Arsenic as an Indicator of Black Shale Assimilation by Nickel-Bearing Mafic-Ultramafic Intrusions, Muremera-Rujungu, Burundi

Abstract

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.