SHANDITE AND OTHER NICKEL MINERALS FROM CHROMITITES OF OPHIOLITE ASSOCIATION IN THE SOUTHEAST OF EAST SAYAN

Shandite (Ni₃Pb₂S₂) was first found by P. Ramdohr in serpentinites of the Trial Harbor deposit, Tasmania, in 1949. Until now, this has been virtually the only finding of the mineral. We found a new occurrence of shandite in chromitites of ophiolites of the Ospa-Kitoi region (southeastern East Sayan). It is shown that natural and synthetic shandites are close in chemical composition. Shandite occurs primarily in association with awaruite as elongate or drop-like segregations rimming its grains or filling its differently oriented microcracks. The mineral was also found as individual isometric or slightly elongate grains measuring to 500 µm. Along with awaruite, shandite occurs in association with heazlewoodite, orcelite, polydymite, and bravoite. Chromitites of the Ospa-Kitoi region bear two types of awaruite with different Ni/Fe ratios and two types of orcelites – with high and low contents of Ni (Ni₂As and Ni_5–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 °C. Serpentinization proceeded in back-arc spreading zones or in subduction zones.