Fluid inclusion study of quartz from Sendachiyama hydrothermal clay deposit, Fukushima Prefecture

Abstract

Sendachiyama clay deposits which consist of kaolinite, halloysite, sericite and associated quartz, occur in hydrothermally argillized zone of andesite and green tuff of Miocene age. Since quartz veining seems to be genetically related with the clay minerals, filling temperatures, salinities and chemical compositions of fluid inclusions in quartz were studied by means of the heating-and cooling-stage and IMA.
Filling temperatures range from 200°C to 300°C and salinities are less than 6 wt.% NaCl equivalent con-centration. Ratios of K/Na of solutions in inclusions are in order of from 0.1 to 1.0. Average value of filling temperatures of the samples from kaolinite-rich zone is 261°C and the one from halloysite-rich zone is 236°C. Judging from the fact that the thermal variation with depth is not recognizable, ascending thermal waters were not diluted with meteoric water of low temperatures as far as the present level of the deposit is concerned. The data obtained suggest not only the conditions of the formation of this deposit, but also the conditions of wall-rock alteration of kaolinite-halloysite-sericite-quartz combination.