Hydrothermal Dolomite-rocks Associated with Hydrothermal-Metasomatic Deposits of the Inner Zone of Northeastern Japan

Abstract

In a previous paper, the writer described the geology and ore deposits, with special references to the, paragenetic relation of ores and altered rocks in some iron, lead-zinc and fluorspar deposits of hydrothermal origin, of northeastern Japan, for the purposes of considering the problems of hydrotherma dolomitization of limestone and the alteration of skarn. In this article, the petrography, chemica analyses, spectrographic analyses, X-ray examinations and differential thermal analyses of limestonee and the dolomitized equivalents in these deposits are given. The results obtained from these studie are summarised as follows:
1) The dolomite-rocks in these deposits are similar to those of hydrothermal origin in foreign countries in respect to megascopic features and physical and chemical characteristics; in some respects, however, distinct differences are recognized, i, e., a) the original rock of hydrothermal dolomite-rocks in these deposits is thermally metamorphosed crystalline limestone; b) the dolomite-rocks are rich in variations of rock types, for instance, there is a highly porous dolomite-rock whose maximum porosity amounts to 36 per cent; dense "marble"-like dolomite-rock with equigranular texture is also found.
2) During dolomitization, somewhat later than the conversion of calcite to dolomite, the formation of apatite in the Akatani mine and the Tide mine, and the formation of Mg(?)-chlorite and chrysotilen in the Sen-nin mine have taken place, together with the introduction of silica.
3) Chemical changes during dolomitization are additions of MgO, FeO, Fe₂O₃, CO₂ and MnO and the. leaching of CaO, AL₂O₃ and SiO₂ in the Akatani mine and the Iide mine. But, in the Sen-nin mine, the later addition of SiO₂ is always marked.
4) In accordance with the changes of major components, spectographic analyses show the additions: of Be and F, and the decreases of Ba, Pb, Sr, Ag, Na and Ti.
5) The dolomitization of limestone wallrock in these deposits is considered to be volume-for-volume replacement, judging from the relict structure of dolomitized limestone. The increase of porosity in the course of dolomitization is, therefore, to be regarded as a result of excessive leaching of lime carbonate by the dolomitizing solution; the pores are "solution cavities".
6) The data of chemical analyses and DTA curves show that the dolomite-rocks in these deposits. consist. largely of low-iron dolomite, but it is noteworthy that, in the Igashima mine, white creamcolored dolomite-rock is composed of ferroan dolomite (para-ankerite).