A fluid inclusion study of an amethyst deposit in the Cretaceous Kyongsang Basin, South Korea

Abstract

ABSTR ACTThe Eonyang amethyst deposit is thought to be spatially and temporally associated with the biotite granite of the Cretaceous Kyongsang Basin, South Korea. The euhedral quartz crystals in cavities in the aplite which intrudes biotite granite are colour-zoned from white at the base to amethystine at the top. Fluid inclusions from rock-forming quartz in granitic rocks and euhedral quartz crystals in cavities were examined. Three types of primary inclusions were observed and three isochores for inclusions representing each type are constructed to constrain the trapping conditions and fluid evolution involved during the formation of the amethyst. The intersection of the isochore representing the early fluid inclusions with solidus temperature of the host granite indicates initial quartz formation at~6008C and 1.0 1.5 kbar. Intermediate quartz formation, associated with the high-salinity inclusions, occurred at somewhat lower temperatures (4008C) and pressures of~1 kbar. The amethystine quartz formed from H 2 O-CO 2 -NaCl fluids at temperatures between 280 and 4008C, and pressures of~1 kbar. Based on the texture and mineralogy of host minerals and on the fluid inclusion characteristics, the euhedral quartz began growing at near solidus conditions of the granite and the pressure did not vary significantly until the end of crystallization of amethystine quartz crystals in cavities. Early quartz in cavities formed from moderately saline fluids that either exsolved from or were in equilibrium with the granite, whereas the amethystine quartz apparently grew from fluids of at least partial sedimentary origin. The granite crystallized at considerable depth under relatively low water pressures probably in the root zones of porphyry-type systems. Hydrothermal activities, fluid compositions and erosion factors combined to provide favourable conditions for the formation of the Eonyang amethyst deposit and its presence near the Earth's surface today.

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

“…Therefore, the true trapping temperature (T t ) for Type 1 fluids is~390 • C. This modelled crystallisation temperature is within the 370 • C to 420 • C range published by Thomas et al [13] from fluid inclusion studies of Brazilian quartz amethyst geodes. Similar fluid inclusion studies by Yang et al [21] reported temperatures of between 280 • C and 400 • C for amethyst crystallisation in granite miaroles of the Eonyang deposit, South Korea. …”

“…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].…”

Fluid inclusions in amethyst quartz of different geological environments from Brazil

“…Gilg et al (2003) and Commin-Fischer et al (2010) reported maximum trapping temperatures of 100°C and low salinity fluids for amethyst hosted in basalt cavities of Rio Grande do Sul, Brazil. For amethyst related to Cretaceous granite in South Korea, Yang et al (2001) discussed higher formation temperatures, between 280°C and 400°C, and average salinity of 4 wt.% NaCl equivalent. Dumanska-Slowik et al (2017) studied fluid inclusions trapped in amethyst associated with Lower Cambrian siltstone-sandstone, and report homogenization temperatures between 210°C and 330°C and salinity values between 6 and 13 wt.% NaCl equivalent.…”

“…Gilg et al (2003) and Commin-Fischer et al (2010) reported maximum trapping temperatures of 100°C and low-salinity fluids for amethyst hosted in basalt cavities at Rio Grande do Sul, Brazil. For amethyst related to Cretaceous granite in South Korea, Yang et al (2001) discussed higher formation temperatures, between 280°C and 400°C, and average salinity of 4 eq. wt.% NaCl.…”

Periodically Released Magmatic Fluids Create a Texture of Unidirectional Solidification (UST) in Ore-Forming Granite: A Fluid and Melt Inclusion Study of W-Mo Forming Sannae-Eonyang Granite, Korea

“…The Sannae W-Mo deposit is a vein-type scheelite-wolframite-molybdenite-bearing deposit [27,28]. The Eonyang amethyst deposit [29][30][31][32] is associated with an aplitic part in the granite and contains a series of UST sequences consisting of quartz and feldspar, in which quartz crystals have developed a clear zonation along the c-axis (Figure 2). The quartz crystals in the USTs and phenocrysts of the associated aplite contain melt and fluid inclusions.…”