The Yonaihata mine is mining several massive bodies of gypsum with sphalerite, galena and chalcopyrite in a Miocene mudstone. The bodies are thought to belong to a type of Black Ore in Japan. They also contain, besides gypsum and sulphide minerals, small amounts of fluorite, calcite, barite and anhydrite. The gypsum bodies consist of massive or disseminated alabaster accompanied by veinlets of fibrous gypsum at their margin. The alabaster replaces calcite, fluorite, barite and anhydrite, and often penetrates the sulphide minerals in the form of veinlets. It suggests that the gypsum was formed by epithermal solutions at the latest stage of the mineralization. The ore solutions have replaced black mudstone more than tuffaceous mudstone which is inserted in this black mudstone. In this case, the tuffaceous mudstone plays the role of cap rock for ore bodies. The mudstones surrounding ore bodies are widely changed into clays by hydrothermal solutions. In the clay zone, an increase of magnesia and a decrease of silica compared with the original mudstones are conspicuous. The occurrence of magnesium chlorite and montmorillonite in the clay zone is ascertained from the results of X-ray powder photographs and differential thermal analyses.
The writers would like to emphasize that the deposition of gypsum is due to the breakdown of the chemical equilibrium by a change of physical conditions in the ascending solution as shown below.
For example,
Na₂SO₄+Ca(HCO₃)₂+H₂O CaSO₄⋅2H₂O+Na₂CO₃+CO₂
(gypsum)
In the formula, the reaction will progress toward the formation of gypsum, if the pressure of the environment decreases to allow the escape of CO₂ gas. By this consideration, it may be shown that ore bodies are formed only within the limits of a geological horizon in this mine.