Genesis of the limonite deposits of the Akita iron mine

Abstract

There are many field evidences suggesting that the limonite deposits of the Akita iron mine were formed by sedimentation of hydrous ferric oxides separated from iron-containing spring waters. The ferruginous precipitates adsorbed various substances from the spring waters, being amorphous when formed. We have, however, no information on the environmental conditions of the precipitation and the mode of crystallization of the precipitates. It is of particular importance from a geochemical point of view to define the temperature and the pH of the solution from which the precipitates formed, and to elucidate the processes of crystallization after the sedimentation of the precipitates.
The mineralogical and chemical features of the limonite ores are much suggestive of the genesis of the deposits. The ores, composed mainly of goethite and jarosite, have low crystallinity, which may provide an evidence that they were formed at ordinary temperatures. The chemical compositions of the ores are shown in Table 1. Considering the results of investigations on aging of hydrous ferric oxides and on coprecipitation of anions with insoluble hydrous oxides, the present author concludes that the precipitation and crystallization of hydrous ferric oxides took place at ordinary temperatures and at low pH, presumably at pH 2 to 3.
In consequence, we may summarize the formation of the limonite deposits as follows : acidic spring waters, containing ferrous iron, phosphates and sulfates, precipitated hydrous ferric oxides during their flow on the ground surface. A certain microbiological process accelerated the oxidation of ferrous iron. The accumulation and the subsequent crystallization of the precipitates containing phosphates and sulfates have formed finally the limonite deposits composed of goethite, jarosite and some iron phosphate minerals.