The giant "Shinano vein" was discovered in 1986, at the southeastern-most part of the Toyoha vein swarm, by systematic underground diamong drillings for a target extracted from the polyascendant mineralization model. Its minable ore reserve is estimated to be approximately 4.5 million tons, with an average grade of 270 g/t silver, 0.7% copper, 2% lead and 13% zinc.
Subsequent exploration by crosscuts and drifts has revealed that the complicated occurrence of ore minerals is attributed to overlapped mineralization of several stages. The ores are classified into four types; pyrite ore, copper ore, zinc ore, and banded lead-zinc ore, each of which is characterized by different mineral assemblages and contents of minor element such as silver, indium, tin, cobalt, nickel, galium, etc.
A symmetrical zoning of ore types is recognized in the successive Soya, Izumo and Shinano veins. The core of the zoning is at the lowermost part of the Izumo vein, where pyrite ore is dominat. This suggests that the ore fluids which have been fed to the Izumo vein have flowed laterally toward both side of Soya and Shinano veins.
Active geothermal system is present in the Toyoha mining area, where some deep drills encountered rock temperatures as high as 230°C. Boiling thermal fluids discharge from the drill holes through the Shinano vein. In addition to the indicated late Pliocene to Pleistocene age of the ore formation, the chemical and isotopic similarities between present-day geothermal fluids in the Shinano vein and the ore fluids in fluid inclusions indicate that the current hydrothermal fluid is survived ore solution.
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