Geochemistry of alteration and host rock lithologies to the Bisie tin deposit, North Kivu Province, DR Congo

Abstract

Whole rock geochemistry is presented for lithologically-varied igneous and meta-sedimentary rocks associated with the Bisie tin deposit, and combined with geochronological data and petrographic and field-based observations to develop insights into tectono-magmatic environment(s) of deposition as well physico-chemical conditions and timing of ore mineralisation.

Bisie tin mine, located in the Mesoproterozoic Kivu Belt (Democratic Republic of the Congo), is the world’s highest-grade tin mine (average grade 4.5 % Sn). Tin mineralisation consists of botryoidal cassiterite veins within an envelope of chlorite alteration, and is situated in a steeply east-dipping shear zone running parallel to the contact to a multi-phase granite massif. The hosting sequence comprises: A lowermost unit, hosting the tin mineralisation and containing mica schists with intercalations of sandstones, felsic tuffs and metabasaltic flows as well as (meta-)gabbro intrusions. Meter-scale pyrite-pyrrhotite-sphalerite-galena lenses of presumed syn-sedimenatry exhalative origin are intercalated in the vicinity of the Bisie deposit. An overlying “metasedimentary unit” comprises tuffaceous shales, siltstones, sandstones and conglomerates. The uppermost unit consists of carbonaceous shales. Peak metamorphic grades in the mica schists are recorded by staurolite-garnet-biotite paragenesis, and decrease towards the top of the sequence and away from the granite massif based on the absence of metamorphic porphyroblasts in the carbonaceous shale. Along the shear zone, peak metamorphic parageneses retrogressed to muscovite (±chlorite) in an alteration zone with a length of > 14 km, which also coincides with a soil Sn anomaly.

Zircon U-Pb information regarding the granitoid massif includes a granite age of 1518 Ma (pre-Kibaran), a marginal monzogranite age of 1274–1271 Ma and gneissic granite age of ca. 1000 Ma. Cassiterite from Bisie was previously dated at 1068–1029 Ma, consistent with formation during the G4 granite stage, associated with Sn, Nb-Ta, W and Au mineralisation across the Kibaride Belt.

Trace element signatures of the monzogranite as well as the metabasites indicate formation in an intraplate setting, consistent with deposition of central Kivu Belt rocks in an extensional basin during Cycle 2 of the Kibaran Orogeny. Compositions of the mica schist and carbonaceous shale indicate both collisional and rift environments, probably pointing to mixed sources. Similarity in trace element signatures prove that the chlorite schists, forming the alteration of the Bisie tin deposit, are alteration products derived from the hosting mica schists. All altered and unaltered metasedimentary samples as well as the monzogranite have positive Sn and negative Sr and Ba anomalies in Primitive Mantle normalisation, with the most pronounced anomalies observed in the chlorite schists, probably representing an alteration halo associated with the Bisie tin mineralisation. The halo has a larger extent than the zone affected by mineralogical changes, possibly registering destruction of feldspar by acidic tin-bearing fluids. Compared to crustal abundances the chlorite schists are strongly enriched in Sn with moderate enrichment in W. Copper and Zn concentrations are enriched in the orebody in the vicinity of later faults.