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.