Magmatic-Hydrothermal Fluid Processes of the Sn-W Granites in the Maniema Province of the Kibara Belt (KIB), Democratic Republic of Congo

Abstract

The Kibara belt (KIB) in the Maniema province hosts orebodies bearing cassiterite-wolframite, which are associated with equigranular to pegmatitic late Mesoproterozoic (1094–755 Ma) granites and Sn-W bearing quartz veins that cut through metasedimentary country rocks. Alteration assemblages of muscovite-quartz (±topaz-fluorite-tourmaline) occur in the granites, and muscovite-sericite-quartz occurs in Sn-W quartz veins. Petrographic analyses, including cathodoluminescence (SEM-CL) on cassiterite grains, reveal two types of cassiterite: yellow transparent cassiterite (lighter under SEM-CL: type I) and dark translucent cassiterite (darker under SEM-CL: type II). These types are organized in micro-textures as oscillatory (growth) zones and replacement zones (type II replaces type I). Unlike cassiterite, wolframite is texturally homogenous. LA-ICP-MS results reveal that type II cassiterite is relatively enriched in Fe, Al, Ga, In, As, Pb, Zn, and U, whereas type I is enriched in V, Ti, Zr, Ta, Hf, and Nb. Contrasting Ce anomaly values in the cassiterite types suggest a transition of redox potentials during the Sn precipitation. Fluid inclusion assemblages (FIAs) in quartz, fluorite, and cassiterite are dominantly aqueous, liquid- or vapor-rich, and rarely carbonic-bearing aqueous inclusions. These often texturally coexist in a single “boiling” assemblage in granites. Raman spectroscopy on the bubble part of fluid inclusions in quartz and cassiterite shows various gas species, including CO2, CH4, N2, and H2. Boiling assemblages in the granites suggest that fluid phase separation occurred at about 380–610 bars, which is about 1–2 km (lithostatic) or 3–5 km (hydrostatic) in apparent paleodepth. FIAs in the granites show ranges of salinities of 4–23 wt.% (NaCl equivalent) and homogenization temperatures (Th) of 190–550 °C. FIAs hosted in cassiterite displayed distinctively lower and narrower ranges of salinities of 2–10 wt.% and Th of 220–340 °C compared to the FIAs hosted in quartz in the granites (salinity of 4–23 wt.%, Th of 190–550 °C) and the quartz veins (salinity of 1–23 wt.%, Th of 130–350 °C). This suggests a less salinized and cooler fluid during the cassiterite precipitation. We suggest that magmatic-derived Sn-W bearing fluids be mixed with less saline and cooler aqueous fluids, possibly meteoric water, during the major cassiterite and possibly wolframite depositions in the KIB. This is based on (1) temperature and salinities, (2) hydrothermal alterations, (3) cassiterite micro-textures, and (4) trace element distributions.