Os-isotope study of platinum-group minerals in chromitites in Alpine-type ultramafic intrusions and the associated placers in Borneo KE1KO HATTORI Ottawa-Carleton Geoscience Centre and Dept. of Earth Sciences, University of Ottawa, Ottawa KIN 6N5, Canada KLAUS-PETER BURGATH Bundesanstalt fiir Geowissenschaften und Rohstoffe, Postfach 510153, D-3000 Hannover 51, Germany AND STANLEY R. HART Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, U.S.A. Abstract tS7Os/lS6Os 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 t87Os/186Os ratio for the 100 Ma mantle source of 187 t86 ms nfir essential derlv ton of these latlnum rou c. 1.04. Thelow Os/ Os ratios in all gra" co mthe " " at" p " -g p elements (PGE) from the mantle. A minor variation in 1870S/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. KEYWOROS: osmium isotopes, platinum-group minerals, laurite, chromitites, Borneo, Indonesia, Malaysia. Introduction THE 1870S/186Os ratios of crustal rocks are much greater than 10, and more variable than the ratios of the mantle (c. 1.0) due to the high and varied Re/Os ratios of the crustal rocks (e.g. All~gre and Luck, 1980; Morgan et al., 1981; Chou et al. 1983; Luck and Turekian, 1983). The large difference in 187Os]]86Os ratios between crustal and mantle- derived rocks may be useful in evaluating the origin and history of the rocks and minerals. Os- isotopes may be particularly interesting for the study of the genesis of PGM and for distinguish- ing between crustal and mantle sources for the PGE. This paper presents tS7Os/IS60s ratios of in- situ laurite in chromitites of an Alpine-type ultramafic-mafic complex and of PGM from the associated placers on Borneo Island, Indonesia/Malaysia. Mineralogical Magazine, June 1992, Vol. 56, pp. 157-164 @~Copyright the Mineralogical Society Geology of the area Most of the samples were collected from the Meratus-Bobaris region in Southeast Kaliman- tan, southern Borneo (Fig. 1A)~ The placers in Southeast Kalimantan occur along river systems which pass through a series of Middle to Late Cretaceous deep-sea sediments and basic volcanic rocks (Alino Formation), Alpine-type ultramafic complexes, elastic sedimentary rocks (Paniiungan Beds), and a series of probably Cretaceous crystalline schists. The rocks in the area are highly deformed and segments of different lithologies are juxtaposed during regional tectonic activity. The ophiolite rocks are believed to be a mixture of an obducted oceanic crust and trench sedi- ments formed in the Middle to Upper Lower Cretaceous at around 114 Ma (Burgath, 1988). The area was later intruded by numerous calc-