Crystal System: Tetragonal
Status of Occurrence: Confirmed Occurrence
Chemical Composition: Tin oxide
Chemical Formula: SnO2
Method(s) of Verification: Snowdon - EMPA (Colman & Appleby, 1991).
- Oxides & Hydroxides
Introduction: cassiterite occurs primarily in medium to high-temperature ore deposits spatially and often genetically associated with granites and other acid-intermediate igneous rocks. The mineral association frequently includes tungsten minerals. Cassiterite is also a common placer mineral in rivers draining areas with primary bedrock tin mineralization.
Occurrence in Wales: Wales may host many styles of mineralization but the classic tin-tunsten (Sn-W) granite association is virtually absent. The only area in which it has been identified (and as a very minor occurrence) is within the Snowdon Caldera in North Wales (Colman & Appleby, 1991). An old report of cassiterite from Treffgarne in Pembrokeshire (Perceval, 1866a) was quickly corrected as brookite (Perceval, 1866b).
- Snowdon, Gwynedd: the bed of Shadow Gully, Cwm Llan, high on the slopes of Snowdon, is floored by a magnetite-cemented breccia (Colman & Appleby, 1991). Hematite and pyrite (with minor chalcopyrite) are also present. Cassiterite occurs within this mineralization as microscopic grains associated with scheelite, the latter confirmed in samples collected during MINESCAN (Bevins & Mason, 1998). The breccia has been interpreted as forming during hydrothermal activity related to the evolution of the volcanic caldera and the Sn/W component to indicate a degree of magmatic input to the hydrothermal fluids (Colman & Appleby, 1991).
- Bevins, R.E. & Mason, J.S., 1998. Welsh Metallophyte and metallogenic evaluation project: Results of a Minesite Survey of Gwynedd. National Museums of Wales, Cardiff.
- Colman, T.B. & Appleby, A.-K., 1991. Volcanogenic quartz-magnetite-hematite veins, Snowdon, North Wales. Mineralogical Magazine, 55, 257-262.
- Perceval, S.G., 1866a. Discovery of wulfenite, etc., in Pembrokeshire. Geological Magazine, 3, 377-378.
- Perceval, S.G., 1866b. Correspondence. Geological Magazine, 3, 518.