| Summary: | The St. Austell pluton is part of the large Hercynian batholith of south-west England. The pluton exhibits a complex sequence of mineralisation events, which occur as distinct hydrothermal vein generations associated with characteristic alterations of the host granites. Geological and petrographic observations indicate that the first mineralisation event following the consolidation of the granites consists of the intrusion of quartz porphyry (rhyolite) dykes and the formation of quartz-tourmaline veins. The latter are associated with the extensive greisenisation of the host granites. This study is concerned mainly with the subsequent evolution of the pluton, which is summarised in the following sequence of events: a. Quartz-fluorite veins, associated with hydrothermal alteration of the host topaz gran-ites and resulting in the formation of the "fluorite granite" petrographic type. b. Formation of quartz-haematite veins. The mineral assemblage of these veins consists of kaolin+illite+haematite+/-chlorite. The wall-rock granites of the quartz-haematite veins are altered to an assemblage consisting of kaolin+illite+haematite. c. Formation of kaolin+/-quartz veins, associated with the extensive kaolinisation of the host granites. The alteration assemblage consists of kaolin and smectite. Petrographic observations indicate that clay mineral authigenesis associated with the illitisation of the parent granites is controlled by differences in the precipitation kinetics of kaolin and illite. Similarly, the kaolinisation of the granites is mainly con-trolled by the slow precipitation kinetics of quartz, thus permitting SiO2 content of the fluids to exceed quartz saturation and allowing smectite to precipitate. Geochemical modelling of the kaolinisation allows the identification of the fluid/mineral equilibrium relations required for the formation of a kaolin+smectite mineral assemblage from the feldspar mineral assemblage of the parent granites. Paragenetic and fluid inclusion evidence suggest that successive mineralisation events occurred at progressively lower temperatures, culminating with the kaolinisation of the granites at relatively elevated temperatures of between 100 and 50°C. The nature of the hydrothermal fluids responsible for the formation of different alteration assem-blages is difficult to assess because of substantial uncertainties involved in the interpretation of the clay minerals' stable isotope compositions. Overall, the geological, petro-graphic, and fluid inclusion data favour the involvement of elevated temperature, moderately to highly saline fluids in both the kaolinisation and illitisation of the St. Austell granites. It is also considered that the kaolinisation of the granites did not take place due to the interaction of the parent granites with meteoric waters. The correlation of the geological and thermal history of the pluton during the Mesozoic suggests that the kao-linisation took place prior to the unroofing of the pluton and during a period of rapid uplift and cooling in the Middle Jurassic to Early Cretaceous. This uplift event is linked to the rifting associated with the opening of the Atlantic Ocean. The alteration of the parent granites is attributed to their interaction with either sedimentary brines expelled from the Plymouth Basin or with evolved (modified) meteoric waters.
|
|---|
