Extreme fractionation of platinum group elements in volcanogenic massive sulfide deposits

Platinum group elements (PGE) and gold in selected ore samples and associat ed lithologies from four well-known volcanogenic massive sulfide (VMS) dist ricts (i.e., the kuroko Zn-Pb-Cu deposits of Hokuroku, Japan; the Besshi Cu -Zn deposit of Shikoku, Japan; the Cu-Zn-Au-Ag deposits of Manitouwadge, On tario, Canada; and the Cu-Zn-Co deposits of Outokumpu, Finland) have been d etermined by nickel sulfide fire assay pre concentration, tellurium copreci pitation, and inductively coupled plasma mass spectrometry analysis. The ch alcopyrite-rich samples associated with mafic-ultramafic rocks from Besshi, Manitouwadge, and Outokumpu locally contain elevated contents of Pd (up to 1.8 ppm), Rh (up to 0.8 ppm), and Au (up to 14 ppm). whereas those of the kuroko deposits hosted by felsic volcanic rocks are poor in PGE. Moreover, the chalcopyrite-rich samples and cordierite-orthoamphibole gneisses show e xtreme fractionation of Au, Ir, Pd, and Pt (Au/Ir values up to 108,000; Pd/ Ir, up to 29,500; and Pd/Pt. up to 2,100), which are somewhat similar to pr eciously reported Au/Ir and Pd/Ir values in modern sea-floor hydrothermal s ulfides but are significantly higher than those in magmatic Ni-Cu sulfides. The extreme fractionation of Au, Ir, Pd, and Pt in these volcanogenic mass ive sulfide deposits cannot be explained by tile relative metal solubilitie s in sea-floor hydrothermal fluids but may be related to local remobilizati on of PGE and Au during late hydrothermal alteration and/or metamorphism.