Detrital Platinum-Group Minerals and Gold In Placers of Southeastern Samar Island, Philippines

Abstract

Abstract The present work provides a comprehensive description of the assemblage of detrital platinum-group minerals (PGM) and gold found in the Llorente river and its tributaries draining the area west of the town of Llorente in southeastern Samar, Philippines. Altogether, 610 grains of detrital PGM were studied by reflected light and scanning electron microscopy, and 464 quantitative electron-microprobe compositions were obtained from 265 grains of PGM in polished sections. The concentrates are dominated by grains of gold, followed by accessory PGM grains. The assemblage of detrital PGM is composed of mainly monophase grains of Os-Ir-Ru-(Pt) alloys (41.3%), Pt-Fe alloys and platinum (together 40.5%), laurite–erlichmanite (16.6%), and rare other PGM (1.6%). Grain sizes of the PGM range from ∼50 μm up to 2–3 mm; median sizes are 250 μm for Pt-Fe and Os-Ir-Ru alloy grains, and 420 μm for laurite–erlichmanite grains. Gold grains are commonly silver-rich and display leached outer zones of Ag-poor gold. Grain sizes range from ∼50 μm to 6.8 mm (median about 400–500 μm). Notable is the presence of rare tetraauricupride. Both the regional bedrock geology and the distinct PGM assemblage indicate that the PGM originated from Alpine-type ophiolite rocks, probably chromitites, which are common in the study area drained by the Llorente river and its tributaries in the Eastern Bicol-Eastern Mindanao ophiolite belt of Samar. Gold and tetraauricupride may originate from diverse sources. Rare inclusions of tellurides and chalcopyrite in detrital gold grains suggest epithermal gold vein deposits or gold-bearing porphyry copper mineralization as sources of the gold, whereas the tetraauricupride grains may originate from hydrothermal mineralization, different from the gold lode deposits, in mafic/ultramafic rocks of the ophiolite belt.