John Robert McArthur HBSc thesis abstract
The Thunder Bay Amethyst Mine is composed of vein and breccia mineralization, containing amethyst, colourless quartz, prasiolite and citrine. The mine occurs in a strike-slip fault situated within a Archean batholitic intrusion of granodiorite.
During the present study fluid inclusions in amethyst indicated temperatures on the order 60 to 90°C during vein deposition. Eutectic temperatures in fluid inclusions suggest the solution composition consisted of a CaCl2 + NaCl system. Salinity of the solution decreases in the direction of vein growth from 22.9 to 15.3 equiv. wt.% NaCl. The presents of Na in the system may assist in the production of hole and electron colour centers by allowing for a charge compensation in the substitution of Fe3+ for Si4+ in the crystal lattice of quartz. Variations in salinity in the direction of growth may reflect changes in amethystine colour density.
Eh-pH conditions, calculated on the basis of paragenetic sequence within Cu-sulfide inclusions, suggest a reducing thrend during deposition.
Sulfur isotopic composition of pyrite in amethyst (delta34S = -0.6 to -0.4) was compared to the sulfur isotopic composition of lead-zinc-barite veins in the Dorion area (delta34 S = -0.1). The similarity in isotopic signature suggest that both types of deposit have similar origin.