Abstract
The Upper Cretaceous Sannae-Eonyang granite crystallized approximately 73 Ma and hosted the Sannae W-Mo deposit in the west and the Eonyang amethyst deposit in the east. The granite contained textural zones of miarolitic cavities and unidirectional solidification texture (UST) quartz. The UST rock sampled in the Eonyang amethyst mine consisted of (1) early cavity-bearing aplitic granite, (2) co-crystallization of feldspars and quartz in a granophyric granite, and (3) the latest unidirectional growth of larger quartz crystals with clear zonation patterns. After the UST quartz was deposited, aplite or porphyritic granite was formed, repeating the prior sequence. Fluid and melt inclusions occurring in the UST quartz and quartz phenocrysts were sampled and studied to understand the magmatic-hydrothermal processes controlling UST formation and W-Mo mineralization in the granite. The composition of melt inclusions in the quartz phenocrysts suggested that the UST was formed by fractionated late-stage granite. Some of the melt inclusions occurring in the early-stage UST quartz were associated with aqueous inclusions, indicating fluid exsolution from a granitic melt. Hypersaline brine inclusions allowed the calculation of the minimum trapping pressure of 80–2300 bars. Such a highly fluctuating fluid pressure might be potentially due to a lithostatic-hydrostatic transition of pressure-attending fluid loss during UST formation. Highly fluctuating lithostatic-hydrostatic pressures created by fluid exsolution allowed shifting of the stability field from a quartz-feldspar cotectic to a single-phase quartz. The compositions of brine fluid assemblages hosted in the quartz phenocrysts deviated from the fluids trapped in the UST quartz, especially regarding the Rb/Sr and Fe/Mn ratios and W and Mo concentrations. The study of melt and fluid inclusions in the Eonyang UST sample showed that the exsolution of magmatic fluid was highly periodic. A single pulse of magmatic fluids of variable salinities/densities might have created a single UST sequence, and a new batch of magmatic fluid exsolution would be required to create the next UST sequence.
Highlights
The ice melting temperatures ranged from −3 to −21 ◦ C in the unidirectional solidification texture (UST) quartz and −11 to −21 ◦ C in the phenocryst, which corresponded to an apparent phenocrysts (Table S1 and Figures 4 and 5)
Our microanalytical study of melt and fluid inclusions in the UST in the Eonyang area suggests that periodic fluid exsolution from the upper part of the crystallizing magma created a UST band sequence (Figure 11)
Replacement of hydrostaticlithostatic pressure continued during UST formation until the cessation of magmatic fluid release (Figure 11)
Summary
Magmatic fluid exsolution processes in the upper part of a crystallizing magma are critical for forming magmatic-hydrothermal ore systems, such as Sn-W granites and porphyry-style deposits [1,2,3,4]. UST is caused by anisotropic mineral crystallization directed toward the inner part of a magma chamber, which has been identified globally at intrusions generating economic hydrothermal deposits of Cu-Au [10,11,12,13,14,15], Mo [6,16,17], and Sn or W [18,19,20,21]. Consists sedimentary sequences and volcanic-volcaniclastic rocks, Peninsula, consistsofofnon-marine non-marine sedimentary sequences and volcanic-volcaniclastic collectively called the Gyeongsang. Supergroup, which was deposited above a Precambrian rocks, collectively called the Gyeongsang Supergroup, which was deposited above a Premetamorphic basementbasement (Yeongnam massif).
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