11:15 〜 11:30
[HRE13-08] Serial sampling of a gold-bearing vein and observation of mineral texture, adularia-quartz ratio and Au grade in the Hishikari epithermal deposits, Japan
キーワード：Hishikari Au-Ag epithermal deposits, Serial sampling of a vein, Au grade, Adularia-quartz ratio, Quartz and adularia textures
Variations in the textures and the ratio of adularia and quartz in a Au-bearing vein of the Hishikari Au-Ag epithermal deposits were systematically analyzed. Samples from working faces of Hosen 8-2 vein with a thin high Au-grade ore zone, an average vein width of 8 cm, and usually consists of only a single homogenous white to transparent band at the Main sub-deposit were collected continuously at close proximity with each other (about 2.4 meters interval) along the vein strike. The length of serial sampling along the vein is ca. 68 m. Eleven distinct textures were observed in quartz (massive, cockade, comb, zonal, microcrystalline, crustiform, mosaic, feathery, flamboyant, ghost-bladed, and pseudo-acicular) and 4 textures were identified for adularia (sub-rhombic, rhombic, tabular, and pseudo-acicular). The earliest stage of vein growth is usually where Au mineralization occurs as electrum and is composed of microcrystalline quartz mixed with adularia±illite±smectite followed by either tabular/sub-rhombic adularia or equigranular quartz bands with crustiform texture.
Petrographic observation of quartz and adularia textures can suggest a possibility of non-boiling, gentle boiling, and violent boiling (flashing) (cf. Dong and Morrison, 1995; Moncada et al., 2012; Shimizu, 2014). In this study, microcrystalline quartz and tabular adularia were observed abundantly within the vein. Dong and Morrison (1995), Moncada et al. (2012), and Shimizu (2014) interpreted that the presence of microcrystalline quartz and tabular adularia in the early stage of the vein suggests violent boiling (or intense boiling) due to fluids moving up to a more permeable environment. Conversely, the presence of crustiform texture reveals a variety of reactions which includes varying degrees of boiling while coarse-grained sub-rhombic adularia requires slow crystallization conditions (Dong et al., 1995). Comb quartz is the most common feature among all the samples while sub-rhombic and tabular adularia occurs abundantly throughout the vein. The evidence of boiling and non-boiling should be further checked by the fluid inclusion study in future.
The vein samples were also subjected to XRD analysis to calculate the adularia-quartz ratio. The adularia-quartz ratio ranges from 2 to 95 %, but the samples with the highest gold grades were found to reside in the veins with about 50-70 % adularia-quartz ratio. Statistical analyses of quartz and adularia features show that microcrystalline quartz (with the samples containing an average of 1253 ppm Au) and rhombic adularia (with an average of 1376 ppm Au) are the textures most associated with elevated Au content. This denotes that the presence of these two textures can aid greatly in differentiating the high-grade ores from non-economic ones. This also suggests that one of the mechanisms responsible for the deposition of metals (Au-Ag) in the Hosen 8-2 vein might be violent boiling, which resulted in the precipitation of amorphous silica and metals from the hydrothermal fluid. Although textures that indicate gentle boiling such as mosaic, feathery, flamboyant, and replacement textures are present, metals were not observed to be associated with textures signifying such environments.
References
Dong, G. and Morrison, G. (1995). Adularia in epithermal veins, Queensland: morphology, structural state and origin. Mineralium Deposita 30, 11-19.
Dong, G., Morrison, G., and Jaireth, S. (1995). Quartz textures in epithermal veins, Queensland – classification, origin, and implication. Economic Geology 90, 1841-1856.
Moncada, D., Mutchler, S., Nieto, A., Reynolds, T. J., Rimstidt, J. D., Bodnar, R. J. (2012). Mineral textures and fluid inclusion petrography of the epithermal Au-Ag deposits at Guanajuato, Mexico: Application to exploration. Journal of Geochemical Exploration 114, 20-35.
Shimizu, T. (2014). Reinterpretation of quartz textures in terms of hydrothermal fluid inclusions at the Koryu Au-Ag Deposit, Japan. Economic Geology 109, 2051-2065.
Petrographic observation of quartz and adularia textures can suggest a possibility of non-boiling, gentle boiling, and violent boiling (flashing) (cf. Dong and Morrison, 1995; Moncada et al., 2012; Shimizu, 2014). In this study, microcrystalline quartz and tabular adularia were observed abundantly within the vein. Dong and Morrison (1995), Moncada et al. (2012), and Shimizu (2014) interpreted that the presence of microcrystalline quartz and tabular adularia in the early stage of the vein suggests violent boiling (or intense boiling) due to fluids moving up to a more permeable environment. Conversely, the presence of crustiform texture reveals a variety of reactions which includes varying degrees of boiling while coarse-grained sub-rhombic adularia requires slow crystallization conditions (Dong et al., 1995). Comb quartz is the most common feature among all the samples while sub-rhombic and tabular adularia occurs abundantly throughout the vein. The evidence of boiling and non-boiling should be further checked by the fluid inclusion study in future.
The vein samples were also subjected to XRD analysis to calculate the adularia-quartz ratio. The adularia-quartz ratio ranges from 2 to 95 %, but the samples with the highest gold grades were found to reside in the veins with about 50-70 % adularia-quartz ratio. Statistical analyses of quartz and adularia features show that microcrystalline quartz (with the samples containing an average of 1253 ppm Au) and rhombic adularia (with an average of 1376 ppm Au) are the textures most associated with elevated Au content. This denotes that the presence of these two textures can aid greatly in differentiating the high-grade ores from non-economic ones. This also suggests that one of the mechanisms responsible for the deposition of metals (Au-Ag) in the Hosen 8-2 vein might be violent boiling, which resulted in the precipitation of amorphous silica and metals from the hydrothermal fluid. Although textures that indicate gentle boiling such as mosaic, feathery, flamboyant, and replacement textures are present, metals were not observed to be associated with textures signifying such environments.
References
Dong, G. and Morrison, G. (1995). Adularia in epithermal veins, Queensland: morphology, structural state and origin. Mineralium Deposita 30, 11-19.
Dong, G., Morrison, G., and Jaireth, S. (1995). Quartz textures in epithermal veins, Queensland – classification, origin, and implication. Economic Geology 90, 1841-1856.
Moncada, D., Mutchler, S., Nieto, A., Reynolds, T. J., Rimstidt, J. D., Bodnar, R. J. (2012). Mineral textures and fluid inclusion petrography of the epithermal Au-Ag deposits at Guanajuato, Mexico: Application to exploration. Journal of Geochemical Exploration 114, 20-35.
Shimizu, T. (2014). Reinterpretation of quartz textures in terms of hydrothermal fluid inclusions at the Koryu Au-Ag Deposit, Japan. Economic Geology 109, 2051-2065.