Geology of the Oppu mining districts, Aomori Prefecture, with special reference to the structural control of mineral deposits

Abstract

The mineral deposits of the Oppu mine, Aomori Prefecture, consist of epithermal fissure-filling chalcopyrite-sphalerite-galena veins in the Neogene sediments. On the basis of lithology and characteristics of sedimentation, stratigraphy of the Oppu mining districts is divided into the following formations: Quaternary Terrace deposits and Talus deposits Oppudake formation Tertiary Miocene Oppu formation Maenosawa formation Koyunosawa formation Yunosawa formation Pre-Tertiary Basement complex
The country rocks of the mineral deposits are Oppu formation (andesitic-dacitic lapilli tuff, coarse tuff and siltstone) and Oppudake formation (salic pumiceous tuff). The geological structures are characterized by large domes which are sometimes accompanied with faults and foldings. Faults are common in the area studied and can be classified into the following four major systems-NE-SW system, WNW-ESE system, ENE-WSW system and N-S system. The distribution of mineral deposits and ore shoots have close relations with the some geological structures. For instance, main mineral deposits are located usually in the combination zone of normal and low dipping reverse faults in this mining area. Especially, the veins are controlled by the sedimentary structures and thrust faults (Figure 5). These structural conditions which control the ore shoots have been stressed in this paper.