Mechanisms for anhydrite and gypsum formation in the Kuroko massive sulfide–sulfate deposits,north Japan |
| |
Authors: | Yasumasa Ogawa Naotatsu Shikazono Daizo Ishiyama Hinako Sato Toshio Mizuta Takanori Nakano |
| |
Institution: | (1) Department of Applied Chemistry, Faculty of Science and Technology, Keio University, Hiyoshi 3-14-1, Kohoku-ku, Yokohama, Kanagawa 223-8522, Japan;(2) Division of Applied Earth Science, Department of Earth Science and Technology, Faculty of Engineering and Resource Science, Akita University, Tegatagakuen-machi 1-1, Akita, Akita 010-8502, Japan;(3) Research Institute for Humanity and Nature, Inter-University Research Institute, Ministry of Education, Culture, Sports, Science, and Technology, Takashima-cho 305, Marutamachi-dori, Kawaramachi Nishi-iri, Kamigyo-ku, Kyotom, Kyoto, Kyoto 602-0875, Japan;(4) Present address: Graduate School of Environmental Studies, Tohoku University, Aoba, Aramaki, Aoba-ku, Sendai, Miyagi 980-8579, Japan |
| |
Abstract: | The Sr, Ba, and rare earth elements (REEs) concentrations and Sr isotopic composition of anhydrite and gypsum have been determined
for samples from the Matsumine, Shakanai, and Hanaoka Kuroko-type massive sulfide–sulfate deposits of northern Japan to evaluate
the mechanisms of sekko (anhydrite and gypsum) ore formation. The Sr isotopic compositions of the samples fall in the range
of 0.7077–0.7087, intermediate between that for middle Miocene (13–15 Ma) seawater (0.7088) (Peterman et al., Geochim Cosmochim
Acta, 34:105–120, 1970) and that for country rocks (e.g., 0.7030–0.7050) (Shuto, Assn Geol Collab Japan Monograph 18:91–105, 1974). The Kuroko anhydrite samples exhibit two types of chondrite-normalized REE patterns: one with a decrease from light REEs
(LREEs) to heavy REEs (HREEs) (type I), and another with a LREE-depleted pattern (type II).
Based on the Sr content and isotopic ratio (assuming an Sr/Ca (mM/M) of 8.7 for seawater), anhydrite is considered to have
formed by mixing of preheated seawater with a hydrothermal solution of Sr/Ca (mM/M) = ca. 0.59–1.36 under the condition in
which the partition coefficient (Kd) ranges between ca. 0.5 and 0.7. This results in the formation of anhydrite with higher
Sr content with an Sr isotopic value close to that of seawater under seawater-dominant conditions.
Larger crystals of type II anhydrite are partly replaced by smaller ones, indicating that anhydrite dissolution and recrystallization
occurred after or during the formation of sekko ore. Gypsum, which partially replaces anhydrite in the Kuroko deposits, also
exhibits two distinct chondrite-normalized REE patterns. Because LREEs are likely to be more readily mobilized during dissolution
and recrystallization, it is hypothesized that LREEs are leached from type I anhydrite, resulting in the formation of type
II anhydrite with LREE-depleted profiles. |
| |
Keywords: | Kuroko deposits Anhydrite Gypsum Sr isotope ratio Rare earth elements (REEs) |
本文献已被 SpringerLink 等数据库收录! |
|