Abstract The Muremera-Rujungu Ni-Cu sulfide mineralized chonolithic bodies are part of the well-endowed East African nickel belt of mafic-ultramafic intrusions in central-east Africa, dating from 1390 ± 10 Ma. Associated with the small, pod-like to tubular mineralized mafic-ultramafic intrusions is a suite of thin but extensive mafic sills that have geochemical patterns very similar to those of the marginal rocks of the chonolithic bodies (and are therefore assumed to be cogenetic) but that are generally unmineralized. The sills intrude pyrrhotite-rich phyllites (metasiltstones) that overlie a thick package of graphitic andalusite mica schists and quartzites (carbonaceous mudstones and sandstones) that are regionally enriched in As and Sb. The studied sills vary from 1 to 12 m thick, and they are distinctly differentiated into a coarse acicular-textured upper part of broadly gabbroic to quartz gabbroic composition and a granular, melanocratic pyroxene cumulate lower part, containing trace amounts of blebby sulfide and euhedral chromite. The geochemistry of the sills is typical of crustally contaminated high-Mg basaltic magma and is notable for the depletion of all chalcophile elements relative to primitive mantle melts. The chalcophile metals can be divided into two groups showing different behaviors: Cu, Bi, and Te follow S, which is preferentially concentrated in the upper, fractionated part of the sills, whereas Ni, Pt, and Pd follow As and Sb in being concentrated in the lower, cumulate part. Comparison of Ni/Cr and Ni/Mg ratios shows that this concentration of Ni exceeds that expected from accumulation in cumulus pyroxenes or chromite. The strong correlation of Ni (and to a lesser extent Pt and Pd) with As in the sills suggests a primary control by accumulation of small amounts of Ni-S-As liquid in the basal parts of the sills. We conclude that the parental magma of the sill became saturated in an immiscible S-As liquid by passing through the As-rich carbonaceous shales below and had most of its chalcophile elements stripped out by accumulation of this liquid at lower levels in the mineralizing system. Some of this early, As-rich mineralization was then reentrained by later batches of magma and incorporated into larger, composite chonolith-hosted mineralization such as at Kabanga North in Tanzania. This finding may have relevance for the methodology of future exploration in the East African nickel belt.

中文翻译：

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砷作为含镍镁铁质-超镁铁质侵入体同化黑色页岩的指标，布隆迪 Muremera-Rujungu

摘要 Muremera-Rujungu Ni-Cu硫化物粒石体是中东非镁铁质-超镁铁质侵入体资源丰富的东非镍带的一部分，距今1390±10 Ma。与小的、豆荚状到管状矿化镁铁质-超镁铁质侵入体相关的是一套薄而广泛的镁铁质岩床，其地球化学模式与圆石体边缘岩石的地球化学模式非常相似（因此被认为是共生的），但一般是未矿化的。岩床侵入了富含磁黄铁矿的千枚岩（变粉砂岩），覆盖在厚厚的石墨红柱石云母片岩和石英岩（碳质泥岩和砂岩）上，区域富含 As 和 Sb。研究的窗台厚度从 1 到 12 m 不等，它们明显区分为粗糙的针状纹理上部，由宽辉长岩到石英辉长岩组成，下部为粒状黑色辉石堆积，含有微量的气泡硫化物和自面体铬铁矿。岩床的地球化学是典型的地壳污染的高镁玄武质岩浆，并以相对于原始地幔熔体而言所有嗜热元素的消耗而显着。嗜硫金属可分为两组，表现出不同的行为：Cu、Bi 和 Te 跟随 S，优先集中在基台的上部分馏部分，而 Ni、Pt 和 Pd 跟随 As 和 Sb 富集在较低的累积部分。Ni/Cr 和 Ni/Mg 比率的比较表明，Ni 的这种浓度超过了在辉石积云或铬铁矿中积累的预期浓度。Ni（以及在较小程度上 Pt 和 Pd）与窗台中的 As 的强相关性表明主要控制是通过在窗台的基部中积累少量 Ni-S-As 液体。我们得出的结论是，通过下面的富 As 碳质页岩，基台的母岩浆在不混溶的 S-As 液体中变得饱和，并且由于该液体在矿化系统中较低水平的积累而剥离了大部分的嗜铜元素。其中一些早期的富砷矿化随后被后来的几批岩浆重新夹带，并被纳入更大的、以复合陨石为主体的矿化中，例如坦桑尼亚的 Kabanga North。