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.
Highlights
Porphyry Copper Deposits (PCDs) are classically regarded as large upper crustal volumes of hydrothermally altered, veined and mineralized rocks centred on small appendices of magmatic rocks extruded from a larger magma body lying at greater depth[1,2] (Fig. 1)
The extraction of fluids and metals from magmas is the first critical step for the genesis of magmatic-hydrothermal ore deposits[1,2] because it controls the amount of “ore ingredients” transferred to PCD formation levels
Quantifying the chemistry and rate of fluids released during the cooling of magmas in the Earth’s crust is key to identify magmatic systems with economic potential
Summary
Isobaric cooling of a thermally homogeneous volatile-saturated volume of magma leads to crystallisation and to the progressive exsolution of bubbles containing supercritical fluids (Fig. 2). For a unit volume of magma this outgassing process occurs repeatedly in a series of pulses with decreasing amplitude as cooling and crystallization proceeds and continues until the magma is solid and the only fluids present in the system are those structurally bound to hydrous minerals (e.g., amphibole, biotite; Fig. 2). For this scenario, the outgassing events occur at fixed crystallinities (fixed bubble/residual melt ratio) and at fixed temperatures (Fig. 2). For a thermally zoned magma intrusion cooling in the crust, the formation of permeability compatible with outgassing occurs simultaneously along isotherms corresponding to outgassing fronts (Fig. 3a–c)
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