Crustal magmatic controls on the formation of porphyry copper deposits

Abstract

Porphyry deposits are large, low-grade metal ore bodies that are formed from hydrothermal fluids derived from an underlying magma reservoir. They are important as major sources of critical metals for industry and society, such as copper and gold. However, the magmatic and redox processes required to form economic-grade porphyry deposits remain poorly understood. In this review, we synthesize advances in understanding crustal magmatic conditions that favor the formation of porphyry Cu deposits at subduction zones. Chalcophile metal fertility of mantle-derived arc magmas is primarily modulated by the amount and nature of residual sulfide phases in the mantle wedge during partial melting. Crustal thickness influences the longevity of lower crustal magma reservoir and sulfide saturation history. For example, in thick crust, prolonged magma activity with hydrous and oxidized evolving magmas increases ore potential, whereas thin crust favors high chalcophile element fertility owing to late sulfide saturation. In the shallow crust (<7 km), depth of hydrothermal fluid circulation and porphyry formation influences metal extraction from melts and ore precipitation from fluids because it controls the chemistry and phases of the fluids. The formation of porphyry Cu deposits is a result of series of optimum crustal magmatic processes. Our knowledge of the processes has been currently expanding. Future studies should focus on the nature of sulfide phases and sulfide melt-fluid interaction in felsic, hydrous magmatic systems and the enriched lithospheric mantle sources and metal transfer mechanisms for alkaline magmatic systems in post-collisional settings. Key points-Prolonged injection of hydrous basaltic magmas and accumulation of andesitic magmas in the mid to lower crust are prerequisites to forming large porphyry deposits because these processes are required to maintain a long-lived magmatic system and associated hydrothermal activity in the shallow crust.-Crustal thickness influences the duration and volume of magma activity, timing of sulfide saturation, chalcophile element fertility, and emplacement depth of porphyry intrusions.-Thick crusts (> 40 km) increase porphyry Cu ore potential by producing voluminous and hydrous magmas in long-lived (≥ 2-3 Ma) mid to lower crustal magma reservoirs at ~30-70 km depth, which can result in the formation of supergiant to giant porphyry Cu deposits if a combination of other ore-forming conditions is fulfilled.-In thin crust (< 40 km), late sulfide saturation and high chalcophile element fertility in shallow magma reservoirs (~5-15 km depth) increase Au-rich porphyry Cu ore potential.-Immiscible sulfide melts can act as temporary metal storages when the sulfide melts and exsolved fluids interact in the shallow magma reservoirs -Depth of porphyry emplacement (~1-7 km), magma alkalinity, and Au fertility control Au endowments in porphyry Cu deposits

Machine learning for geochemical exploration: classifying metallogenic fertility in arc magmas and insights into porphyry copper deposit formation

“…We find that porphyry Cu deposits associated with calc-alkaline magmas evolved at deeper levels in the crust (lower PC2 scores) than those associated with alkaline magmas. This is in line with the link between porphyry deposits in thicker arc crust (typically Cu-rich and Au-poor) with calc-alkaline magmas, and between porphyry deposits in thinner arc crust (typically Au-rich) with more alkaline magmas (Sillitoe 2000;Richards 2009;Chiaradia 2020;Park et al 2021).…”

Physical transport of magmatic sulfides promotes copper enrichment in hydrothermal ore fluids

“…This amount is comparable to the deficit of Cu in volcanic rocks above thick continental crust compared to arc rocks above thin crust (∼70%; Chiaradia, 2014) and the Cu/Ag deficit in the bulk continental crust compared to primitive arc basalts (∼50%; Chen et al, 2020). Selective depletion of the most chalcophile precious metals (Au, Pd, and Pt) by removal of a small amount of sulfide may explain the giant Cu-only deposits of Chile (Park et al, 2021). However, the low concentration of Pd and Pt in ore deposits is not a conclusive argument against sulfide entrainment and subsequent transfer to magmatic fluid: all these elements are highly soluble at magmatic temperatures (e.g., Sullivan et al, 2022) and their abundance in ore deposits therefore depends on selective precipitation efficiency.…”

“…The preceding evolution, by contrast, is currently envisioned to be a partial depletion step that is caused by sulfide saturation sequestering chalcophile metals originally present in primary arc basalts (e.g., Chen et al, 2020). A global deficit of Cu in arc volcanics above thick continental crust and in the bulk continental crust (Chiaradia, 2014), combined with common sulfide presence and partial Cu enrichment in lower crustal cumulates (Métrich et al, 1999;Rezeau and Jagoutz, 2020), is believed to indicate significant loss of Cu and other chalcophile metals by recycling to the mantle (Lee et al, 2012;Jenner, 2017;Park et al, 2021). Paradoxically, the Cu deficit of magmatic arc rocks is largest in tectonic settings that also host the world's premier porphyry copper provinces (Chiaradia, 2014;Loucks, 2021).…”

“…The suggested roles for magmatic sulfide in the formation of porphyry copper deposits have focused on upper crustal processes; e.g., transient chalcophile element storage, sulfide accumulation and remobilization, bubble-flotation of composite particles, and wholesale or fractional sulfide decomposition upon fluid saturation, which can affect the bulk Au/Cu ratio of resulting ore deposits (Hattori and Keith, 2001;Halter et al, 2004;Nadeau et al, 2010;Wilkinson, 2013;Mungall et al, 2015;Yao and Mungall, 2020). Most recent research has instead focused on identifying tectonic and magmatic conditions that minimize lower crustal Cu loss and optimize chances of making ore deposits from the remaining Cu in evolving magmas (Richards, 2015;Lee and Tang, 2020;Rezeau and Jagoutz, 2020;Chelle-Michou and Rottier, 2021;Park et al, 2021).…”

Discovery of Yaozhuang Stock and Deep Ore Prospecting Implication for the Western Mangling Orefield in North Qinling Terrane, Central China

“…The formation of porphyry Mo deposits is controlled by many factors, including the content of water and sulfur in magmatic-hydrothermal solution, oxygen fugacity of magma (Burnham and Berry, 2012), and the migration of metal elements (Sillitoe, 2010;Richards et al, 2012;Park et al, 2021). Porphyry metallogenic magmas not only in subduction settings but also in collision settings have significant water-rich (>4.0%) (Wang et al, 2014a) and high-oxygen fugacity (Delta FMQ > 1.5) (Richards, 2011;Wang et al, 2014b).…”