Shandite and other nickel minerals from chromitites of ophiolite association in the southeast of East Sayan

Shandite (Ni3Pb2S2) was first found by P. Ramdor in serpentinites of the Tr ial-Harbor deposit (Tasmania) in 1949. Up to now, this has been virtually t he only finding of the mineral. We found a new occurrence of shandite in ch romitites of ophiolites of the Ospa-Kitoi region (southeast of East Sayan). It is shown that natural and synthetic shandites are close in chemical com position. Shandite occurs primarily in awaruite as elongated or drop-like s egregations on the periphery of its grains or fills its differently oriente d microcracks. The mineral was also found as individual isometric or slight ly elongated grains measuring to 500 mu m. Along with awaruite, shandite oc curs In association with heazlewoodite, orcelite, polydymite, and bravoite. Chromitites of the Ospa-Kitoi region bear two types of awaruite with diffe rent Ni : Fe ratios and two types of orcelites - with high and low contents of Ni (Ni2As and Ni5-xAs2, respectively). Formation of nickel minerals in these chromitites proceeded at low temperatures in the stability region of serpentinites with the upper bound of 500 degrees C. Serpentinization proce eded in back-are spreading zones or in subduction zones.