15:45 〜 16:00
[HRE13-06] 新潟県佐渡島鶴子銀山鉱石の鉱物学的・地球化学的特徴
キーワード：鶴子銀山、佐渡金銀鉱床、銀鉱物、SEM-EDS、ICP-MS
The Sado gold-silver deposits, operated for about 400 years since the Edo Period until 1989, are known for their second largest gold production in Japan's mining history. However, the formation mechanism of the Sado gold-silver deposits has not been fully understood. Although about 30 deposits are distributed in Sado Island, geochemical studies have been limited to the Aikawa deposit, the largest deposit in the island. This study focuses on the mineralogical and geochemical characteristics of the Tsurushi silver deposit, one of the representative deposits among the Sado gold-silver deposits and is located near the Aikawa deposit [1]. The Tsurushi silver deposit is apart from the Aikawa gold-silver deposit by only about 3 km, without connecting/overlapping their mineralization areas and veins.
In this study, ore samples of the Tsurushi silver deposit, provided by Sado City, were analyzed. Total of 11 polished pieces were prepared from the ore samples. A polarization-reflection microscope and scanning electron microscope (SEM-EDS) were used to identify the minerals, observe their mineralogical distribution, and perform elemental mapping. Seven of these polished pieces were selected as representative samples, and the pieces of their counterparts were sectioned by mineralogy within the ore sample. Each rock fragment was powdered, and a total of 28 solution samples were analyzed by inductively coupled plasma-mass spectrometry (ICP-MS) after the acid digestion procedure with some modification from [2].
Pyrargyrite, chalcopyrite, pyrite, sphalerite, galena, covellite and quartz were observed by the polarization-reflection microscopy. In addition to these minerals, polybasite, argentite and adularia were observed by the SEM-EDS. The results of the whole-rock chemical composition analysis by ICP-MS showed that the maximum and median concentrations of Ag were 389 ppm and 21.9 ppm, respectively. The maximum and median concentrations of Au were 28.7 ppm and 0.203 ppm, respectively. Eleven elements of the siderophile and chalcophile elements, excluding Ni and As, showed a correlation with Ag, corresponding to the presence of sulfide minerals. We call them as "sulfide group elements" in this study. For each element in this group, two types of trends were observed in elemental scatter diagrams. One is a "correlative trend," in which the concentration of the element increases in proportion to Ag concentration, and the other is an "independent trend," in which the concentration of the element increases regardless of the Ag concentration. The samples belonging to the independent trend consist of mainly massive sulfide minerals, whereas those belonging to the correlative trend showed distinct quartz veins. The relationship between Ag concentration and total element concentrations suggests a dilution effect by quartz in the correlative trend. Some of the samples belonging to the correlation trend have particularly high concentrations of both Ag and Au. Since the quartz veins in the samples in this study cut the massive sulfide minerals, it is suggested that there were two stages of mineralization at the Tsurushi deposit: mineralization that produced many kinds of sulfide minerals including Ag and mineralization associated with quartz veins that precipitated high concentrations of Ag and Au.
[1] MITI (1987) Report of regional geological survey, Sado Region.
[2] Kato et al. (2005) Geochem. Geophys. Geosyst. 6, Q07004.
In this study, ore samples of the Tsurushi silver deposit, provided by Sado City, were analyzed. Total of 11 polished pieces were prepared from the ore samples. A polarization-reflection microscope and scanning electron microscope (SEM-EDS) were used to identify the minerals, observe their mineralogical distribution, and perform elemental mapping. Seven of these polished pieces were selected as representative samples, and the pieces of their counterparts were sectioned by mineralogy within the ore sample. Each rock fragment was powdered, and a total of 28 solution samples were analyzed by inductively coupled plasma-mass spectrometry (ICP-MS) after the acid digestion procedure with some modification from [2].
Pyrargyrite, chalcopyrite, pyrite, sphalerite, galena, covellite and quartz were observed by the polarization-reflection microscopy. In addition to these minerals, polybasite, argentite and adularia were observed by the SEM-EDS. The results of the whole-rock chemical composition analysis by ICP-MS showed that the maximum and median concentrations of Ag were 389 ppm and 21.9 ppm, respectively. The maximum and median concentrations of Au were 28.7 ppm and 0.203 ppm, respectively. Eleven elements of the siderophile and chalcophile elements, excluding Ni and As, showed a correlation with Ag, corresponding to the presence of sulfide minerals. We call them as "sulfide group elements" in this study. For each element in this group, two types of trends were observed in elemental scatter diagrams. One is a "correlative trend," in which the concentration of the element increases in proportion to Ag concentration, and the other is an "independent trend," in which the concentration of the element increases regardless of the Ag concentration. The samples belonging to the independent trend consist of mainly massive sulfide minerals, whereas those belonging to the correlative trend showed distinct quartz veins. The relationship between Ag concentration and total element concentrations suggests a dilution effect by quartz in the correlative trend. Some of the samples belonging to the correlation trend have particularly high concentrations of both Ag and Au. Since the quartz veins in the samples in this study cut the massive sulfide minerals, it is suggested that there were two stages of mineralization at the Tsurushi deposit: mineralization that produced many kinds of sulfide minerals including Ag and mineralization associated with quartz veins that precipitated high concentrations of Ag and Au.
[1] MITI (1987) Report of regional geological survey, Sado Region.
[2] Kato et al. (2005) Geochem. Geophys. Geosyst. 6, Q07004.