Petrology and geochemistry of some alteration processes in Cornish granites, south-west England

Hammad, Hanem Mohammed (1987) Petrology and geochemistry of some alteration processes in Cornish granites, south-west England. University of Southampton, Doctoral Thesis.

Abstract

The present study deals with the greisenisation, tourmalinisation and kaolinisation processes in the granites of Cligga Head, Cape Cornwall and Wheal Remfry, respectively in Cornwall, south-west England. The granites studied are part of the Cornubian batholith which was emplaced during the Hercynian Orogeny. General geologic features of the three localities were studied in the field and ninety rock samples were collected. Detailed petrographic studies were carried out using the polarizing microscope. The Cligga Head granites consist of unaltered granites, greisenised granites and greisens. Cape Cornwall granites comprise porphyritic granites, tourmaline-bearing granites and tourmaline-quartz rocks and veins. The Wheal Remfry comprises fresh to slightly kaolinised granites, moderately kaolinised granites, and highly kaolinised granites in addition to tourmaline breccia body. XRD technique was used to differentiate between the clay minerals during the kaolinisation process. Major and trace elements were analysed using Phillips PW 1400 X-Ray Fluorescence spectrometer. The trace element analyses were performed for Pb, Th, Rb, Sr, Ba, U, Zr, Nb, Ga, Cr, V, As, Y, Mo, Ni, Cu, Sn, W, Zn, La, and Ce. Rare earth elements were determined by inductively coupled plasma spectrometry (ICP) on samples from the three examined areas. The behaviour of REE group in the granites during the alteration process has been investigated. All REE fractionation patterns are similar, exhibiting a prominent negative Eu anomaly. During the greisenisation and tourmalinisation processes, the granites suffered a marked depletion in the REEs, which is due to the dilution effect of the new quartz. In contrast the kaolinisation shows an enrichment character, reflecting the physical concentration in an insoluble residue and adsorption by the clay minerals. The geochemistry of tourmalines from the series tourmaline-quartz rocks, tourmaline-quartz veins and from the tourmaline-quartz pegmatitic pods within the early granites at Cape Cornwall have been determined, using (SEM) technique. They show substitution towards the schorl end-members of the schorl-dravite series. Fluid inclusion studies were carried out on samples from the kaolinised granites and tourmaline-breccia body, using a Linkam TH microthermometric stage attached to a Leitz Dialux 20 microscope. The formation of breccia took place between 300-400 C and with salinity between 14-24 eq.wt% NaCl and the fluids responsible for the kaolinisation of granites were very dilute 1.5-3.5 eq.wt% NaCl and homogenized at < 200^oC. (D82695)

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