Relation between Ores and Altered Rocks in so-called Contact Deposits of the Inner Zone of North Eastern Japan

Abstract

The problems of "Dolomitization and Ore Deposition" in the deposits of foreign countries, especially of the United States, have been discussed by many investigators. In our country, however, little attention has been paid to the hydrothermal dolomite-rock associated with metalliferous deposits.
In his research on the geology and ore deposits of the Akatani-Iide iron and lead-zinc field, Niigata Prefecture, the writer has been confronted with these problems. He was also able to study these problems during visits in 1952 to the Sennin iron mine, Iwate Prefecture, and to the Igashima fluorspar mine, Niigata Prefecture. The purposes of the papers are to present these topics and to point out their geologic significance. This paper describes the paragenetic relation of ores and altered rocks in each mine, and reaches the following conclusions:
1) These iron, lead-zinc, and fluorspar deposits under discussion are not contact-metasomatic but are hydrothermal-metasomatic deposits. The hydrothermal mineralizations that formed workable ores are thought to be intimately related to the acidic volcanism (liparite eruption and intrusion) of the middle Miocene; on the other hand, the contact-metasomatism is thought to be related to the acidic plutonism (granite intrusion) of the early Tertiary. Based on these views, the rocks in these deposits have undergone two distinctly separated periods of mineralization.
2) The iron deposits with specular hematite in our country have been long believed to be of the contact-metasomatic type. However, from the present-day views as mentioned above, such a genetic consideration must be wholly revised. The main hematite-mineralization in our country probably is included in the Neogene metallization that formed numerous epithermal deposits in the "Green Tuff Region".
3) In these deposits, the close association of ores with dolomitized limestone and/or "altered skarn" is very marked. For instance, in the Akatani mine some of the specular hematite orebodies are enveloped in dolomitized limestone. In a few places, however, the limestones in contact with the ores are definitely unaltered. The dolomitization of limestone involves a change in color, texture and porosity. The front of dolomitization in limestone is usually sharply defined, this tendency is especially conspicuous in dark-colored limestone.
4) In specular hematite deposits of the Akatani and the Sennin mine, the alteration of skarn is very intense over an extensive area, and no orebodies are found where the skarn is unaltered. In the Sennin mine, the selective metasomatism of "altered skarn" is always found.