石狩炭田地域に産する菱鉄鉱質岩の成因(1)一前駆物質の形成過程

摘要

Thin beds, lenses, and nodules of sideritic rocks occur abundantly in fresh water sediments of the Ishikari coalfield. The sideritic rocks were derived from an iron-rich hydroxide precursor that underwent early diagenesis. In this paper, the origin of the sideritic rock precursor is discussed on the basis of the whole-rock chemical compositions of the rocks. The majority of the elements analyzed, except for manganese and phosphorus, show a negative correlation with the Fe_2O_3 content and a positive correlation with the SiO_2 content. The sideritic rocks are chemically composed of two substances as a first approximation, namely, clayey or sandy detritus and a silicate-free material composed essentially of iron with a small amount of manganese and phosphorus. The iron and manganese in the sideritic rocks were derived from bivalent ions dissolved in the river water. When river water flows in a plain field, dissolved iron and manganese ions are oxidized due to extensive exposure to the atmosphere, whereby ferric iron hydroxides and hydrates of manganese oxide precipitate at the bottom of the river or lake. Iron phosphate (FePO_4) co-precipitates with iron hydroxide in this process. The iron of the sideritic rocks in the Ishikari coalfield does not originate from laterite but from ferrous ions dissolved in river water.