Gold and silver ores from the Ezuri Kuroko ore deposits, Akita Prefecture, Japan

Abstract

The Ezuri Kuroko deposits are located approximately at 140°36'of east longitude and 40°15' of north latitude in the Hokuroku district of Akita Prefecture. The ore deposits are hosted by basaltic tuff and dacitic rocks of the Miocene age, and they are composed mainly of stratiform black ores.
   In this paper, the features of Au- and Ag-bearing ores and minerals are described, and the environment of the mineralization is estimated from the FeS content of sphalerite and the data of fluid inclusions in sphalerite.
   Gold- and/or silver-bearing minerals identified by microscopic examination and electron microprobe analysis are electrum, stromeyerite, pearceite, mckinstryite, argyrodite and fahlore (up to 7.8 atomic % Ag). These minerals are particularly found in fine-grained massive black ore. Most of electrum contains 30-40 atomic % Ag, and Ag distribution of an individual grain is comparatively homogeneous. Silver content of black ore (20-1, 260 ppm) is dependent on the amount of fahlore as well as of other silver minerals.
   The ranges of homogenization temperatures of fluid inclusions in sphalerite are 236°-283°C (breccia-like black ore), 210°-279°C (massive black ore) and 165°-230°C (coarse-grained sphalerite filling cavities in massive black ore), respectively. The FeS content of sphalerite coexisting with electrum is less than 0.2 mole%. Taking account of the effect of pressure, it is deduced from these data that Au- and Ag-bearing ores were formed in a range of ca. 240°-290°C under the condition of a high activity of sulfur. Furthermore, it appears that gold was precipitated from Au(HS)₂⁻ complex mainly owing to cooling and a decrease in H₂S concentration of ore-forming fluids caused by mixing with seawater and/or by precipitation of metal sulfides.