HFSE and REE mineralization at Machinga prospect in the Chilwa alkaline province, southern Malawi

Abstract

Peralkaline rocks from Machinga prospect, situated at the north-eastern margin of Zomba-Malosa syeno-granitic pluton, were investigated to characterize high field strength elements (HFSE) and rare earth elements (REE) mineralization and elucidate the genesis of HFSE- and REE-bearing rocks based on mineralogical and geochemical methods. The investigation utilized two sets of samples: (1) quartz syenite, granite and granitic pegmatite and (2) a second set that includes eudialyte-bearing granite and pegmatite.A range of REE and HFSE minerals was observed in the rocks including monazite, bastnäsite, synchysite, pyrochlore, zircon and eudialyte. Pseudomorphs after eudialyte, mainly comprising zircon and quartz, occur abundantly in the pegmatite and to a lesser extent in alkaline granite. HFSE and REE are variably distributed in the three rocks types (quartz syenite, granite and granitic pegmatite). However, the most elevated concentration of total REE (between 1.0 wt % and 5.2 wt %) corresponds to highest Nb + Ta + Zr concentration (between 1.5 wt % and 7.5 wt %). The elevated concentration largely occurs in metasomatized granitic pegmatite and metasomatized quartz syenite (?) (intruded by granitic pegmatite in parts) but also corresponds to relatively fresh eudialyte bearing alkaline granite.Using geochemistryand field mapping results; the study demonstrates that the rocks comprising the Machinga prospect are a product of extended fractionation of silica saturated alkaline magma. As the magma fractionation proceeded, alkaline granite was emplaced adjacent to the quartz syenite as an outer ring. Later associated granitic pegmatite lenses crystalized as final stage of emplacement of the pluton.Based on petrography and mineral chemistry, three mineral paragenetic stages associated with HFSE and REE minerals were distinguished. The first magmatic stage crystallized zircon and other igneous minerals. The second stage is characterized by precipitation of eudialyte, and the third by decomposition and replacement of eudialyte by zircon and quartz and other secondary HFSE and REE repositories. These last two stages occurred during late magmatic to early metasomatic-hydrothermal activities, which were closely associated and likely occurred concurrently.