The geology of the china clay deposits of Southwest England with particular reference to their chemical properties
The geology and commercial properties of china clay deposits in Cornwall and Devon have been investigated in the field and by laboratory methods. Two deposits were studied, the Lower Bostraze pit on Land's End and the larger Lee Moor pit on southwest Dartmoor. These china clay deposits are hosted in biotite monzogranites which have been subject to a history of hydrothermal alteration. SEM and XRD studies show that extreme argillic alteration is characterised by total destruction of the feldspars which are replaced by a fine-grained assemblage dominated by kaolinite and secondary mica. Partial alteration leads to assemblages of secondary mica, smectite and kaolinite. Pervasive alteration leads to a reduction of bulk rock density as a result of the leaching of alkali-metals and silica from the system. Kaolinite forms euhedral to anhedral platelets with larger booklet or vermiform stacks. TEM work has revealed that the kaolinite populations are composed of a mixture of euhedral and anhedral crystallites, a low disorder clay will be composed of euhedral crystallites with a subordinate number of anhedral kaolinites. The stack and booklet forms are common at higher structural levels within the deposits and are probably formed under supergene conditions. Fluid inclusion studies indicate a complex pattern of fluid/rock interaction. Inclusion abundances provide a measure of the amount of fluid that has passed through the granite, the most intensely altered granites having the highest values. The inclusion populations in kaolinized granites are dominated by regular to irregularly shaped liquid-vapour and vapour-only inclusions that sometimes contain a phase identified by SEM-EDS as kaolinite. The earliest episode of fluid activity was responsible for the formation of hydrothermal sheeted vein swarms, breccias, microfracturing and pervasive phyllie alteration. These formed from high temperature, dominantly magmatic fluids from between 250 to > 400°C with moderate to high salinities (10 to 40 eq.wt. % NaCl). These fluids were followed by cooler < 100 to 200°C, less saline (< 13 eq.wt. % NaCl) meteoric fluids that were responsible for the first stages of kaolinization. A small number of low/moderate temperature (180-220°C) high salinity (25-35 eq. wt. % NaCl) inclusions were found which are most likely to represent the influx of basinal brines during wrench fault re-activation and thus played a role in the kaolinization process. Homogenization temperature and salinity signatures for weakly and intensely kaolinized granites are similar and suggest that the degree of kaolinization is controlled by the volumes of fluid that pervade the granites. Stable oxygen and hydrogen isotope studies show that the isotopic compositions of kaolinite plot close to the kaolinite weathering line, indicating growth/re-crystallization in a supergene environment. Evidence for the epithermal regime under which kaolinization took place is found in both pits where late quartz-filled cross-course veins contain a significant amount of silica, iron oxides and re-mobilized/re-crystallized kaolinite. Stable oxygen isotope analyses of cross-course vein quartz from Lower Bostraze pit supports the fluid inclusion data which suggest a low temperature epithermal environment of deposition. The flow of fluids through the granite was controlled by the intensity of fracture systems present. Both pits host sheeted vein swarms and breccias which acted as regions of enhanced permeability and are coincident with zones of intense kaolinization. Pre-granite wrench-faults were re-activated following granite emplacement and played an important part in providing zones of high permeability during kaolinization. Laboratory studies have shown that the commercial properties of paper coating clays are controlled by their mineralogy, which is determined by the precursor mineralogy of the granite and its hydrothermal history. The yellowness of a clay is related to the amount of iron oxide present either adsorbed onto the kaolinite surfaces or as discrete particles. In general terms the higher the yellowness values the lower the brightness values. Abrasion is related to the levels of quartz and feldspar present. Anomalously high abrasion values at Lee Moor have been shown by SEM to be due to the presence of silica coatings on kaolinite and K-feldspar laths from antiperthites. Viscosity is related to the levels of mixed-layer smectite/mica phases and smectite in the ultrafine fraction. Viscosity is also related in a complex way to the particle size and shape of kaolinites. An intensely altered granite will contain well-formed blocky crystallites which will have good viscosity values, weakly altered granite will contain smaller thinner particles the viscosity of which is poor. The zones of intense kaolinization around vein swarms are dominated by blocky, well-formed low disorder kaolinite crystallites which have good viscosity and brightness values. Partially altered granite contains less kaolinite and more mica and smectite. The kaolinite will be dominated by less well-formed high disorder crystallites which may be stained by iron oxides thus increasing yellowness.