HYDROTHERMAL BLEACHING OF AMETHYST AT THE THUNDER BAY AMETHYST MINE, ONTARIO

The Thunder Bay Amethyst Mine in northwestern Ontario contains two periods of amethyst deposition in which an earlier phase is contained as breccias fragments enclosed in a later phase. The zoned amethyst in the earlier phase is bleached to cloudy grey to colorless. Absorption spectra in the visible range indicate the absence of Fe(IV) peaks responsible for amethyst color, although absorption peaks attributed to Fe(III) are present. Infrared absorption spectra reveal the presence of molecular water and hydroxyl, in agreement with previous studies. Molecular water can be attributed to the presence of fluid inclusions, which are present in both phases of deposition. The bleached earlier phase, however, contains abundant secondary and pseudo-secondary fluid inclusions, whereas the amethyst-bearing phase contains relatively scarce primary inclusions.

Observations with the FTIR microscope were able to avoid most fluid inclusions in both amethyst and the bleached phase, revealing nearly identical absorptions from molecular water and hydroxyl. The cause of the bleaching of the early phase of amethyst deposition may be the result of instability of the Fe(IV) caused by the heat of solutions depositing the latter amethyst phase. However, temperatures previously determined for this phase of deposition are lower than experimentally determined temperatures for the upper stability of amethyst coloration. Alternatively, incorporation of molecular water along microfractures in the early bleached phase, producing secondary and pseudo-secondary inclusions, may have resulted in increased hydroxyl incorporation resulting in possible instability of the color centers.