Geology and Ore Deposits of the Yatani Mine, with Special Reference to the Gold and Silver Veins

Abstract

The Yatani mine is located in the southwestern part of the Yonezawa Basin, Yamagata Prefecture, which is known as the so-called "green tuff" area.
The geology of this area composed mainly of the Miocene pyroclastic and sedimentary rocks. The stratigraphy is in ascending order as follows : the Koarasawa formation, the Karasugawa formation the Yatani formation and the Otoge formation.
The ore deposits in the Yatani formation (acidic pyroclastic rocks and black mudstone) are of epithermal fissure-filling vein type. They may be classified into two types by their mineral constituents, Pb-Zn veins, and Au-Ag veins.
The Pb-Zn veins consist of galena, sphalerite, pyrite, small amounts of chalcopyrite, native gold, argentite and Mn-minerals. They have a general strike of E-W.
The Au-Ag veins consist of native gold, argentite, chalcopyrite, quartz, johannsenite, rhodochrosite, rhodonite, calcite, small amounts of pyrite, sphalerite, galena, adularia and chlorite. They have a general strike of NE.
The writer described mainly the occurrences of the Au-Ag veins.