Abstract
The Asahi mine is located in Kusamoto, Yamakuni Village, in the northwestern part of Oita Prefecture. The ore deposits are narrow epithermal fissure-filling veins related to the eruption of Tertiary apo-andesite. Veins were formed by the successive deposition of coarse-grained ores, carbonate-minerals, fine-grained rich ores and barren ores. The mineralization is characterized by the complex association of sulphide minerals, particularly, argentiferous sulphantimonides. Examination under the microscope makes it possible to recognize two main stages of mineralization in the rich ores :
1st-Sulphide stage. This stage consists mainly of pyrite, sphalerite, chalcopyrite, galena and argentite which were formed together with some polybasite and pyrargyrite.
2nd-Sulphosalt stage. Though some pyrite and chalcopyrite are deposited early in this stage, abundant silver-bearing minerals such as pyrargyrite, tetrahedrite and miargyrite were formed in the later stage.
In each stage, the following sequences of mineralization are noted: (1) sulphides→sulphantimonides and (2) iron→zinc→(copper, lead)→silver→antimony.
From the observation of 520 gold particles on polished sections it is concluded that there are three stages of gold-mineralization in the formation of these gold deposits:
The earliest gold, associated with galena and chalcopyrite, occurs as rounded grains which include more than half of the gold particles. The next stage is represented by the deposition of gold of network-type replacing argentite, and by gold veinlets penetrating polybasite. The last stage is shown as minute inclusions in tetrahedrite and pyrargyrite, comprising only 14 percent of all gold particles. Most of the gold was pre-cipitated in the relatively earlier stage, and deposition of silver was concentrated in the last stage of mineralization.
