Uranium Deposit at Nakamaruke, Sekigawa-mura, Iwafune-gun, Niigata Prefecture

Abstract

The uranium deposit at Nakamaruke, Sekigawa-mura, Iwafune-gun, Niigata Prefecture, was discovered in 1960 and surveyed by the authors from 1960 to 1964, as a part of regional prospecting for uranium in Neogene Tertiary sediments accompanied with the so-called "Green Tuff", with is widely distributed in the border district between Niigata and Yamagata Prefectures.
The tabular-form uranium deposits of Japan in Neogene Tertiary sediments, the so-called "Ningyotoge type" deposits, are in general considered to have been controlled by the basement structures and are epigenetic deposits related to such an environment where groundwater circulates or stagnates.
The Nakamaruke deposit, one of these tabular-form deposits in Neogene Tertiary sediments, differs to a considerable extent from the "Ningyo-toge type" deposits in mode of occurrence, paragenesis of minerals, and other features, and can be said to be a peculiar type deposit newly found in Japan.
(1) In the Nakamaruke area, the Kitaoguni formation (lower Miocene), the Tsugawa formation (middle Miocene), and the Nanatani formation (upper Miocene) rest in ascending order unconformably upon the basement composed mainly of granite, and these Neogene Tertiary sediments show a monoclinic structure with a relatively gentle dip.
(2) Uranium is concentrated in arkose just below the bentonite bed in the upper part of the marine Tsugawa formation directly overlying the basement granite. This uranium deposit might have a close genefical relation to apatite contained in the arkose.
(3) Such occurrences of uranium associated with apatite in Neogene Tertiary sediments are found in some places within 40 km from north to south around Nakamaruke, and deposits of this type may be expected to exist widely throughout this district, as well as the possibility of existence of the "Ningyo-toge type" deposits.