[H-RE11] Earth Resource Science

[HRE11-P04] Microscopic observation of mineral assemblage in the ore of the Kushikino deposit using SEM- EDS: Implications for silver isotopic analysis to elucidate the mineralization process

*Sho Yoshida¹, Yasuhiro Kato¹, Kentaro Nakamura¹, Koichiro Fujinaga¹, Kazutaka Yasukawa¹, Junichiro Ohta¹, Mizuki Ishida¹, Ogasawara Mitsuki¹ (1.School of Engineering, The University of Tokyo)

Keywords:Kushikino deposit, epithermal deposit, silver minerals, microscopic observation, SEM-EDS

The global demand for precious metals such as gold and silver is forecasted to increase in the future because of their importance in high- and green-tech industry. Hydrothermal deposits are the major sources of gold and silver worldwide. Deciphering the formation process of hydrothermal deposits can provide us a key to constrain geological factors that cause concentration of valuable elements, which would be an important clue for mineral exploration. Since temperature is one of the most important factors controlling the dissolution and precipitation of elements in ore genesis, various methods have been used to estimate the mineralization temperature [1,2,3]. While these previous studies have estimated the mineralization temperatures of gangue minerals, the precipitation temperatures of useful metals have hardly estimated using the target metals themselves in ore deposits.
Recently, based on theoretical calculations of silver isotopic ratios of various silver minerals, a "silver isotope geothermometer" was proposed to estimate the temperature at which silver precipitates [4]. The silver isotope geothermometer is based on the silver isotopic fractionations between argentite and stephanite [4]. This could make it possible to directly determine the mineralization temperature of silver minerals in ore. However, there is no previous study that applied this silver isotope geothermometer to an actual ore sample. Thus, it is necessary to validate the applicability of this method experimentally.
In this study, we focused on the Kushikino deposit, which has been known as one of the largest gold-silver deposits in Japan and for the production of many types of silver minerals. To constrain the conditions of ore samples suitable for silver isotope analysis, we conducted detailed observations of ore minerals from the Kushikino deposit by SEM-EDS and reflection microscopy. In the presentation, the results will be described along with suggestions for the optimal method for silver isotope analysis of the ore samples from the Kushikino deposit.