Potarite (Pd-Hg) in thermally metamorphosed dunite from the Inazumi-yama ultramafic complex, southwestern Japan: an implication for the behaviour of mercury in PGE mineralization in peridotite

Pd-rich PGM (platinum-group minerals) have been found in sulphide-bearing d unites and harzburgites from the Inazumi-yama ultramafic complex in southwe stern Japan. In the dunite, potarite (PdHg0.86-0.88) is the most abundant P GM but other associated PGM include stibiopalladinite (Pd5Sb2) and rare spe rrylite (PtAs2) and Pd-rich alloys. A Pd telluride has been found in the ha rzburgite. The PGM are enclosed usually by pentlandite-heazlewoodite compos ite grains and, rarely, by altered chrome-spinel. These minerals are charac teristic of an ultramafic assemblage but they are accompanied by ubiquitous galena and minor sphalerite not usually associated with these ultramafic a ssemblages. The ultramafic part of this complex has been thermally metamorp hosed (olivine-talc zone) within the contact aureole of an adjacent granite . The PGM, sulphides and altered spinel are all intergrown with antigorite and/or chlorite, indicating a metamorphic overprint on the primary igneous mineralogy. The Pd/Pt ratios of 9 suggest a process of hydrothermal concent ration for the Pd and it is proposed that the Pd has been remobilized and r econcentrated by hydrothermal solutions derived from the granitic magma whi ch reacted with the Pd concentrated in the primary magmatic sulphides. Merc ury may have been leached from surrounding sediments by the hydrothermal so lutions. It is possible that potarite and the other PGM formed at temperatu res of up to 650 degrees C and are likely to be found in other thermally me tamorphosed ultramafic rocks showing a depleted character (spinel Cr/[Cr+Al ]>0.7), common in orogenic belts.