Abstract
Porphyry deposits, our main source of copper and of significant amounts of Mo, Re and Au, form at convergent margins in association with intermediate-felsic magmas. Although it is accepted that copper is transported and precipitated by fluids released by these magmas, the magmatic processes leading to the formation of economic deposits remain elusive. Here we perform Monte Carlo petrological and geochemical modelling to quantitatively link crustal magmatic processes and the geochemical signatures of magmas (i.e., Sr/Y) to the formation of porphyry Cu deposits of different sizes. Our analysis shows that economic deposits (particularly the largest ones) may only form in association with magma accumulated in the lower-middle crust (P > ~0.5 GPa) during ≥2–3 Ma, and subsequently transferred to and degassed in the upper crust over periods of up to ~2.0 Ma. Magma accumulation and evolution at shallower depths (<~0.4 GPa) dramatically reduces the potential of magmatic systems to produce economic deposits. Our modelling also predicts the association of the largest porphyry deposits with a specific Sr/Y interval (~100 ± 50) of the associated magmatic rocks, which is virtually identical to the range measured in giant porphyry copper deposits.
Highlights
Porphyry deposits, our main source of copper and of significant amounts of Mo, Re and Au, form at convergent margins in association with intermediate-felsic magmas
Our results suggest that the formation of arc magmatic systems associated with porphyry Cu deposits occurs in two steps: 1) long-lasting (>~2.5 Ma) injection of hydrous basalts in the mid- to lower crust (>~17 km) leading to the formation of large amounts (>800 km3) of andesitic magma (SiO2 = 57–64 wt.%) with a specific interval of Sr/Y ratios (50–150) and high H2O concentrations (85% of the simulations of the most fertile magmas, with exsolvable Cu > 30 Mt, are between 5.5–13 wt.% H2O; Supplementary Information 4, Figure S4.1) under an average magmatic arc flux (e.g., 0.0009 km3/a); 2) subsequent transfer of this magma to mid-/upper crustal levels (~8–18 km), from where magma may provide copper-bearing fluids, during recurring episodes of mineralization composition
The largest systems correspond to Sr/Y values that overlap with the average (±2σ) Sr/Y values of major porphyry Cu deposits (Table S1.1; (b) gigatons of exsolvable H2O contained in the hybrid melts accumulated at different crustal depth versus their Sr/Y compositions
Summary
Our main source of copper and of significant amounts of Mo, Re and Au, form at convergent margins in association with intermediate-felsic magmas. Annen et al.[16] have developed a thermal model to quantify the generation and evolution of intermediate to felsic magmas, i.e., those typically associated with porphyry deposits, which is valid for a broad range of physico-chemical conditions (P, T, H2O contents, magma fluxes) deemed appropriate for subduction-related magmatic systems. This thermal modeling shows that, for a given magma flux, the size of crustal magmatic systems, which controls the maximum amount of Cu that they can deliver, is controlled by the depth (pressure, P) and duration of magma accumulation (time, t). Using a Monte Carlo approach, we combine the model of ref. 16 with models of H2O solubility in silicate melts[17], petrological and geochemical modelling, as well as mass balance calculations, to quantify crustal www.nature.com/scientificreports/
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