Contribution to the petrogenesis of the platinum palladium-bearing black dunites of the Acoje block, Zambales ophiolite, Philippines: selenium and sulfur-contents, clinopyroxenes, initial weathering

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Black serpentinites (BS), hosting the NiCu sulfide, platinum and palladium mineralisation in the Acoje ophiolite block in contact with chromitite concentrations, were investigated in comparison to the non-mineralised green serpentinites (GS), concerning their selenium and sulfur bulk rock contents, their secondary clinopyroxene, pyroaurite and hematite formation. Se/S-ratios in the black serpentinites range from 3.4 × 10 −3 to 4.8 × 10 −4. They are in the same order of magnitude as Se/S-ratios analysed in individual sulfides (Orberger and Taxel 1991) and consistent with the suggested mantle Se/S-ratios of 10 −4 (Eckstrand R., Goldschmidt Conference, Baltimore, 1988). A positive correlation of selenium and platinum/palladium is found for this ophiolitic environment. This correlation is proposed for high PtPd potential magmatic sulfide deposits in layered intrusions (Eckstrand 1988); thus, this study confirms the magmatic character of the NiCu-sulfide mineralisation proposed by Abrajano and Pasteris ( Tectonophysics 168, 65–100, 1989). Two types of diopsides characterise mainly the black serpentinites. Diopside I is present in green and black serpentinite and shows CaO contents of about 21 wt%, and Cr 2O 3 contents of 0.54–0.92. Diopside II is mainly present in black serpentinite. It is extremely CaO-rich (24.0–25.4 wt% CaO) and significantly lower in Cr 2O 3 (0.04–0.54 wt%). Initial weathering is mainly observed in BS and is expressed in the formation of pyroaurite and locally Fe-oxide and -hydroxide, surrounding the base metal sulfides, and the formation of Fe-rich “serpentines” (up to 11 wt%), forming large black platy aggregates in the clay fraction of BS. Pyroaurite-formation buffers pH and retains PtPd during weathering, whereas hematite and goethite formation provide sufficiently oxidising conditions for a PtPd-transport. The black platy aggregates in the clay fraction of the black serpentinites might be responsible for its black color.