PLATINUM GROUP ELEMENT (PGE) SYSTEMATICS OF ARC SULFIDES: A CASE STUDY FROM CUMULATES OF THE CHILAS COMPLEX, KOHISTAN ARC, PAKISTAN

Mafic-ultramafic cumulate sequences of the Chilas Complex, Kohistan island arc, are generally sulfide deficient, with layered hornblende-pyroxene-plagioclase-gabbros containing less than 0.03 modal volume percent sulfides, with an average of 0.06% S in whole-rocks. Chalcophile and siderophile element analyses of these sulfides indicate 0.05-25 ppb Os, 0.005-29 ppb Ir, 1.4-697 ppb Pt, 0.3-325 ppb Pd, 12-27 ppb Au, and 74-644 ppm Cu. These concentrations are significantly higher than whole-rock values (0.007-0.9 ppb Os, 0.001-1 ppb Ir, 0.2-30 ppb Pt, 0.07-22 ppb Pd, 1-4 ppb Au, and 7-119 ppm Cu).

A small occurrence of dunite (500 m x 1000 m) hosts locally higher abundances of sulfides (up to ~5 volume %), and hence elevated concentrations of Cu (4500 ppm), Ni (2010 ppm), PGEs (533 ppb Pt, 747 ppb Pd), and Au (310 ppb). Moncheite ((Pt,Pd)(Te,Bi)₂) and Au-Ag alloy inclusions were observed in Cu-Fe sulfides (chalcopyrite and bornite). The dunites intrude the layered gabbro sequence, and are volumetrically a small part of the total ultramafic-mafic sequence (~15 vol.%).

These data provide evidence for the saturation of arc magmas in small volumes of sulfides in the deep crust. Mass balance calculations suggest that the bulk of highly siderophile elements (Au, PGE) are retained in these sulfide phases, with derivative (fractionated) magmas containing less than 4.8 ppb Au, 3.5 ppb Pt, and 4.3 ppb Pd. In contrast, Cu is likely minimally affected by fractionation of these small volumes of sulfide, and derivative magma concentrations likely remained close to assumed primary concentrations of 50-100 ppm Cu; these magmas could have gone on to form Au-PGE-poor porphyry Cu deposits. The residual Cu-Au-PGE-enriched sulfides could be remobilized into low-volume partial melts during later anatectic events, and represent a potential source of post-orogenic ore-forming magmas.