Early Miocene metallogenic event formed the Bosawa low‐sulfidation epithermal gold deposit, Northeast Japan arc

Abstract

There are a number of epithermal Au‐Ag deposits in the Tohoku region of the Northeast Japan arc; however, these deposits have undergone limited exploration for over the past seven decades. This reconnaissance research study of the Bosawa deposit in Akita Prefecture was conducted to support future gold exploration in the Tohoku region. The Bosawa deposit is a vein‐type Au‐Ag deposit, with a production of 42,486 t of ore at 7.01 g/t Au and 14.9 g/t Ag recorded for the period from 1936 to 1957; mining started in the 18th century. The deposit consists of the Ohgiri vein system, which is hosted in felsic volcanic rocks of the Early Miocene Katsurabuchi Formation. The study included geological prospecting, ore description, K‐Ar dating, and fire assay analyses. The deposit is characterized by (a) strongly silicified host lapilli tuff fringed by pyrite‐adularia‐illite alteration; (b) presence of sulfide‐poor banded quartz veins with adularia; (c) abundance of bladed quartz in veins, pseudomorphs after bladed calcite; (d) presence of colloform opal texture in veins; and (e) simple ore mineralogy consisting of gold and argentite. These characteristics are typical of low‐sulfidation epithermal Au deposits. The K‐Ar age of adularia (19.33 ± 0.45 Ma) indicates that mineralization at Bosawa is distinctly older than the ages of other epithermal deposits in the Tohoku region but similar to the age of the initial mineralization episode of the Sado and Tsurushi deposits (24.4–22.1 Ma), Japan's second largest gold producer. This mineralization age coincides with the start of rifting of the Northeast Japan arc and separation from the Asian continent. As the felsic rocks related to this rifting event are widespread in the arc, and some are associated with epithermal gold deposits, the confirmation of typical low‐sulfidation style of mineralization in the Early Miocene provides evidence for a previously unrecognized metallogenic event in the Tohoku region.