The character and evolution of hydrothermal fluids associated with the kaolinized St. Austell granite, SW England

Abstract

A complex assemblage of fluid inclusions is present in quartz from the St. Austell granite. Homogenization temperatures range from below 100°C to above 570°C, and salinities vary from <l to 42 wt % NaCl equivalents. On a regional scale the fluid inclusion populations show marked variations in their abundance and distribution of types. These variations can be correlated with the petrology of the host rock. Low-temperature, low-salinity fluids are most abundant in extensively-kaolinized granite. These represent the latest phase of fluid activity in the St. Austell granite and were probably responsible for the formation of the extensive ‘china clay’ deposits in the region, at temperatures ranging from about 150°C to below 70°C. The most saline fluids (with halite as a daughter mineral) are restricted to the lithionite and fluorite granites. They could either represent the earliest fluids associated with these granites or the concentrated residue formed by boiling of an earlier, more dilute fluid. In the biotite granite, the earliest fluids have salinities of about 10–20 wt % NaCl equivalents, and it can be demonstrated that these fluids boiled at temperatures around 520°C. Abundant gas-rich inclusions in all rock types show further that episodic boiling did occur during the evolution of the hydrothermal phase associated with all these granites.