Geology and Ore Deposits of the Shakanai Mine (2) Alteration of the Country Rocks

Abstract

Country rocks in the environs of the Shakanai Mine are composed chiefly of rhyolite and related pyroclastic rocks.
The horizon of ore bodies of economic value of the Shakanai ore deposits lies in the boundary between rhyolite and pyroclastic rocks in the upper part of the Shakanai formation (Nishikurosawa stage correlated to lower Burdigalian), but horizon of another mineralizations can also be observed to be situated in the Sainokami formation (Onnagawa stage, upper Burdigalian to Helvetian).
The hydrothermal alteration which took place during the formation of deposits has the following characteristics.
1. Elongated texture of pumice in the tuffaceous country rocks.
2. Existence of illcrystalline sericite group.
3. Impregnation of fine-grained pyrite in the country rocks.
4. Natural radioactive anomaly.
5. High concentration of mercury.
6. High concentration of various trace elements.
Zone of montmorillonite-zeolite facies as the diagenetic alteration is highly disturbed close to the ore, deposits. This evidence should be thought to have been mainly caused by repetition of post-mineralization alterations.