11:00 AM - 1:00 PM
[SCG47-P17] Mineralogical characteristics of the ores from the Arasedani and Renge mines in the Kuga group, eastern Yamaguchi Prefecture
Keywords:manganese deposit
Many bedded manganese deposits distribute in the Kuga group, eastern Yamaguchi Prefecture (Miyamoto, 1953). It is known that the manganese ores in such deposits originated from manganese nodules and crust-bearing siliceous sediments on deep-sea floor, and metamorphism during the subduction-accretion process converted the Mn-enriched original substances to manganese ores (e.g. Nakagawa et al. 2011). The manganese deposits in the Kuga area are classified into two types: Mn-carbonate type and Mn-silicate type. It is considered that the primary Mn-carbonate ores were converted to the Mn-silicate ores under the effect of the contact metamorphism by Cretaceous granitoids (e.g. Nishimura et al. 2012). Therefore, to investigate the mineral assemblages and their chemical characteristics of both types of ores can contribute to the further understanding of the evolutional process of the bedded manganese deposits. In this study, we targeted the Mn-carbonate ores from the Arasedani mine and the Mn-silicate ores from the Renge mine distributed in the Kuga group as case study approach. They locate north and south across the Habu granodiorite. The Renge mine, which is closer to the diorite body, is expected to be more affected by the contact metamorphism.
The Mn-ores were collected from the mine dumps in each deposit. The ores from the Arasedani mine are mainly composed of rhodochrosite >> rhodonite + quartz for types A-I, and -II ores, and hausmannite + alleghanyite + rhodochrosite for type A-III ore. Types A-I and -II ores are expected to be formed under high temperature and fCO2 conditions rather than type A-III one based on the study of Brusnity et al. (2017). In types A-I and -II ores, rhodochrosite was partly replaced by rhodonite, indicating the following chemical reaction due to increasing temperature; rhodochrosite + quartz à rhodonite + CO2 (Yoshinaga, 1958). Chemical compositions of rhodochrosite in types A-I, -II, and -III ores are similar to each other represented as (Mn0.77-0.85Ca0.10-0.21Fe0.00-0.03Mg0.01-0.03)CO3. Rhodonite shows the almost ideal composition, MnSiO3, and its Fe content is negligible. The ores from the Renge mine are mainly composed of Mn-pyroxenoids (pyroxemangite >> rhodonite) > garnet >garnetz > cummingtonite~grunerite (types R-I, -II and -III ores). Type R-IV ore, however, is different, in which garnet is the most abundant and Mn-pyroxenoids are absent. In addition, native bismuth is remarkably observed. Pyroxemangite [(Mn0.60-0.63Fe2+0.27-0.31Mg0.05-0.06Ca0.03-0.04)Si1.00O3], rhodonite [(Mn0.63-0.74Fe0.11-0.16Ca0.09-0.18)Si1.00O3)] and garnet (Sps80-71Alm19-12Adr8-3Grs4-2) in the Renge ores are characterized by relatively high Fe content. As ore minerals, linnaeite, chalcopyrite, galena, altaite , and cobaltpentlandite were observed from the Arasedani ores. Cobalt and nickel in cobaltpentlandite, (Co8.41Ni0.39Mn0.16Fe0.06)S9.02S8, and linnaeite, (Co1.84-2.35Ni0.52-1.09Cu0.06Fe0.06Mn0.01-0.05)S2.98-3.00S4. Their formation seems to be related to the elements supplied by Mn-enriched original substances such as Mn-nodules and crusts. In contrast, gersdorffite, cobaltite, pyrite, pyrrhotite, chalcopyrite, native bismuth and bismuthinite were found in the Renge ores. Some Bi-bearing minerals were also described from the copper skarn deposits in the Mine area, central Yamaguchi Prefecture (Nagashima et al. 2016; 2021). Such skarn deposits were formed by the contact metamorphism of ilmenite-series granitoids. Since the Habu granodiorite, which is assumed to be the heat source of the Renge mine, is also located in the distribution are of ilmenite-series granitoids (Ishihara, 1977), it is hypothesized that this granodiorite is the source of bismuth.
The Mn-ores were collected from the mine dumps in each deposit. The ores from the Arasedani mine are mainly composed of rhodochrosite >> rhodonite + quartz for types A-I, and -II ores, and hausmannite + alleghanyite + rhodochrosite for type A-III ore. Types A-I and -II ores are expected to be formed under high temperature and fCO2 conditions rather than type A-III one based on the study of Brusnity et al. (2017). In types A-I and -II ores, rhodochrosite was partly replaced by rhodonite, indicating the following chemical reaction due to increasing temperature; rhodochrosite + quartz à rhodonite + CO2 (Yoshinaga, 1958). Chemical compositions of rhodochrosite in types A-I, -II, and -III ores are similar to each other represented as (Mn0.77-0.85Ca0.10-0.21Fe0.00-0.03Mg0.01-0.03)CO3. Rhodonite shows the almost ideal composition, MnSiO3, and its Fe content is negligible. The ores from the Renge mine are mainly composed of Mn-pyroxenoids (pyroxemangite >> rhodonite) > garnet >garnetz > cummingtonite~grunerite (types R-I, -II and -III ores). Type R-IV ore, however, is different, in which garnet is the most abundant and Mn-pyroxenoids are absent. In addition, native bismuth is remarkably observed. Pyroxemangite [(Mn0.60-0.63Fe2+0.27-0.31Mg0.05-0.06Ca0.03-0.04)Si1.00O3], rhodonite [(Mn0.63-0.74Fe0.11-0.16Ca0.09-0.18)Si1.00O3)] and garnet (Sps80-71Alm19-12Adr8-3Grs4-2) in the Renge ores are characterized by relatively high Fe content. As ore minerals, linnaeite, chalcopyrite, galena, altaite , and cobaltpentlandite were observed from the Arasedani ores. Cobalt and nickel in cobaltpentlandite, (Co8.41Ni0.39Mn0.16Fe0.06)S9.02S8, and linnaeite, (Co1.84-2.35Ni0.52-1.09Cu0.06Fe0.06Mn0.01-0.05)S2.98-3.00S4. Their formation seems to be related to the elements supplied by Mn-enriched original substances such as Mn-nodules and crusts. In contrast, gersdorffite, cobaltite, pyrite, pyrrhotite, chalcopyrite, native bismuth and bismuthinite were found in the Renge ores. Some Bi-bearing minerals were also described from the copper skarn deposits in the Mine area, central Yamaguchi Prefecture (Nagashima et al. 2016; 2021). Such skarn deposits were formed by the contact metamorphism of ilmenite-series granitoids. Since the Habu granodiorite, which is assumed to be the heat source of the Renge mine, is also located in the distribution are of ilmenite-series granitoids (Ishihara, 1977), it is hypothesized that this granodiorite is the source of bismuth.