Origin of high-Cr podiform chromitites from Kabaena Island, Southeast Sulawesi, Indonesia: constraints from mineralogy and geochemistry

The East Sulawesi Ophiolite, one of the three largest ophiolites in the world, contains important podiform chromitites. However, the origin of these chromite deposits is not well constrained. Here, we present the detailed mineralogy and in situ chemistry of chromite and solid inclusions within chromite for podiform chromitites from the Kabaena Island, Southeast Sulawesi. Chromite grains have low TiO₂ (0.17–0.27 wt%) and Al₂O₃ (13.4–15.1 wt%) contents and high Cr# [Cr/(Cr+Al), molar] of 0.70–0.74 and Mg# [Mg/(Mg+Fe²⁺), molar] of 0.69–0.75, which resemble those of other high-Cr podiform chromitites worldwide. The MORB-normalized patterns for some selected major-trace elements of chromite show slightly positive slopes from Al₂O₃ to Mn. Chromite-hosted silicate inclusions include olivine, clinopyroxene, orthopyroxene and amphibole and are characterized by higher Mg/Fe ratios than those of host peridotites. The geothermobarometry of chromite-hosted clinopyroxene-orthopyroxene pairs indicates that the estimated T-P conditions of the parental magma of the Kabaena chromitites are 950–1010°C and 7.0–8.4 Kbar. The extreme-Mg-rich olivine inclusions (Fo &gt;96) with extremely high Ni (5300–8200 ppm) and Cr (1500–6700 ppm) contents are interpreted to have crystallized from high-Mg and Cr boninitic melts, and the Fo contents were then elevated by subsolidus re-equilibration (Fe–Mg and Fe–Ni exchange) with chromite. Sulfide inclusions contain base metal minerals containing millerite with rare pentlandite and chalcopyrite, and platinum-group minerals that include the laurite-erlichmanite series, Ir–Ni monosulfide, irarsite and cuproiridsite. This sulfide assemblage reveals that their parental magmas experienced the evolution of high T and low <i>f</i>(S₂) to low T and high <i>f</i>(S₂) from the early to late stage. Finally, combined with the palaeogeographic reconstruction in Sulawesi, we propose that the high-Cr chromitites in this region were formed by the reaction of depleted mantle and boninitic magma beneath a juvenile island arc, implying the initiation of a new subduction in the Miocene.

东苏拉威西蛇绿岩（East Sulawesi Ophiolite）作为全球三大蛇绿岩之一，赋存有重要的豆荚状铬铁矿床（podiform chromitites）。然而，这类铬铁矿床的成因尚未得到有效约束。本文以苏拉威西东南部卡巴埃纳岛（Kabaena Island）的豆荚状铬铁矿床为研究对象，系统报道了其铬铁矿及铬铁矿内部固态包裹体的详细矿物学特征与原位化学组成。铬铁矿颗粒具有较低的TiO₂（0.17~0.27 wt%）与Al₂O₃（13.4~15.1 wt%）含量，较高的铬数[Cr/(Cr+Al)，摩尔比]（0.70~0.74）与镁数[Mg/(Mg+Fe²+)，摩尔比]（0.69~0.75），与全球其他高铬型豆荚状铬铁矿床的特征相似。对铬铁矿部分主微量元素的洋中脊玄武岩（MORB）标准化配分模式显示，从Al₂O₃到Mn元素呈现轻微的正斜率变化。铬铁矿宿主的硅酸盐包裹体包括橄榄石、单斜辉石、斜方辉石与角闪石，其Mg/Fe比值高于寄主橄榄岩的对应比值。基于铬铁矿宿主的单斜辉石-斜方辉石对的地质温压计计算结果，卡巴埃纳岛铬铁矿床母岩浆的估算温压条件为950~1010℃与7.0~8.4千巴（Kbar）。富镁极端型橄榄石包裹体（Fo>96）具有极高的Ni（5300~8200 ppm）与Cr（1500~6700 ppm）含量，被认为是从高镁、富铬的玻安质熔体（boninitic melts）中结晶形成，其后通过与铬铁矿发生固溶再平衡（subsolidus re-equilibration，Fe-Mg与Fe-Ni交换）作用提升了Fo值。硫化物包裹体包含含针镍矿（millerite）的贱金属矿物组合，伴生少量镍黄铁矿（pentlandite）与黄铜矿（chalcopyrite），以及铂族矿物（platinum-group minerals）组合，包括硫钌矿-硫锇矿（laurite-erlichmanite）系列、铱镍单硫化物（Ir–Ni monosulfide）、铱砷矿（irarsite）与铜铱硫盐（cuproiridsite）。该硫化物组合表明，其母岩浆从早至晚经历了从高温低硫逸度（f(S₂)）到低温高硫逸度的演化过程。最后，结合苏拉威西地区的古地理重建（palaeographic reconstruction）结果，本文提出该区域的高铬型铬铁矿床形成于幼年岛弧（juvenile island arc）下方亏损地幔与玻安质岩浆的反应过程，暗示中新世（Miocene）时期该区域发生了新的俯冲作用起始事件。