Characterization of low temperature fluid processes associated with epithermal quartz veining iron mineralization and kaolinization in S.W. England
Low temperature fluids of diverse origin are widely believed to have played a dominant role in the formation of both primary kaolin and iron vein deposits in S.W. England. Most workers believe that dilute, low temperature fluids of dominantly meteoric origin were responsible for the extensive kaolinization of the Comubian batholith. Recent fluid inclusion studies undertaken at Goonbarrow china clay pit by Bray (1980) suggest however, that kaolinization was linked to the infiltration of a hot (400-450[degrees]C), low density vapour phase. Some uncertainty exists therefore, as to the precise nature and origin of the kaolinizing fluids at this locality. In order to investigate these aspects and additionally ascertain how kaolinizing fluids were able to pervasively infiltrate the Comubian granites, detailed petrographic, fluid inclusion and microstructural studies have been undertaken at Goonbarrow during the current study. In the St. Austell region of S. W. England kaolinization is spatially associated with iron vein mineralization. This spatial relationship is noted on two markedly different scales. On a pit scale, "economic" kaolinization is frequently spatially coincident with quartz-haematite-amethyst) veins exhibiting "epithermal" type textures. On a regional scale, larger analogues of these veins, known locally as iron lodes, are disposed about the extensively kaolinized St. Austell granite. This recurrently observed spatíal association between kaolinization and iron-bearing veins has led several workers (Hawkes, 1974; Jackson et al., 1989; Bristow, 1989) to speculate that Fe-mineralization and argillic alteration were genetically linked. To investigate this potential link, detailed petrographic, fluid inclusion, D-ICP, stable isotope and bulk geochemical analyses have additionally been undertaken on both vein and lode samples. In contrast to the previous findings of Bray, studies of secondary fluid inclusions in granite quartz from Goonbarrow, indicate that kaolinization was associated with the influx of large volumes of dilute, low temperature fluids (Th usually <200[degrees]C, salinity <5 wt% NaCl eq.). The phenomenal abundance of low temperature, secondary inclusions in granite samples suggests that kaolinizing fluids utilised an extensive micro fracture network to pervasively infiltrate the granite. Combined fluid inclusion-microstructural studies indicate that this microfracture network developed during at least two distinct episodes of microfissuring. Two distinct primary fluids have been distinguished in both vein and lode samples from S.W. England; (1) an early, dilute, moderate to low temperature fluid that is thought to represent heated meteoric fluids and (2) a younger, saline, low temperature Ca-enriched fluid that is thought to represent trapping of a basinal brine. The latter fluids are thought to have been tapped from compacting offshore sedimentary basins via major NW-SE oriented wrench faults. The currently available data suggests that basinal brines were the most likely iron mineralizing fluids, negating the speculated genetic link between kaolinization and iron mineralization. The source of the iron is uncertain, but geological considerations suggest it was probably derived from red-bed sequences within the flanking sedimentary basins. Mixing between the iron enriched basinal brines and dilute groundwaters at structural intersections is inferred to have been the trigger for iron precipitation in S.W. England.