Os-isotope Study of Platinum-Group Minerals in Chromitites in Alpine-Type Ultramafic Intrusions and the Associated Placers in Borneo

Keiko Hattori, Klaus-Peter Burgath and Stanley R. Hart
Ottawa-Carleton Geoscience Centre and Dept. of Earth Sciences, University of Ottawa, Ottawa K1N 6N5, Canada
Bundesanstalt für Geowissenschaften und Rohstoffe, Postfach 510153, D-3000 Hannover 51, Germany
Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, U.S.A.

Abstract: 187Os/186Os ratios were determined for in-situ laurite grains in Alpine-type chromitites and platinum-group minerals (PGM) in the associated alluvial placers in Borneo, Indonesia/Malaysia. The Os-isotope ratios of laurite grains in chromite defne an 187Os/186Os ratio for the 100 Ma mantle source of c. 1.04. The low 187Os/186Os ratios in all grains confirm the essential derivation of these platinum-group elements (PGE) from the mantle. A minor variation in 187Os/186Os ratios was detected among PGM from placers, but no variation was found within individual grains, including a grain with chemical inhomogeneity. The values are similar to those for PGM in the associated chromitites. The data are consistent with a detrital origin of PGM in placers: the placer PGM originated in the ultramafic section of ophiolities and the release of these grains from igneous rocks and their deposition in placers was almost entirely by mechanical processes.

Keywords: osmium isotopes • platinum-group minerals • laurite • chromitites • Borneo • Indonesia • Malaysia

Mineralogical Magazine; June 1992 v. 56; no. 383; p. 157-164; DOI: 10.1180/minmag.1992.056.383.02
© 1992, The Mineralogical Society
Mineralogical Society (www.minersoc.org)