Magmatic-hydrothermal evolution of Sn-W granites in the Kibara belt, Democratic Republic of Congo: 40Ar-39Ar dating and LA-ICP-MS microanalysis of cassiterites and cassiterite-hosted fluid inclusions

Abstract

• Early Neoproterozoic age for Sn-W deposits in the Kibara belt. • Ta/Nb ratios in cassiterite differentiate Sn-granites from Sn-W quartz veins. • Cassiterite-hosted fluid inclusions reveal significant batholith-scale fractionation. • Alteration, fluid mixing, and magma fractionation drive cassiterite mineralization. The northwestern part of the Kibara Belt (KIB) in the Democratic Republic of Congo (DRC) consists of highly altered (greisenized) Sn-bearing granites “the G-4 granite” and associated mineralized Sn-W quartz veins. To understand the magmatic-hydrothermal processes, including the sources of metals, intrusion-vein relationships, and metal precipitation, we investigated Ar-Ar ages of mica minerals and the geochemistry of cassiterites and fluid inclusions using in-situ LA-ICP-MS microanalysis. The Sn-W province in the KIB has been determined to be Early Neoproterozoic, with the Sn-bearing granites (phlogopite Ar-Ar: 974 ± 1 Ma) being slightly younger than the Sn-W quartz veins (muscovite Ar-Ar: 990 ± 1 Ma). Primary and pseudo-secondary fluid inclusion assemblages (FIAs) in cassiterites from the Sn-bearing granites and the Sn-W quartz veins show an elevated incompatible element ratio, including Li/Na, Cs/Na, and Rb/Sr, suggesting fractional crystallization in the causative magma. These ratios are relatively higher in the FIAs from the Sn-bearing granites compared to the Sn-W quartz veins, indicating more “fractionated granite-derived” and postdating magmatic-hydrothermal fluids in the Sn-bearing granites. Tantalum-rich (Ta/Nb > 1) cassiterites in the Sn-bearing granites and relatively Nb-rich (Ta/Nb < 1) cassiterites in the Sn-W quartz veins further support the presence of more fractionated magmatic fluids in the Sn-granites compared to the Sn-W quartz veins. A positive correlation trend of homogenization temperatures versus salinities, with relatively consistent Cl/Br ratios in all the studied FIAs, suggests a mixing of fluids derived from magmatic and possibly meteoric sources. Fluctuations and positive correlations of redox-sensitive trace element ratios (e.g., Fe/Mn and V/Ti ratios) in the cassiterite further indicate the mixing of fluids with contrasting redox potentials. This mixing could enhance the oxidation of the magmatic-hydrothermal fluids and promote a cassiterite precipitation in the KIB.