Mineralisation, Greisenisation and kaolinisation at Goonbarrow china clay pit, Cornwall, U.K.
Goonbarrow, a China Clay pit situated within the St. Austell granite China Clay region, is the subject of a varied geochemical and isotopic study to determine the mode of genesis of the kaolinite and associated tin/tungsten mineralisation. Detailed geological mapping in conjunction with a geochemical study of the micas indicated that Goonbarrow is situated at the junction of the petrographically distinct phases of the St. Austell granite. An unusual assymmetric curved-feldspar-crystal pegmatite is found at the junction. Elvans at Goonbarrow and three other locations within the St. Austell granite are shown to be intruded during hydrothermal activity and in some cases after major vein formation. Three main types of vein were recognised in Goonbarrow, the major ones being spatially associated with zones of kaolinised granite. A potassium/ argon age study showed that the granites, pegmatite and greisens (and by inference tin/tungsten mineralisation) were formed at about 280 ±10 m.y. Four elvans, including Goonbarrow, were dated at around 272 m.y. Three of these elvans crosscut major vein, swarms. Age determinations on fine grained muscovite produced predominantly during kaolinisation and several kaolinised potassium feldspars also gave Hercynian eges. Scanning electron microscopy studies on daughter minerals in fluid inclusions indicated the presence of Al, As, Ca, Cl, Cu, Fe, K, Kg, Mn, Na, S, Sn and Zn although many of these elements were not present in minerals in the pit. Temperatures, pressures and salinities of vein fluids were determined by conventional fluid inclusion studies, which also indicated that the veins vere boiling. Hydrogen and oxygen isotope studies on vein quartz and greisen muscovites coupled with a re-interpretation of previous work and the fluid inclusion study produced a new model for the genesis of Cornish China Clay. Kaolinite genesis from the vapour phase of hot boiling fluids intimately associated with greisen bordered quartz/tourmaline veins of Hercynian age is favoured.