Geochemical Constraints on Kaolinization in the St Austell Granite, Cornwall, England
Psyrillos, Agamemnon, Manning, David A. C., Burley, Stuart D., Journal of the Geological Society
Abstract: The St Austell Granite contains important kaolin deposits, the formation of which is chronologically the last identifiable mineralization episode in the pluton. Field evidence demonstrates that kaolinization is spatially associated with hydrothermal quartz veins and progressed through the migration of alteration fronts away from reactivated pre-existing hydrothermal vein structures. Kaolin veins, sometimes with a quartz core, also occur. Kaolinization results in the formation of a kaolin+smectite assemblage, with no precipitation of quartz. Geochemical models of kaolinization at 25-100 deg C show that appropriate mineral assemblages are produced from parent granite by reaction with meteoric water at these temperatures, and with high salinity brine at 75 and 100 deg C. The kaolin+smectite mineral assemblage does not reflect conditions of equilibrium, but reflects elevation of fluid silica activities in excess of quartz saturation as a consequence of feldspar dissolution. Quartz fails to nucleate because of slow precipitation kinetics at the temperatures of kaolinization (<100 deg C). Kaolinization of the granites was a dynamic process, controlled by feldspar dissolution kinetics, the precipitation kinetics of silica minerals and the initial log a (K^sup +^/H^sup +^) of the fluids that interacted with the granites. Kevwords: Cornwall England, granites, kaolinization.
Kaolinization of permeable quartz-feldspar rocks is a process of considerable economic importance throughout the world. In addition to the formation of potentially valuable kaolin resources, this process affects clastic petroleum reservoirs with consequences for petroleum production (such as either secondary porosity formation or changes in petrophysical properties; e.g. Giles et al. 1992). A fundamental understanding of the process of kaolinization can be obtained through the study of extensively kaolinized granitic rocks, with application to other lithologies in less accessible locations (such as buried aquifers). In this paper, the process of kaolinization in the St Austell Granite is constrained using a combination of petrographic information and theoretical geochemical models of fluid-rock reaction.
The granite batholith of southwest England formed during the latest stages of the Variscan orogeny in the Late Carboniferous to Early Permian, as suggested by geological and isotope dating evidence (Darbyshire & Shepherd 1985; Chen et al. 1993, 1996; Chesley et al. 1993). Kaolin deposits of international importance occur within the Cornubian batholith, with the most extensive occurring in the St Austell pluton, one of six major outcrops of the batholith. Where exposed, the pluton exhibits a number of different granite types, formed as a result of multiple intrusive episodes (Hill & Manning 1987; Hill 1988; Manning et al. 1996; Fig. 1).
The post-consolidation evolution of the St Austell pluton is characterized by four distinct episodes of hydrothermal mineralization (Bristow & Exley 1994). The mineralization sequence identified by these authors comprises the following events (from older to younger).
(1) Quartz tourmaline veining, associated with metasomatic alteration (greisen) of the host granites and some Sn-W mineralization. Greisening of the parent granitic rocks is also associated with the pervasive alteration of plagioclase to a relatively coarsely crystalline white mica, termed 'sericite' by Exley (1959, 1976). In order to avoid confusion with magmatic muscovite, this mineral is referred to hereafter as `hydrothermal muscovite'.
(2) Intrusion of quartz porphyry (rhyolite) dykes, which cut quartz-tourmaline veins and are often altered to give a sericitic groundmass (part of the pervasive greisening process refered to above).
(3) Quartz-hematite veins (so-called cross-courses), associated with the destruction of feldspars and the formation of illite and smectite.
Questia, a part of Gale, Cengage Learning. www.questia.com
Publication information: Article title: Geochemical Constraints on Kaolinization in the St Austell Granite, Cornwall, England. Contributors: Psyrillos, Agamemnon - Author, Manning, David A. C. - Author, Burley, Stuart D. - Author. Journal title: Journal of the Geological Society. Volume: 155. Publication date: September 1998. Page number: 829+. © Geological Society Publishing House Jan 2009. Provided by ProQuest LLC. All Rights Reserved.
This material is protected by copyright and, with the exception of fair use, may not be further copied, distributed or transmitted in any form or by any means.