Porphyry copper deposit formation in arcs: What are the odds?

Abstract

Arc magmas globally are H2O-Cl-S–rich and moderately oxidized (ΔFMQ = +1 to +2) relative to most other mantle-derived magmas (ΔFMQ ≤ 0). Their relatively high oxidation state limits the extent to which sulfide phases separate from the magma, which would otherwise tend to deplete the melt in chalcophile elements such as Cu (highly siderophile elements such as Au and especially platinum-group elements are depleted by even small amounts of sulfide segregation). 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 magmatic-hydrothermal fluid phase has the potential to form ore deposits (most commonly porphyry Cu ± Mo ± Au deposits) if its metal load is precipitated in economic concentrations, but there are many steps along the way that must be successfully negotiated before this can occur. 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. The cumulative probability of forming a large porphyry Cu deposit at any given time in an arc magmatic system (i.e., a single batholith-linked volcanoplutonic complex) is estimated to be ~0.001%, and less than 1/10 of these deposits will be uplifted and exposed at shallow enough depths to mine economically (0.0001%). Continued uplift and ero­sion in active convergent tectonic regimes rapidly remove these upper-crustal deposits from the geological record, such that the probability of finding them in older arc systems decreases further with age, to the point that porphyry Cu deposits are almost nonexistent in Precambrian rocks. A key finding of this paper is that most volcanoplutonic arcs above subduction zones are prospective for porphyry ore formation, with prob­abilities only falling to low values at late stages of magmatic-hydrothermal fluid exsolution, focusing, and metal deposition. This is in part because of the high threshold required in terms of grade and tonnage for a deposit to be considered economic. Thus, the probability of forming a porphyry-type system in any given arc segment is relatively high, but the probability that it will be a large economic deposit is low, dictated to a large extent by mineral economics and metal prices.

Formation of Gold Alloys during Crustal Differentiation of Convergent Zone Magmas: Constraints from an AU-Rich Websterite in the Stanovoy Suture Zone (Russian Far East)

“…In particular, gold-bearing alloys in the Ildeus intrusion co-exist with a wide range of native metals (Pt, W, Bi, Pb, Zn, Au, Ag) and intermetallic compounds (Fe-Pt, Ni-Rh-Pt, Pd-Pt, Cu-Ag-Au, Cu-Ag, Ni-Cu-Zn-Ag-Au, Cu-Zn-Ag, Cu-Zn-Au, Cu-Sn, Cu-Zn-Sn, Pb-Sb) indicating reduced conditions during the development of crustal magmatic conduits in the Triassic Stanovoy subduction zone [36]. Our data from the Ildeus intrusion suggest that, besides the influence of pressure and temperature, variations in the redox conditions between deep lithosphere and shallow, oxidized upper crust in subduction-related settings [36,70,71,73], where gold and other ore metals are usually deposited in the porphyry and epithermal environments [6,26,91,92] will also play an important role in the modification of Au-bearing alloys under a range of crustal conditions.…”

Numerical Modeling of Structurally Controlled Ore Formation in Magmatic‐Hydrothermal Systems

“…In magmatic‐hydrothermal deposits, field observations and numerical modeling show that fluid release from magmatic bodies can be focused through a cupola region (e.g., Burnham, 1979; Dilles, 1987; Richards, 2021; Shinohara & Hedenquist, 1997). Fluids are released directly into the host rocks in the porphyry‐like setup and into a permeable structure in the horizontal vein‐type deposit setup.…”

Distinct magma evolution processes control the formation of porphyry Cu–Au deposits in thin and thick arcs

“…2a). Longlived compression in the overriding plate typically occurs during subduction (Richards, 2021) and slows down the ascent of mantlederived melts, favoring their evolution at deep crustal levels. In agreement with this, geochemical indices of bulk rocks (e.g., increasing Sr/Y, La/Yb) indicate that magmas undergo an average deeper fractional crystallization as the arc crust thickness increases (Mantle and Collins, 2008;Profeta et al, 2015;Chiaradia, 2015Chiaradia, , 2021b.…”

“…Additionally, the maximum potential endowments occur for systems that are continuously exsolving fluids at high pressures (>∼0.6 GPa) for durations of ∼1.5 Myr (Fig. 5); it is unlikely that fluids exsolving so deep can be focused into the same upper crustal volume during 100 s of kyr and up to ∼1.5 Myr to form an economic deposit (Richards, 2021). Therefore, process 1a is overall unlikely to form porphyry Cu-Au deposits of any size.…”