990), and large volumes of hydrothermal fluid will not be exsolved upon depressurization and crystallization. Their much lower viscosity and density compared to silicate magmas suggest that they may rise through the crust as a separate plume, but it is also possible that much of this deep fluid is absorbed by reaction with hot wall rocks to form carbonate and hydrous silicate alteration minerals (e.g., Rosing and Rose, 1993). The relatively high oxidation state of Phanerozoic arc magmas (FMQ +1 to +2) limits the tendency of these S-rich magmas to undergo early sulfide saturation with resultant removal of chalcophile and siderophile elements from the melt (Mitchell and Keays, 1981; Hamlyn et al., 1985; Peach et al., 1990; Li and Audtat, 2015). These processes are all geologically normal, although some may be rare and/or random in their occurrence, but they must align; missing any one step, or its inefficient operation, will reduce or eliminate the potential for ore formation. In some of the most extreme examples, such as the Oyu Tolgoi porphyry Cu-Au deposit in Mongolia, quartz veins exceed 90% of the rock volume, and grades of 3%4% Cu plus 12 g/t Au were encountered over 100 m intervals during drilling (Figs. This additional step reduces the overall probability of finding an economic porphyry Cu deposit in any given arc batholithic system to perhaps 0.0001%. Such systems are very common in arc volcanoplutonic complexes and are probably the default product of batholithic devolatilization. These zones may be the equivalent of feeder systems below large composite volcanoes (e.g., Sillitoe, 1973), although surface volcanism does not appear to be necessary for porphyry formation (Sillitoe, 2010). Lower-temperature fluids may continue to flow over longer periods (105106 yr) as the system wanes and fluid drains from distal parts of the magma chamber (Candela, 1997; Cathles and Shannon, 2007). This exercise shows that, although Phanerozoic arcs are inherently fertile for ore formation, the actual process of generating an economic porphyry deposit depends on multiple steps that cumulatively reduce the probability to very low values. These characteristics place significant constraints on the processes that give rise to ore formation. Hildreth and Moorbath (1988, p. 483) envisaged the MASH zone to be a plexus of dikes, sills, pods, small chambers, and mushy differentiated intrusions, where basaltic magmas evolve by fractional crystallization and mix with felsic crustal melts to form hybrid magmas of intermediate composition and leave ultramafic to mafic lower-crustal cumulate zones (as observed in the Talkeetna arc, Alaska, the Kohistan arc, Pakistan, and the Sierra Valle Frtil arc, Argentina; Greene et al., 2006; Jagoutz et al., 2007; Walker et al., 2015). High Sr/Y values (>20) in igneous rocks have been widely used as an indicator of magmatic fertility for porphyry Cu formation (Thiblemont et al., 1997; Sajona and Maury, 1998; Oyarzun et al., 2001; Chiaradia et al., 2012; Loucks, 2014; Bissig et al., 2017), most likely reflecting high magmatic water content as a prerequisite for forming magmatic-hydrothermal systems (Lpez, 1982; Dilles, 1987; Lang and Titley, 1998; Richards et al., 2001, 2012; Rohrlach and Loucks, 2005; Schutte et al., 2010; Richards, 2011b). Erosional loss of upper-crustal porphyry Cu deposits increases with geological age (Kesler and Wilkinson, 2006, 2008; Wilkinson and Kesler, 2007, 2009) and in large part explains the increasing rarity of these deposits in Mesozoic, Paleozoic, to Proterozoic rocks, and their almost complete absence in the Archean. Such magmas tend to display listric-shaped normalized REE patterns, which flatten in the MREE and may increase again slightly in the HREE (Sisson, 1994; Nandedkar et al., 2016). (C) Multiple sets of sheeted quartz-molybdenite-pyrrhotite veins cut by a dacite porphyry dike, itself cut by late veins (high-grade ore zone,>2 wt% MoS2, MAX porphyry Mo deposit, British Columbia). Andesitic magmas with resultant high Sr/Y and La/Yb values are commonly referred to as adakites, but this term has been specifically associated with the melting of subducted oceanic crust (Kay, 1978; Defant and Drummond, 1990), which can lead to similar trace-element characteristics. For the purposes of this paper, I will focus on factors that control the fertility of magmatic systems, and I will separately distinguish factors that might then act on a fertile system to cause ore formation. The probability of ore formation is also a function of human valuation, because this defines what is considered to be ore (i.e., material from which metals can be profitably extracted). Instead, it appears that the bulk of the fluid must be released suddenly and the source magma chamber essentially devolatilized within a few tens of thousands of years, if ore formation is to be successful (Huber et al., 2012; Chelle-Michou et al., 2014; Schpa et al., 2017). Samples of porphyry Cu ore. (A and B) Drill core samples from Oyu Tolgoi, Mongolia, showing intense stock work and sheeted veining in quartz monzodiorite intrusions, with abundant chalcopyrite (Cp; yellow) and bornite (Bn; purple). 5; Redmond et al., 2004; Landtwing et al., 2005; Klemm et al., 2007; Cernuschi et al., 2018). Most such deposits are mined from large open pits, benefiting from economies of scale and efficient mineral processing technologies (to concentrate small percentages of sulfide minerals from large volumes of waste material; mostly silicate gangue). Most of this material, including up to 11% of the volatiles (Kimura and Nakajima, 2014), eventually sinks deep into the mantle and may reappear at Earth's surface later as components of mantle plumes (Hofmann and White, 1982; van der Hilst et al., 1997; Zhao, 2004). These two criteria, large volumes and rapid emplacement of magma, are critical for subsequent ore formation (Richards, 2003; Chiaradia and Caricchi, 2017; Schpa et al., 2017). However, in the MiocenePliocene El IndioPascua belt of Chile-Argentina, porphyry and epithermal Au-Ag-Cu deposits have been linked to either deep crustal dehydration melting during flat subduction and crustal thickening (Kay et al., 1999; Kay and Mpodozis, 2001; Muoz et al., 2012), direct release of flat slab-derived fluids into the lower crust, causing partial melting (Bissig et al., 2003), or tectonic changes immediately prior to or during the initial stages of subduction flattening (Skewes et al., 2002). Mid- to upper-crustal batholithic magma chambers (510 km depth) are thought to exist for most of their super-solidus lives as crystal mushes (Gelman et al., 2013; Klemetti, 2016), in which significant volumes of fluid may be trapped as interstitial bubbles (seen as vesicles or miarolitic cavities in granitic rocks; Candela and Blevin, 1995; Candela, 1997; Edmonds et al., 2014; Edmonds and Wallace, 2017) or in supersaturated viscous silicate melt (Gardner et al., 2000).
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