3:45 PM - 4:00 PM
[HRE12-07] Occurrence of calcium silicate hydrate minerals in a high Au-grade zone of the Kinsen 1-1 vein at the Hishikari deposit, Kagoshima, Japan
Keywords:Hishikari deposit, low-sulfidation epithermal deposit, high Au-grade zone, calcium aluminum silicate hydrates, prehnite-ferriprehnite solid solution
Occurrence of calcium silicate hydrate minerals in a high Au-grade zone of the Kinsen 1-1 vein at the Hishikari deposit, Kagoshima, Japan
Naohiro Kishi1, Jonathan Macuroy2, Ryohei Takahashi1, Akira Hara3, Yoshinori Okaue3, Akira Imai4, Pearlyn Manalo1, Hinako Sato1, and Andrea Agangi1
1Graduate School of International Resource Sciences, Akita University; 2Center for Regional Revitalization in Research and Education, Akita University, Japan, 3Sumitomo Metal Mining Co., Ltd., 4Faculty of Engineering, Kyushu University, Japan
The Hishikari low-sulfidation Au-Ag deposit is located in southern Kyushu and is the largest gold producer in Japan. This study aims to investigate the relationship between calcium aluminum silicate hydrate minerals and a high Au-grade zone along the strike of the Kinsen 1-1 vein at +85 ML. Fifty-seven quartz vein samples were continuously collected with ca. 2 m intervals along the vein strike and were subjected to optical and electron microscopy as well as bulk and mineral composition analyses.
Bulk chemical compositions of the vein samples yielded Au grades ranging from 4.10 to 3267 ppm with an average of 408 ppm. The southwestern part of the studied zone has medium Au grades (100-500 ppm), whereas the central part has high Au grades (> 500 ppm). The northeastern part has relatively low Au grades (< 100 ppm).
Main gangue minerals observed are quartz, calcite, and adularia with locally abundant prehnite (prehnite-ferriprehnite solid solution), truscottite, and gyrolite. Prehnite occurs as (1) aggregates of acicular crystals and (2) relatively coarse individual crystals and those with mosaic textures, with the former having a higher birefringence than that of the latter. Acicular truscottite is always intergrown with sheet-like gyrolite. Major ore minerals observed are electrum, minor chalcopyrite, bornite, sphalerite, and galena. Abundant occurrence of electrum is associated with prehnite, truscottite, and gyrolite.
Mole fraction of Fe3+ expressed as Fe3+/(Fe3++AlVI) in octahedral site of acicular prehnite ranges from 0.001 to 0.744 with an average of 0.361, whereas that in individual and mosaic prehnite ranges from 0.000 to 0.869 with an average of 0.285. It is recognized that incorporation of Fe3+ into the octahedral site of prehnite-ferriprehnite solid solution is attributed to temperatures (Liou et al., 1983). Assuming the same oxygen fugacity, we consider that the individual and mosaic prehnite most likely formed at relatively higher temperatures than those of acicular prehnite in the Kinsen 1-1 vein at +85 ML.
Keywords: Hishikari deposit, low-sulfidation epithermal deposit, high Au-grade zone, calcium aluminum silicate hydrates, prehnite-ferriprehnite solid solution
References
Liou, J.G., Kim, H.S. and Maruyama, S. (1983) Prehnite-epidote equilibria and their petrologic applications. J. Petrol., 24, 321-342.
Naohiro Kishi1, Jonathan Macuroy2, Ryohei Takahashi1, Akira Hara3, Yoshinori Okaue3, Akira Imai4, Pearlyn Manalo1, Hinako Sato1, and Andrea Agangi1
1Graduate School of International Resource Sciences, Akita University; 2Center for Regional Revitalization in Research and Education, Akita University, Japan, 3Sumitomo Metal Mining Co., Ltd., 4Faculty of Engineering, Kyushu University, Japan
The Hishikari low-sulfidation Au-Ag deposit is located in southern Kyushu and is the largest gold producer in Japan. This study aims to investigate the relationship between calcium aluminum silicate hydrate minerals and a high Au-grade zone along the strike of the Kinsen 1-1 vein at +85 ML. Fifty-seven quartz vein samples were continuously collected with ca. 2 m intervals along the vein strike and were subjected to optical and electron microscopy as well as bulk and mineral composition analyses.
Bulk chemical compositions of the vein samples yielded Au grades ranging from 4.10 to 3267 ppm with an average of 408 ppm. The southwestern part of the studied zone has medium Au grades (100-500 ppm), whereas the central part has high Au grades (> 500 ppm). The northeastern part has relatively low Au grades (< 100 ppm).
Main gangue minerals observed are quartz, calcite, and adularia with locally abundant prehnite (prehnite-ferriprehnite solid solution), truscottite, and gyrolite. Prehnite occurs as (1) aggregates of acicular crystals and (2) relatively coarse individual crystals and those with mosaic textures, with the former having a higher birefringence than that of the latter. Acicular truscottite is always intergrown with sheet-like gyrolite. Major ore minerals observed are electrum, minor chalcopyrite, bornite, sphalerite, and galena. Abundant occurrence of electrum is associated with prehnite, truscottite, and gyrolite.
Mole fraction of Fe3+ expressed as Fe3+/(Fe3++AlVI) in octahedral site of acicular prehnite ranges from 0.001 to 0.744 with an average of 0.361, whereas that in individual and mosaic prehnite ranges from 0.000 to 0.869 with an average of 0.285. It is recognized that incorporation of Fe3+ into the octahedral site of prehnite-ferriprehnite solid solution is attributed to temperatures (Liou et al., 1983). Assuming the same oxygen fugacity, we consider that the individual and mosaic prehnite most likely formed at relatively higher temperatures than those of acicular prehnite in the Kinsen 1-1 vein at +85 ML.
Keywords: Hishikari deposit, low-sulfidation epithermal deposit, high Au-grade zone, calcium aluminum silicate hydrates, prehnite-ferriprehnite solid solution
References
Liou, J.G., Kim, H.S. and Maruyama, S. (1983) Prehnite-epidote equilibria and their petrologic applications. J. Petrol., 24, 321-342.