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GREENISH QUARTZ FROM THE THUNDER BAY AMETHYST MINE PANORAMA, THUNDER BAY, ONTARIO, CANADA

Laura Baker Hebert and George R. Rossman

Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125–2500, USA

E-mail address: labaker{at}gps.caltech.edu

Naturally occurring greenish quartz found within the context of amethyst-bearing deposits is not simply the result of the exposure of amethyst to thermal bleaching or exposure to the sun. Rather, it can represent a set of distinct color-varieties resulting from the changing chemical and thermal nature of the precipitating solution. Greenish quartz occurs at the Thunder Bay Amethyst Mine Panorama (TBAMP), Thunder Bay, Ontario, Canada, in several distinct varieties. Yellowish green quartz and dark green quartz with purple hues occur as loose detritus, and pale greenish gray quartz occurs as part of a color-gradational sequence of mineralization involving macrocrystalline quartz of other colors and chalcedony. The TBAMP system contains a number of color varieties of quartz including greenish, amethyst, colorless, and smoky. Spectroscopic, irradiation and controlled heating studies show that changes in salinity and temperature of the hydrothermal system that produced the TBAMP deposit are reflected in the changing coloration of the quartz. The greenish quartz, especially the greenish gray variety, has increased turbidity and fluid inclusions in comparison with the adjacent amethyst. Analysis of different colors on major (r = {101}) and minor (z = {011}) rhombohedral sectors within the quartz indicates that changes in the growth rate also have influenced color development. As the system evolved, two factors contributed to the color changes. A minor ferric component appears to change position from interstitial to substitutional within specific growth-sectors, and the trace-element composition of the quartz evolved. The samples from the TBAMP deposit are compared to isolated samples of greenish quartz collected from three other amethyst-bearing localities: Farm Kos and Farm Rooisand (Namibia), Kalomo-Mapatiqya (Zambia), and southern Bahia (Brazil). All included similar greenish hues with the exception of the yellowish green variety. Colors within the quartz are consistently correlated with the speciation of hydrous components. Darker green samples incorporate larger amounts of molecular H₂O than either pale greenish gray samples, colorless samples, or amethyst. The appearance of strong hydroxyl peaks in the infrared spectra is limited to amethyst and colorless varieties.

Keywords: amethyst, green quartz, Thunder Bay, Ontario.