Stable-isotope, fluid-inclusion, and mineralogical studies relating to the genesis of amethyst, Thunder Bay Amethyst Mine, Ontario

Abstract

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 brecciated earlier sequence containing chalcedony, colorless quartz, and prasiolite, which appears to be thermally bleached amethyst. The vein system occupies a fault in Archean granodiorite and is associated with a narrow zone of chloritic and hematite alteration overprinted by weak argillic alteration. Fragments of Proterozoic (1339 Ma) Sibley Group rocks occur in the vein system, indicating the former presence of a shallow cover during deposition of quartz and limiting the maximum age of the deposit. These downfallen fragments and the abundance of vugs indicate near-surface formation of the deposit.Main-stage fluid-inclusion homogenization temperatures are in the range from 91.2 to 40.9 °C (mean 68.4 °C) in amethyst, whereas in colorless quartz homogenization temperatures range from 146.5 to 114.7 °C (mean 132.1 °C). Eutectic temperatures fall in three ranges with means of −50.9, −48.7, and −43.9 °C, which are related to paragenetic position and indicate an NaCl–CaCl2–H2O system, with possible additional components in later inclusions. Salinities in amethyst-hosted inclusions decrease in the growth direction from 22.9 to 15.3 equiv.wt% NaCl.Trace sulfide and other mineral inclusions indicate a trend of decreasing Eh and pH from an initially rather oxidized (sulfate stable) to a reduced (sulfide stable) condition during deposition. Sulfur isotopic composition in pyrite and chalcopyrite ranges from δ34S = −0.4 to −1.4‰ and is similar to values obtained from lead–zinc–barite in other vein deposits surrounding the Sibley depositional basin. Oxygen isotopes in quartz range from δ18O = +12.7 to +17.1‰, corresponding to δ18O(H2O) = −2.1 to −12.8‰ using fluid-inclusion temperatures. Fresh quartz monzonite wall rock (δ18O = +11.82‰) and altered quartz monzonite (δ18O = +11.01‰) do not seem to have undergone significant isotopic exchange with the hydrothermal solution, and the trend of isotopic change does not account for the trend of δ18O(H2O) determined in quartz. Rather, mixing of local meteoric water with a basinal brine appears to explain the observed trend.The amethyst deposits are believed to have been formed by basinal brines expelled from Sibley Group sediments. The brines dissolved silica by alteration processes accompanying their passage through granitic basement rocks in basin marginal faults. Amethyst was deposited on mixing with meteoric water. The temperature interval for amethyst formation appears to be restricted to less than ~90 °C. Temperatures causing thermal bleaching of amethyst are as low as 145 °C, and possibly 115 °C, as indicated by these results. This low range of temperature is not in agreement with bench-type experiments indicating bleaching at hundreds of degrees Celsius.

Mineralogy of Agates with Amethyst from the Tevinskoye Deposit (Northern Kamchatka, Russia)

“…A similar association native copper with Cu-and Cu-Fe-sulfides and Cu-oxide (cuprite, CuO) was reported for amethyst-free agates from the Avacha Bay Eastern Kamchatka (Russia) [55][56][57]. Native copper together with Cuand Cu-Fe-sulphides associated with hematite and pyrite have been observed in the amethyst obtained from the Thunder Bay mine (Canada) [58]. Copper minerals are absent in agate geodes derived from the Amethysta do Sul (Brazil) and Ijevan (Armenia) deposits, which are similar in both morphology and texture to examined agates and to agate geodes from Artigas and Los Catalanes (Uruguay) and like amethysts from the Boudi deposit (Morocco) (Table 3) [7,10,11,38,59,60].…”

Quartz-Amethyst Hosted Hydrocarbon-Bearing Fluid Inclusions from the Green Ridge Breccia in the Snoqualmie Granite, North Cascades, WA, USA

“…Gem quality amethyst, e.g., from Brazil and Uruguay come from geodes in basaltic rocks of the Parana Continental Flood Basalt Province [12]. Published crystallisation temperatures of amethyst cover a wide range, e.g., (1) 370 to 420 • C from fluid inclusion studies of Brazilian geodes [13]; (2) <50 • C fluid inclusion data from Brazilian amethyst [14]; (3) stable oxygen isotope data from amethyst and agate hosted in the Prana-Etendka volcanics [14] and in Devonian and Tertiary volcanics of Scotland [15,16] indicate temperatures <150 • C; (4) Granite hosted amethyst veins from Thunder Bay, Canada, are considered to have formed between 40 and 90 • C [17]; (5) amethysts from epithermal polymetallic veins have formation temperatures ranging between 150 and 250 • C based on fluid inclusion studies [18][19][20] and (6) higher fluid-inclusion homogenization temperatures (280-400 • C) were reported from amethyst in granite miaroles of the Eonyang deposit, South Korea [21]. Finally, such high temperatures are also used in laboratory-based hydrothermal amethyst synthesis [18].…”

Spectral, electron microscopic and chemical investigations of gamma-induced purple color zonings in amethyst crystals from the Dursunbey-Balıkesir region of Turkey

“…Geologically, these transparent prismatic crystals can be found in clusters as druses in a nodular stone, commonly known as a geode, and they have been recognized in several widely-dispersed countries around the world (e.g. Brazil, Uruguay, Canada, Turkey, and so on), of which the most popular and richest deposit is in the Rio Grande Do Sul region of Brazil (5)(6)(7)(8)(9). In the gem market, the dark purple-, violet-, and orchid-colored amethyst gems (if the crystals are transparent) are considered to be the most precious by many buyers (3,4,10).…”