Occurrence and genesis of melanterite from the Moji Mine, Kitakyushu City, Southwest Japan

Abstract

The genesis of melanterite (FeSO₄·7H₂O) from an abandoned iron mine was investigated. The ferruginous and/or sulfate mineral zones observed on a wall at the alit was as follows: 1) on the upper part of the wall, the iron sulfide minerals zone with melanterite, 2) the goethite zone overlying skarn minerals, and 3) the goethite and gypsum zone with a small amount of malachite overlying limestone. The pH of the seepage water increases from zone 1) to zone 3). The distribution of ferruginous minerals was controlled mainly by the pH of the seepage water and partly by the rate of the water supplied.
     In the iron sulfide minerals zone, sulfate and Fe(II) ions are derived from the decomposition of the sulfide minerals, and dissolved to acidic water. Melanterite precipitated in place from the acidic water by evaporation. At the lower part of the wall, the pH of seepage water increased by the reaction of the acidic water with Ca bearing minerals, hence the dissolved Fe(II) ion was oxidized rapidly and precipitated as goethite. The rate of oxidation from Fe(II) to Fe(III) also depended on the residence time of the seepage water, and, under acidic conditions, a thin layer of goethite deposited in points where insufficient seepage water was supplied from the upper zone. From the resultant iron free water, gypsum precipitated partly at the bottom of the wall by evaporation.