Ophiolitic chromitites of Timor Leste: their composition, platinum group element geochemistry, mineralogy, and evolution

Ultramafic rocks that crop-out as fault-bounded blocks within Hili Manu, ~50 km east of the capital Dili on the north coast of Timor Leste, have been a topic of considerable debate regarding their ophiolitic affiliation with the Banda Arc or Australian subcontinental lithosphere. Two discrete ultramafic massifs separated by amphibolite of undefined origin, Be Heda Hill and Kerogeol Hill in the east and Subao Highway in the west, have been identified and host tabular and vein-like chromitite morphologies, respectively. The chromitite bodies at Hili Manu occur as small lenses a few meters in size. This paper presents the first detailed investigation of the chromitites from the two massifs at Hili Manu, including their mineralogy, textures, and chemistry. Chemically, the primary chromites from the Hili Manu chromitites comprise both high-Cr type (Cr₂O₃ 40.24-56.95 wt.% and Cr# [Cr/(Cr + Al)] > 0.6) that occur at Kerogeol Hill and Subao Highway and high-Al type (Cr₂O₃ 35.70-37.38 wt.% and #Cr < 0.56) that are restricted to Be Heda Hill. Similarly, the accessory spinels of the enclosing wall rocks also show high-Cr (#Cr = 0.60-0.82) and high-Al varieties (#Cr = 0.17-0.48). Platinum-group minerals (PGM) such as laurite and possible erlichmanite-series minerals, silicates, base metal sulfides (BMS), and arsenides are found as inclusions and within fractures in the chromite or the serpentinite matrix. The platinum-group element (PGE) concentration of the Hili Manu chromitites and its immediate host peridotite at the three localities varies, with total PGE (Os + Ir + Ru + Rh + Pt + Pd) in the ranges 73.1-1295 ppb and 28.5-364 ppb, respectively. The total PGE content is higher and the Pd/Ir ratio is lower in the Cr-rich chromitites compared to the Al-rich ones. Most of the PGM are 3-10 μm in size and occur as primary or composite inclusions in IPGEbearing phases, particularly Os-rich laurite, within unaltered chromite grains. Host peridotite major- and trace-element geochemistry coupled with the chemistry of chrome-spinels for both the chromitites and host peridotites in this study show evidence of formation of the Hili Manu peridotite in an upper mantle in a supra-subduction zone setting, part of the young oceanic lithosphere from the Banda Arc.