Mineral Database

Mineral Database
Mineralogy of Wales

Cassiterite

Crystal System: Tetragonal
Formula: SnO2
Status of Occurrence: Confirmed Occurrence
Distribution: Rare
Chemical Composition: Tin oxide
Method(s) of Verification: Snowdon - EMPA (Colman & Appleby, 1991).
Chemical Group:
  • 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).
Key Localities:
  • 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).
References:
  • 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.