[S-CG47] Petrology, Mineralogy & Resource Geology

[SCG47-10] Zebra rock indicating undiscovered paleo-hydrothermal system in northern Australia: the formation process and Fe-precipitation mechanism

*Hirokazu Kawahara^1,2, Hidekazu Yoshida³, Nagayoshi Katsuta⁴, Shoji Nishimoto⁵, Ayako Umemura³, Ryusei Kuma⁶ (1.Graduate School of Environmental Studies, Nagoya University, 2.Japan Oil, Gas and Metals National Corporation, 3.Material Research Section, Nagoya University Museum, 4.Faculty of Education, Gifu University, 5.Faculty of Law, Aichi University, 6.Institute for Space-Earth Environmental Research, Nagoya University)

Keywords:Liesegang band, Fe-oxide band, pH buffering, Hydrothermal alteration

Zebra rock from the eastern Kimberley region of northern Australia is a Late Proterozoic sedimentary rock with characteristic reddish-brown bands of several mm to 2 cm in width composed of Fe-oxide mineral (hematite). For the formation process of zebra rock, we introduce a new model, suggesting that zebra rock formed in an acidic hydrothermal system and that pH buffering of Fe²⁺- bearing acidic fluid, in a neutralization reaction with primary carbonate minerals, induced rhythmic Fe-precipitation (Kawahara et al., 2022: Chem. Geol.). The Fe profile clearly shows a reaction front, indicating unidirectional diffusive fluid migration along bedding planes. The two types of zebra rock distinguished by clay mineral assemblages (kaolinite-rich type and alunite-rich type) correspond with typical acidic hydrothermal alteration zonation. By confirming the distribution of the two types of zebra rock and the direction of fluid migration in the field, it is expected that the details of the undiscovered hydrothermal system related to the formation of zebra rock will be clarified.