Fractionation of sulfide phases controls the chalcophile metal budget of arc magmas: evidence from the Chilas complex, Kohistan arc, Pakistan

Some arc magmas lead to the formation of porphyry deposits in the relatively shallow upper crust (<5 km). Porphyry deposits are major sources of Cu and an important Au source but lack significant amounts of platinum group elements (PGE). Sulfide phases control the behavior of chalcophile elements and affect the potential to form ore deposits either by remaining in the mantle residue or by fractionating from arc magmas at lower crustal levels, although in detail the role of sulfide saturation in the lower crust remains poorly understood. Lower crustal cumulate rocks from the 85 Ma Chilas Complex of the Kohistan arc, Pakistan, provide insight into processes that occur at depth in arcs. Here we provide Cu, Ni, Au, and PGE concentrations and Os isotope ratios of the Chilas Complex in order to constrain the extent of sulfide saturation in the lower crust and the effect of sulfide saturation on the metal budget of evolved melts that ascend to the upper crust. The Chilas rock suite contains less than 0.17 wt % sulfides and low PGE concentrations. In situ laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) measurements of the sulfide inclusions in silicate minerals show enrichment in several chalcophile elements (up to 34 wt % Cu, 23 ppm Au, 245 ppm Pd, and 20 ppm Pt), whereas iridium group PGE (IPGE- Os, Ir, Ru) are mainly below detection limits. The metal content of the parental melt was modeled based on the elemental concentrations of the sulfides. The modeled parental arc magmas contain 70 to 140 ppm Cu, 0.2 to 1.5 ppb Au, and 1.2 to 8 ppb Pd, but low concentrations of IPGE, suggesting that IPGE were likely retained in the mantle source. Mass balance calculations show that segregation of a sulfide melt in the lower crust could further deplete the melt by more than 95% in Pd and Pt, 33 to 85% in Au, and 13 to 60% in Cu. Thus, magmas that ascend to the upper crust would contain very low concentrations of Au (< 0.2 ppb) and Pd (< 0.04 ppb), but they would retain sufficient concentration of Cu (~45–57 ppm) to form porphyry Cu deposits upon emplacement in the upper crust, as is commonly observed in arc settings