Chalcophile element fertility and the formation of porphyry Cu ± Au deposits

Abstract

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.

Magmatic sulfides in high-potassium calc-alkaline to shoshonitic and alkaline rocks

“…These sulfide types are similar to those found in midocean ridge basalts (MORBs; e.g. Patten et al, 2012;Keith Figure 8. Box plot comparison of the Cu and Ni content (wt %) resulting from individual mineral analyses measured by EPMA for the different sulfide types characterising each study area.…”

The efficiency of copper extraction from magma bodies: Implications for mineralization potential and fluid-silicate melt partitioning of copper

“…The potential of Cu mineralization is therefore suppressed in magmas with low oxygen fugacities (e.g., Zajacz et al 2012). Another possible means of lowering ore-forming potential via low oxygen fugacities is the sequestration of metals by early sulfide fractionation (e.g., Jenner et al 2010;Park et al 2015Park et al , 2019Hao et al 2017), but this proposal is not widely accepted (e.g., Spooner 1993;Keith et al 1997;Larocque et al 2000;Halter et al 2002Halter et al , 2005Stavast et al 2006;Nadeau et al 2010Nadeau et al , 2013Wilkinson 2013;Du and Audétat 2020). Here, we employed the amphibole oxybarometer (Ridolfi et al 2010) to track the oxidation state of the three plutons.…”

Geochronology, Petrogenesis and Oxidation State of the Northparkes Igneous Suite, New South Wales, Australia: Implications for Magma Fertility

“…In a high oxygen fugacity magma (ΔFMQ > +1) at ~0.2 GPa, the dominant sulfur species in the melt will be highly soluble sulfate (Jugo, 2009;Beermann et al, 2011;Matjuschkin et al, 2016), which will delay the sulfide saturation during magmatic evolution. Recent studies have shown that late sulfide saturation allow the chalcophile elements (Au and Cu) to become concentrated in the magma, which favours the formation of porphyry deposits (Crocker et al, 2015;Park et al, 2015Park et al, , 2019Hao et al, 2017).…”