No 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

Chalcophile element fertility, the chalcophile metal abundance in the source magma, is likely to be a critical factor for the formation of porphyry Cu ± Au deposits. In this study, we provide evidence to support this hypothesis by comparing the platinum group element (PGE) geochemistry of barren and ore-bearing Cu ± Au granitic suites. We report the PGE contents of three barren volcanic and subvolcanic suites from Argentina and Japan and two Cu ± Au bearing suites from Indonesia and Chile. These results are compared with those from previous studies of a porphyry Cu-only subvolcanic suite from Chile and three porphyry Cu-Au-bearing suites from Australia and the USA. The barren suites are depleted in PGE abundances by the time of fluid exsolution (< 0.1 ppb Pd and Pd/Pt <~3), which is attributed to early sulfide saturation in a mid to lower crustal magma chamber. In contrast, the Cu ± Au ore-bearing suites contain at least an order of magnitude higher PGE contents than the barren ones at fluid saturation (up to~10 ppb Pd and Pd/Pt of 0.1-40). They are characterized by late sulfide saturation, which allows both chalcophile elements and sulfur to concentrate by fractional crystallization before volatile saturation. We suggest that plots of Pd/ MgO against Pd/Pt for igneous suites can be used to estimate chalcophile element fertility and distinguish between barren, porphyry Cu, and porphyry Cu-Au granitoid systems. The positive correlation of these chalcophile element fertility indicators and ore grades suggests that metal contents in magmas play an important role in controlling ore grade, particularly Au, in porphyry Cu ± Au deposits.

Platinum-group element (PGE) geochemistry may be used to constrain the timing of sulfide saturation in magmas, which influences Cu and Au fertility of evolving magmatic systems. We report new geochronological and geochemical analyses, with emphasis on PGE geochemistry, for a suite of regional hornblende-porphyritic diorite intrusions and ore-bearing porphyries from the super-giant Escondida and smaller Zaldivar Cu deposits of Northern Chile. The regional dioritic intrusions have zircon U-Pb ages between 39.6 to 37.1 Ma, which overlap with ages of ore-bearing Escondida and Zaldivar porphyries (38.1 to 35.0 Ma). Whole rock major and trace element, Sr-Nd-Pb and zircon O-Hf isotope geochemistry indicates that the regional diorites and ore-bearing porphyries are co-magmatic and originated from the same mantle-derived magma by fractional crystallization, with minor contamination by Paleozoic crust (~10%). The low concentrations of PGE in the regional diorites show that they reached sulfide saturation before the MgO content of the melt fell to 4.7 wt.%, the MgO state of the melt, and the magma volume are critical factors in determining the potential to form a porphyry Cu deposit. Plots of Pd/MgO against Y can be used as empirical indicators of magma fertility for porphyry mineralization, and to discriminate between Cu-Au and Cudominated systems, but cannot predict the size of the deposit. The super-giant status of the Escondida deposit is attributed to it being underlain by a large batholith with a calculated minimum mass of 10 12 tonnes (~400 km 3).