14:45 〜 15:00
[HRE11-04] A textural, geochemical, and fluid inclusion study of quartz veins of the Hokuryu Au-Ag deposit, Hokkaido, Japan
キーワード：Hokuryu, epithermal, quartz texture, petrography, cathodoluminescence, EPMA
The Hokuryu Au-Ag deposit is a low-sulfidation epithermal deposit genetically linked to the Middle to Late Miocene back-arc volcanism in northeastern Hokkaido (Watanabe, 1996). It produced 2.2 tons of Au and 11.8 tons of Ag from 1928 to 1943 (Suzuki et al., 1966). In this study, we investigated the crustiform quartz veins of the Hokuryu deposit. Seven quartz vein floats from the dumps of the Hokuryu mine were petrographically analyzed. The quartz veins are mainly composed of quartz occurring in three sequences. Quartz in sequence 1 has pseudo-acicular or ghost-bladed textures with coexisting ghost-sphere, moss, flamboyant or mosaic textures; quartz in sequence 2 has ghost-sphere, moss, flamboyant, colloform or mosaic textures; quartz in sequence 3 has massive or drusy textures which coexist with feathery texture, or chalcedony. The sequence 3 is only occasionally present, and in some cases, it occurs as vein centerlines. Electrum (~64 at.% Ag) is present in the interstices of the quartz crystals of sequences 1 and 2, Ag-sulfoselenides mainly in the sequence 2, and rhombic adularia in the sequences 2 and 3.
These quartz textures, particularly pseudo-bladed and microspherical textures (e.g. ghost-sphere and moss), were confirmed by cathodoluminescence (CL) imaging. Quartz with CL emission band of ~380 nm was commonly observed as cement surrounding the microspheres or as infill along the bladed features. Quartz with such CL emission band shows elevated Al content (av. 3150 ppm). Fluid inclusions in the sequences 1 and 2 are usually significantly necked-down, owing to recrystallization (ghost-bladed, pseudo-acicular quartz textures) and replacement (ghost-sphere, flamboyant, mosaic). Secondary coexisting vapor-rich and liquid-rich inclusions were observed in drusy and feathery quartz in sequence 3. The presence of pseudo-acicular and ghost-bladed textures with other quartz textures indicative of amorphous silica or chalcedonic precursors (e.g. ghost-sphere, moss, flamboyant, colloform, and mosaic) is interpreted to indicate boiling, (e.g. Fournier, 1985; Dong et al., 1995). The relatively high Al concentration of the cementing or replacing quartz in the sequences 1 and 2, and the abundance of rhombic adularia in the sequence 3 corroborate the likely occurrence of boiling (Reed and Spycher, 1985).
References:
Dong, G., Morrison, G. and Jaireth, S. (1995) Quartz textures in epithermal veins, Queensland – classification, origin, and implication. Economic Geology, 90, 1841-1856.
Fournier, R.O. (1985) The behavior of silica in hydrothermal solutions. Reviews in Economic Geology 2, 45-51.
Reed, M.H. and Spycher, N.F. (1985) Boiling, cooling, and oxidation in epithermal systems: A numerical approach model. Reviews in Economic Geology, 2, 249-270.
Suzuki, M., Konoya, M. and Fujiwara, T. (1966) Explanatory text of the geological map Japan, scale 1:50,000, “Omu”. Geol. Surv. Hokkaido (in Japanese with English abstract).
Watanabe, Y. (1996) Genesis of Vein-hosting Fractures in the Kitami Region, Hokkaido, Japan. Resource Geology, 46, 151-166.
These quartz textures, particularly pseudo-bladed and microspherical textures (e.g. ghost-sphere and moss), were confirmed by cathodoluminescence (CL) imaging. Quartz with CL emission band of ~380 nm was commonly observed as cement surrounding the microspheres or as infill along the bladed features. Quartz with such CL emission band shows elevated Al content (av. 3150 ppm). Fluid inclusions in the sequences 1 and 2 are usually significantly necked-down, owing to recrystallization (ghost-bladed, pseudo-acicular quartz textures) and replacement (ghost-sphere, flamboyant, mosaic). Secondary coexisting vapor-rich and liquid-rich inclusions were observed in drusy and feathery quartz in sequence 3. The presence of pseudo-acicular and ghost-bladed textures with other quartz textures indicative of amorphous silica or chalcedonic precursors (e.g. ghost-sphere, moss, flamboyant, colloform, and mosaic) is interpreted to indicate boiling, (e.g. Fournier, 1985; Dong et al., 1995). The relatively high Al concentration of the cementing or replacing quartz in the sequences 1 and 2, and the abundance of rhombic adularia in the sequence 3 corroborate the likely occurrence of boiling (Reed and Spycher, 1985).
References:
Dong, G., Morrison, G. and Jaireth, S. (1995) Quartz textures in epithermal veins, Queensland – classification, origin, and implication. Economic Geology, 90, 1841-1856.
Fournier, R.O. (1985) The behavior of silica in hydrothermal solutions. Reviews in Economic Geology 2, 45-51.
Reed, M.H. and Spycher, N.F. (1985) Boiling, cooling, and oxidation in epithermal systems: A numerical approach model. Reviews in Economic Geology, 2, 249-270.
Suzuki, M., Konoya, M. and Fujiwara, T. (1966) Explanatory text of the geological map Japan, scale 1:50,000, “Omu”. Geol. Surv. Hokkaido (in Japanese with English abstract).
Watanabe, Y. (1996) Genesis of Vein-hosting Fractures in the Kitami Region, Hokkaido, Japan. Resource Geology, 46, 151-166.