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.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). A wealth of geologic and isotopic evidence demonstrates that metals (e.g. Cu, Mo, Au, Zn, Pb) and sulphur found at PCD crustal levels (i.e., 1-5 km depth) originate from degassing magma bodies (plutons) emplaced at 5-15 km depth 1-4 . A fundamental challenge in developing models for the genesis of PCDs is to understand the set of conditions leading to the formation of deposits spanning four orders of magnitude of copper endowment worldwide 5 . The genesis of the largest deposits is favoured by various factors, including long-lived magmatic-hydrothermal activity, high concentrations of "ore ingredients" (i.e., copper and sulphur) in the source magma, efficient partitioning and transport of these elements in the magmatic fluid, and localised precipitation of copper sulphides 6-8 . The common association of high-Sr/Y magmatic rocks with PCDs 9 indicates that specific magmatic processes may generate particularly fertile magmas. Nevertheless, mass balance calculations for the Yerington and Bingham districts (two classical PCDs for which the size of the pluton is known) show that the volumes of the respective underlying plutons are sufficient to form the overlying PCDs, assuming "normal" initial copper and sulphur concentrations 10,11 . Furthermore, the compositions of high temperature single-phase intermediate-density fluids from barren and mineralized systems have broadly similar compositions 8,12 . These da...

Deciphering fluid flow at the magmatic-hydrothermal transition: A case study from the world-class Panasqueira W–Sn–(Cu) ore deposit (Portugal)

“…From this surface and the mean fluid flow velocity (10 -4 m.s -1 ) we obtain a fluid flow duration ranging from to 70 000 years. This is consistent with the short time duration of mineralization emplacement estimated for a single magmatic-hydrothermal event (spanning only about 10 000 to 200 000 years) from numerical modeling (Cathles et al, 1997;Chelle-Michou et al, 2017) and radiometric dating (Parnell, 1998).…”

Gold endowments of porphyry deposits controlled by precipitation efficiency

“…A similar conclusion has also been reached by ref. 22 . In the Cu endowment vs. duration of ore deposit formation plot ( Fig.…”

Cyclic Magmatic-Hydrothermal Evolution in Porphyry Systems: High-Precision U-Pb and Re-Os Geochronology Constraints on the Tibetan Qulong Porphyry Cu-Mo Deposit*

“…Cu, comparable to the amount of Cu at Qulong, can be accumulated within 200 -300 kyrs 604 (Chelle-Michou et al, 2017). 605…”

“…11) suggests that at least some of the prolonged durations 799 proposed for porphyry systems may be an artefact of over interpreting low-precision data 800 without evaluation of their accuracy. Recent simulation studies suggest that the duration of 801 ore formation is potentially one the of controlling factors for the formation of giant deposits 802 (Chelle-Michou et al, 2017;Chiaradia and Caricchi, 2017). However, it is obvious that such 803 a "long" duration (e.g., tens to hundreds of kyr) is still beyond the precision of micro-beam 804 analysis (Chiaradia et al, 2014).…”