STABLE-ISOTOPE, FLUID-INCLUSION, AND MINERALOGICAL STUDIES RELATING TO THE GENESIS OF AMETHYST, THUNDER-BAY AMETHYST MINE, ONTARIO

The Thunder Bay Amethyst Mine exploits a vein system in which the main zoned sequence consists of chalcedony, colorless quartz, and three to four stages of amethyst. The main sequence surrounds fragments of a b recciated earlier sequence containing chalcedony, colorless quartz, an d prasiolite, which appears to be thermally bleached amethyst. The vei n system occupies a fault in Archean granodiorite and is associated wi th a narrow zone of chloritic and hematite alteration overprinted by w eak argillic alteration. Fragments of Proterozoic (1339 Ma) Sibley Gro up rocks occur in the vein system, indicating the former presence of a shallow cover during deposition of quartz and limiting the maximum ag e of the deposit. These downfallen fragments and the abundance of vugs indicate near-surface formation of the deposit. Main-stage fluid-incl usion homogenization temperatures are in the range from 91.2 to 40.9-d egrees-C (mean 68.4-degrees-C) in amethyst, whereas in colorless quart z homogenization temperatures range from 146.5 to 114.7-degrees-C (mea n 132. 1-degrees-C). Eutectic temperatures fall in three ranges with m eans of -50.9, -48.7, and -43.9-degrees-C, which are related to parage netic position and indicate an NaCl-CaCl2-H2O system, with possible ad ditional components in later inclusions. Salinities in amethyst-hosted inclusions decrease in the growth direction from 22.9 to 15.3 equiv.w t % NaCl. Trace sulfide and other mineral inclusions indicate a trend of decreasing Eh and pH from an initially rather oxidized (sulfate sta ble) to a reduced (sulfide stable) condition during deposition. Sulfur isotopic composition in pyrite and chalcopyrite ranges from 634S = -0 .4 to - 1.4 parts per thousand and is similar to values obtained from lead - zinc - barite in other vein deposits surrounding the Sibley dep ositional basin. Oxygen isotopes in quartz range from deltaO-18 = + 12 .7 to + 17.1 parts per thousand, corresponding to deltaO-18(H2O) = -2. 1 to -12.8 parts per thousand using fluid-inclusion temperatures. Fres h quartz monzonite wall rock (deltaO-18 = +11.82 parts per thousand) a nd altered quartz monzonite (deltaO-18 = +11.01 parts per thousand) do not seem to have undergone significant isotopic exchange with the hyd rothermal solution, and the trend of isotopic change does not account for the trend of deltaO-18(H2O) determined in quartz. Rather, mixing o f local meteoric water with a basinal brine appears to explain the obs erved trend. The amethyst deposits are believed to have been for-med b y basinal brines expelled from Sibley Group sediments. The brines diss olved silica by alteration processes accompanying their passage throug h granitic basement rocks in basin marginal faults. Amethyst was depos ited on mixing with meteoric water. The temperature interval for ameth yst formation appears to be restricted to less than approximately 90-d egrees-C. Temperatures causing thermal bleaching of amethyst are as lo w as 145-degrees-C, and possibly 115-degrees-C, as indicated by these results.This low range of temperature is not in agreement with bench-t ype experiments indicating bleaching at hundreds of degrees Celsius.