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B. Sc. Honours Thesis

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The Dunbrack deposit consists of a steeply dipping vein with argentiferous galena and other sulphides and sulphosalts in a gangue of predominantly quartz, hosted by altered adamellite granite. The vein appears to be associated with a regional structural system of lineaments, faults and joint sets of probable post-batholith age.

The identification of an SiO2 polymorph, previously reported as tridymite was not successful. The biaxial mineral occurs twinned into trillings as in tridymite, but other physical properties (x-ray diffraction patterns, specific gravity, indices of refraction) do not correspond to those of tridymite.

Fluid inclusions in quartz and in the alleged tridymite homogenize at ca. 134oC. The pressure of crystallization cannot be ascertained, but the above minimum temperature of entrapment should be corrected for pressure by ca. +56oC for every kilobar.

This study gives weight to previous suggestions that the Dunbrack deposit, despite its location within the Devonian batholith, may owe its genesis to processes not directly related to the emplacement of the granitoids and that it was formed in post-Devonian times.

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Pages: 67
Supervisor: Marcos Zentilli