Oxygen isotopic study of vein quartz in Neogene-Quaternary overprinting hydrothermal systems in the Toyoha-Muine area, Hokkaido, Japan

Abstract

An oxygen isotopic study of vein quartz in the Toyoha-Muine area was conducted using a CO₂ laser microprobe technique, in order to determine origin of hydrothermal fluids. The oxygen isotopic values of the hydrothermal fluids resulting in the vein formations were calculated to be between -10.6 and 1.0 ‰, using the obtained oxygen isotopic values of quartz and previous data on the formation temperature of quartz from fluid inclusion studies. Combined with existing chronological data on the hydrothermal activities related to the vein formations, the isotopic values are classified into two different chronological ranges: -6.3 to 1.0 ‰ in Middle to Late Miocene and -10.6 to -6.7 ‰ in Pliocene-Pleistocene. Based on the previous geological reconstruction at the area, the values for the Middle to Late Miocene event indicate mixtures of fluids with variable ratios of meteoric water, seawater and magmatic water, whereas the values for Pliocene-Pleistocene result from fluids with high ratios of meteoric water to magmatic water.