SHRIMP U-Pb dating of cassiterite: Insights into the timing of Rwandan tin mineralisation and associated tectonic processes

Abstract

• First SHRIMP direct U-Pb timing of tin mineralisation in the Karagwe–Ankole Belt. • SIMS U-Pb dating of hydrothermal cassiterite with sub-ppm U contents. • Tin mineralisation in a tectonic context. • Value of monazite in dating felsic/fractionated igneous rocks in which the zircon has suffered major radiation damage. Secondary Ionisation Mass Spectrometry (SIMS) U-Th-Pb dating of cassiterite with very low to sub-ppm U contents, zircon and monazite has provided direct measurements of the ages of Mesoproterozoic–Neoproterozoic pegmatite and quartz vein Sn mineralisation in the Karagwe–Ankole Belt (KAB), Rwanda, and through dates on local igneous rocks, helped to place the mineralisation in a tectonic context. Three generations of Sn mineralisation have been identified, each associated with magmatic and metamorphic processes during different periods of Rodinian and Gondwanan assembly. The first generation of mineralisation occurred in pegmatite at ca. 1145 Ma. The second generation, the main episode of Sn mineralisation, occurred from ca. 1090 to 960 Ma in quartz veins and intra-pegmatitic greisen. It started during the period ca. 1090–1040 Ma with fault-controlled Sn mineralisation in quartz veins hosted in quartzite and intra-pegmatitic greisen. This was mainly associated with the ca. 1078 Ma D 2 orogenic event in the KAB during the final amalgamation of Rodinia. Peak Sn mineralisation occurred during the period ca. 1040–960 Ma in quartz veins hosted in mica schists related to shear zones. It was associated with G 4 S-type granitic magmatism, with zircon U-Pb and monazite Th-Pb ages between 1011 ± 18 and 976 ± 11 Ma, possibly related to a late/-post-collisional setting linked to the amalgamation of Rodinia. The third and distinct generation of Sn mineralisation occurred at ca. 530 Ma in quartz veins hosted in mica schists within a shear zone located close to reworked basement rocks with meridional foliation. This Sn generation is attributed to hydrothermal fluids, possibly induced by a post-tectonic event associated with the emplacement of G 5 S-type granite at 614 ± 9 Ma, during the Pan-African orogeny. Recognition of the nebulous Sn-mineralised, cross-structural, Satinsyi-Rutongo Fault Zone, and the links established between the mode of Sn mineralisation, its local geological setting, igneous activity and regional tectonism, will help in targeting exploration for ore-grade Sn deposits in Rwanda and nearby countries.