Abstract

Frenchvale quarry, once mined for dolomitic marble, contains pink corundum-bearing, quartz-free/-poor, feldspathic gneiss that is unusually sodic (~7% wt.% Na2O) and iron-poor (~0.6 wt.% Fe2O3), but has silica, alumina and immobile trace-element contents resembling those of suspended fluvial particulate matter (e.g. in the Congo River). The protolith of the gneiss, interpreted as a fine-grained clastic sediment deposited offshore, evidently was albitised prior to deformation and regional metamorphism. Variably-altered gneiss samples show a narrow range of δ18OVSMOW values (8.1 to 10.7‰) and no systematic differences in bulk O isotope composition as a function of alteration intensity. With the exception of an extensively fuchsitised zone adjacent to a thick (1.2 m), cross-cutting quartz vein that contains H2O–NaCl+CO2+CH4-bearing fluid inclusions, the O isotope data do not support interaction of the gneiss with an externally-derived fluid phase except at low fluid:rock ratio, even where granodiorite occurs in direct contact with the gneiss. Fluid inclusions in the quartz vein have bulk forumla$$X_{{\rm H}_2{\rm O}}$$⁠, forumla$$X_{{\rm C}{\rm O}_{\rm 2}}$$ and forumla$$X_{{\rm C}{\rm H}_{\rm 4}}$$ values (in mol.%) of 99.60, 0.14 and 0.26, respectively, as determined by gas chromatography. Although the protolith of the gneiss was associated with carbonate platformal rocks (now marble), corundum is confined to the feldspathic rocks. These feldspathic rocks lack calc-silicate minerals; they are not skarns. As such, they are distinct from well-known Himalayan sapphire and ruby deposits cited previously as analogues of the Frenchvale corundum occurrence.

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