Tempo of magma degassing and the genesis of porphyry copper deposits

Abstract

Porphyry deposits are copper-rich orebodies formed by precipitation of metal sulphides from hydrothermal fluids released from magmatic intrusions that cooled at depth within the Earth’s crust. Finding new porphyry deposits is essential because they are our largest source of copper and they also contain other strategic metals including gold and molybdenum. However, the discovery of giant porphyry deposits is hindered by a lack of understanding of the factors governing their size. Here, we use thermal modelling and statistical simulations to quantify the tempo and the chemistry of fluids released from cooling magmatic systems. We confirm that typical arc magmas produce fluids similar in composition to those that form porphyry deposits and conclude that the volume and duration of magmatic activity exert a first order control on the endowment (total mass of deposited copper) of economic porphyry copper deposits. Therefore, initial magma enrichment in copper and sulphur, although adding to the metallogenic potential, is not necessary to form a giant deposit. Our results link the respective durations of magmatic and hydrothermal activity from well-known large to supergiant deposits to their metal endowment. This novel approach can readily be implemented as an additional exploration tool that can help assess the economic potential of magmatic-hydrothermal systems.

Stochastic modelling of deep magmatic controls on porphyry copper deposit endowment

“…2a). Assuming that these extracted fluids generate mineralization28, the median value of Cu accumulation after the maximum lifetime (~200 ka) permissible for such a sustained flux2026 would be <9 Mt (<4.5 Mt Cu for 50% efficiency: Fig. 2b).…”

“…However, it is the next step (the transfer of this large accumulation of hybrid melt to shallower levels from where fluids can be exsolved) that determines the formation and the ultimate size of the deposit (see also ref. 28). This is suggested by data on Cu endowment and overall duration of the ore deposition period for different deposits (Supplementary Information 1, Table S1.1).…”

Amphibole and apatite insights into the evolution and mass balance of Cl and S in magmas associated with porphyry copper deposits

“…Another volume estimate can be made considering that the ore-related magmatic system assembled over a maximum of 100 ka (Chelle-Michou et al 2015b). Assuming typical magma fluxes of 0.001 to 0.01 km 3 /a in magmatic arc setting (de Saint Blanquat et al 2011), a maximum magma volume of 100 to 1000 km 3 can be estimated (see also Chelle-Michou et al 2017). The differences between mass balance and geochronology-based volume estimates may indicate that (1) only a small fraction of the total budget of Cu, Fe and S could have been extracted and precipitated, and/or (2) other deposits in the immediate vicinity of Coroccohuayco (e.g., Tintaya, Ccatun Pucara, Quechua and Antapaccay with a total of 13 Mt of Cu; Chelle-Michou et al 2015b) may have been sourced by the same body of magma at depth (corresponding to a minimum of 143-570 km 3 using the same constraints as above).…”

“…To compute the mass transfer of volatile and fluid-mobile elements (i.e., S, Cl and Cu) from the cooling pluton to the overlying porphyry-skarn deposit, we used the magma outgassing model of Chelle-Michou et al (2017) for a fluidsaturated magma with 5 wt% of water. This model considers stepwise degassing of the magma at successive fluid percolation thresholds over the course of magma cooling and crystallization.…”

“…These elements are usually sourced from andesitic to dacitic magmas that undergo outgassing upon cooling and crystallization at upper crustal depth, and subsequently transported to higher crustal levels where extensive sulfide precipitation might occur if appropriate conditions are met (Hedenquist and Lowenstern 1994;Dreher et al 2005;Sillitoe 2010;Pettke et al 2010;Simon and Ripley 2011;Chelle-Michou et al 2017). Importantly, chlorine and sulfur are critical elements of ore-forming fluids due to their capacity to form ligands with ore metals (e.g., Communicated by Hans Keppler.…”