Geology of the Bisie wood tin (cassiterite) deposit and its host rocks, North Kivu Province, DR Congo

• Bisie wood tin deposit, the world’s highest-grade tin deposit. • Botryoidal cassiterite veins in chlorite alteration envelope hosted by shear zone. • Shear zone located in mica schist in vicinity to multi-phase granite massif. • Grade controlling factors: vein thickness, presence of folded veins. • Main activity on granite massif 1271–1274 Ma, pre-dating tin mineralization. The Bisie Tin Mine, located in the Mesoproterozoic Kivu Belt in the Democratic Republic of Congo, is the world’s highest-grade tin operation with an average grade of 4.5 % Sn. In this study we provide a geological description of the deposit and its surrounding mine license area and present U-Pb geochronological data for an adjacent granite massif. The metasedimentary host rocks of the Bisie Tin Mine comprise: (1) Mica schists and intercalated mafic and felsic intrusive and extrusive rocks; (2) a “metasedimentary unit” of clastics, containing a mafic tuff component and (3) carbonaceous shale. The metamorphic grade decreases away from the granite massif and upwards in the stratigraphic sequence. The mica schist displays a peak metamorphic assemblage of staurolite, garnet and biotite. overgrowing a muscovite-dominated foliation. Multiple stages of magmatism occurred in the granite massif, including an undated biotite syenogranite as the most voluminous phase. Further sub-intrusions include alkaline granites, syenites and monzogranites. A marginal monzogranite yields 1271–1273 Ma U-Pb zircon ages, indicating that felsic magmatism was partially synchronous with the development of surrounding metasediments. A previous study reported a granite age of 1518 ± 40 Ma, pre-dating the onset of the Kibaran orogeny and gneissic rocks within the granite massif with a maximum age of around 1000 Ma, contemporaneous with the G4 granite stage. The granite massif likely formed in an extensional environment (metamorphic core complex) exhuming pre-Kibaran rocks, and was subsequently intruded by Kibaran granite phases. The Bisie deposit is located in mica schist about 500 m from the granite contact in the northern project area. Tin mineralisation is hosted in a steeply east-dipping shear zone oriented parallel to the contact with the granite massif. Along the shear zone, an alteration halo is developed for >14 km – manifested in the replacement of metamorphic porphyroblasts by muscovite (± chlorite) and development of Sn anomalies in the soil cover. Tin mineralisation occurs as up to ∼1 m wide veins of botryoidal cassiterite (so-called wood tin). The veins are hosted within an up to ∼25 m wide alteration envelope of chlorite (± actinolite) schist. High-grade ore shoots plunge at intermediate angle to the north and contain folded veins, absent in low grade zones, as well as N(NW)-trending veins paralleling the shear zone. Continued deformation in the shear zone caused cassiterite veins to be strongly boudinaged and brecciated. Cassiterite previously dated to ∼1068–1026 Ma, formed contemporaneously with the G4 granite stage. Quartz veining and precipitation of pyrite, chalcopyrite, arsenopyrite (± sphalerite and galena) are associated with later NW-, NNW- and E-W-trending faults that dissect the tin-mineralised shear zone. NW-trending faults segmented the Bisie deposit into the Mpama North and South ore bodies. Multiple phases of normal and strike-slip movement reactivated these faults.. Massive sulphide lenses with an inferred syn -sedimentary origin occur proximal to the tin mineralisation in the Mpama South area.