Genesis of Chromitites in the Andriamena Greenstone Belt (Madagascar): Petrogenetic and Tectonic Implications

Abstract

Apart from the well-known stratiform and podiform chromite deposits, those in the Archean greenstone belts are also of great importance to both researchers and the mining industry due to their unique occurrences and critical economic value. Chromitites from the Andriamena greenstone belt are associated with plutonic mafic-ultramafic rocks within the Antananarivo craton of Madagascar. Chromites in chromitites host diverse mineral inclusions, including olivine, pyroxene, amphibole, and phlogopite. Abundant euhedral to subhedral base metal sulfides and rare platinum group element (PGE) minerals also exist as interstitial phases or inclusions in the chromitites. Electron microprobe results indicate that the Andriamena greenstone belt–hosted chromitites are typical high-Cr type with chromite Cr# values >60. Trace element patterns of chromites are characterized by Ni and Sc depletions and Zn, Co, and Mn enrichments. Orthopyroxenes in the chromitites have high Mg# values of 93.8 to 95.6 and consistent trace element patterns characterized by variable enrichments in light rare earth elements and large ion lithophile elements. The whole-rock PGE patterns exhibit Rh, Pt, and Pd depletion relative to Os, Ir, and Ru, similar to those observed in podiform chromitites. Zircon U-Pb dating yields ages of ~2.76 Ga for the granitic gneiss hosting the orebody and 2.67 Ga for the felsic veins intruding the orebody, thus constraining the formation of chromitite within a time interval of ~2.76 to 2.67 Ga. The mineral and whole-rock geochemistry of chromitites from the Andriamena greenstone belt suggests that the chromitite-forming parental magma was boninitic, indicative of a suprasubduction setting. Using binary mixing modeling, we suggest that the parental melts were formed from pristine boninitic melts that assimilated ~5 to 10% of granitic crustal material. Based on studies of global greenstone belt–hosted chromitites, these deposits are thought to have a complex origin, either in a subduction-related environment or in a subduction-unrelated plume setting. Moreover, the plutonic mafic-ultramafic complexes in global greenstone belts have good potential for chromitite exploration.