Depth of Formation of Some Subvolcanic Hydrotbermal Veins

Abstract

Depth of formation is evaluated on some subvolcanic hydrothermal veins in Akita Prefecture, Northeastern Japan mainly based on isopach maps of Tertiary formations in the area questioned. There are two opinions on the age of metallogenesis in the province ; late Nishikurozawa stage to very early Onnagawa stage (A) or late Funagawa stage to very early. Tentokuji stage (B), both belonging to Neogene. Estimated depth of formation is about 200 meters for the uppermost parts of the veins developed in the Osarizawa, Ani, Miyatamata and Tazawa mines, and about 250-300 meters for those in the Akarimata and Tatemata mines, if we assume that those veins were formed during the A age. The depth is, however, evaluated as several to ten hundreds meters, if we choose the B age instead of the A. In the district near the Tazawa mine, no sedimentation occurred during the Funakawa and Tentokuji stages. Then, the depth of formation of the veins in the mine is clearly defined as 150-300 meters regardless of its age of formation is A or B.