Argillaceous and Zeolitic Alteration Zones Surrounding Kuroko (Black Ore) Deposits in Odate District of Akita Prefecture

Abstract

In the Odate district, typical argillaceous and zeolitic alteration zones develop in the Miocene marine acidic tuffs surrounding Kuroko deposits (Pb-Zn-Cu sulphide ore) which were formed on the sea bottom of middle Miocene age by submarine exhalative activities. The ore deposits are enclosed by the sericite-chlorite zone which changes upward and laterally to the montmorillonite zone. These alteration zones were formed beneath the sea bottom by reaction of acidic tuffs and tuffaceous sediments with hydrothermal solution as the tuffs deposited. The zeolitic alteration zones extend distant from the ore deposits, and they are subdivided into the clinoptilolite-mordenite zone and the analcime-calcite zone. The letter was formed by reaction of acidic tuffs with interstitial solution which was enriched in Na⁺ and Ca⁺ migrating from the argillaceous alteration zones.
Native sulphur was discovered in the sericitized pumice tuff of the marine Shakanai Formation about 50 meters below the No.7 ore body of the Shakanai Mine.