9 WHEAL REMFRY BRECCIA, Wheal Remfry china clay pit This spectacular stone was created by a huge explosion 270 million years ago. As the molten granite cooled and crystallised, a fluid under very high pressure, rich in water, boron, silica and iron separated out and accumulated near the top of the granite. At Wheal Remfry a fault line intersected a reservoir of this pressurised borosilicate fluid, which found its way up this line of weakness to the surface. This caused a rapid drop in pressure and material rushed up the newly formed conduit towards the surface - rather like when the cork pops out of a champagne bottle. Once the pressure was dissipated the material fell back down the conduit and crystallised as a mixture of silica (quartz) and the boron containing mineral tourmaline, with fragments of granite and killas suspended in it (a breccia). As the granite magma cooled and crystallised, separate water based fluid separated out which was immiscible with the granite magma (immiscible fluids will not mix - like oil and water). Boron, silica and iron migrated into this segregation forming a borosilicate-rich fluid. An exceptionally large pool of this fluid accumulated along the line of the Fal Valley Fault Zone where Wheal Remfry china clay pit is now. This fluid was under very high pressure and gradually was injected up the line of the fault towards the surface. Once a conduit to the surface had been established, the pressure caused much of the borosilicate fluid to explosively rush up towards the surface, carrying with it many fragments of granite, some already solid, some molten, together with fragments of killas torn off from the walls of the conduit. As soon as the pressure was dissipated, much of the erupted material fell back down the vent and rapidly crystallised as a matrix of quartz and tourmaline with fragments of several types of granite and killas in the breccia. The pink fine grained granite is sometimes referred to as ‘porphyry’; the pinkness is caused by impregnation with iron oxides. The porphyry is not appreciably kaolinised, possibly because the kaolinising fluids could not penetrate the tough fine grained quartz-tourmaline matrix. A later quartz vein cuts through both the matrix and clasts. Exposures in the china clay pit show that wisps of fluidal porphyry (a form of granite magma) were drawn out into streaks in the quartz-tourmaline breccia, indicating that both were fluid at the same time. The breccia is about 0.5km long and up to 100m wide.