Tourmaline-quartz-cassiterite mineralization of the Land's End Granite at Nanjizal, west Cornwall

Abstract

Nanjizal is one of the few locations in the southern segment of the Land's End Granite (Early Permian) where magmatic-hydrothermal tin mineralization has been worked. Three principal mineralization episodes are recognized. The first episode comprises steeply dipping ENE-WSW striking tourmaline quartz veins precipitated from magmatic-hydrothermal fluids during regional NNW-SSE extension; they are not associated with metalliferous mineralization. The second episode is represented by two sets of vertical/subvertical tourmaline veins striking, respectively, 280degrees and 340degrees. The NNW striking set hosts tourmaline-quartz-cassiterite ( alkali feldspar, apatite, zircon) mineralization and was worked for tin during the 19(th) Century. The first generation of tourmaline in these veins is dominated by the Mg-rich end-member, dravite. The subsequent two generations of tourmaline are dominated by the Ferich end-member, schorl, together with zoned cassiterite with Fe-rich cores and Fe-poor rims. The NNW striking tourmaline-quartz-cassiterite veins at Nanjizal have a similar orientation to cassiterite-bearing veins within the St. just Mining District further north. It is possible that the mineralization at Nanjizal was contemporaneous with mineralization in the St. Just Mining District and hence related to a later magmatic episode and the renewed release of magmatic-hydrothermal fluids during ENE-WSW extension. The third mineralization episode is dominated by steeply dipping WNW-NNW striking quartz-limonite-chalcedonyveins that are associated with wall rock hematisation and kaolinisation. Schorl-quartz-cassiterite mineralisation is relatively uncommon in S.W. England but there are remarkable similarities between the vein parageneses at Nanjizal and the Birch Tor-Vitifer area of Dartmoor; both are likely to have been sourced from predominantly magmatic-hydrothermal fluids.