The Chromite Deposits in the Nittoh Mine

A Study on the Chromite Deposits in the Kamuikotan Zone, Hokkaido

Abstract

In Hokkaido, there are many ultra-mafic igneous bodies along the two parallel tectonic zones which are named the Kamuikotan zone and the Hidaka zone respectively. These tectonic zones composing the geological back-bone of the island, are regarded as the product of Cretaceous orogenic movement which is called the "Hidaka Orogeny". It is believed that Hidaka zone has an orogenic deep core and the Kamuikotan zone is its westward overthrusting zone. Many characteristics of the ultra-mafic igneous rocks are common to both the tectonic zones. But contrasting features have also been observed in them. One of the most disparate features is that the ultra-mafic rocks of the Kamuikotan zone are almost wholly serpentinized, whereas they are always in the unaltered peridotitic state in the Hidaka zone.
It is in the southern part of the great serpentinite belt in the Kamuikotan zone which has long been well-known as a chromite field that the occurrence of lode ore of chromite has been reported.
The Nittoh mine is situated in the southern part of this chromite field.
The geology of the mining district is very complicated, having irregular dome or basin structure of the sedimentary rocks that are belived to be Pre-Cretaceous in age.
From the modes of occurrences, almost all of the chromite deposits are regarded to be deformed secondarily. The deformation is closely related to the secondary structure of the serpentinite, which is regarded to have been caused by the successive movements of the Hidaka orogeny. The serpentinite having primary structure is massive, and retains the characteristic texture of peridotite, even when the olivine is completely altered to serpentine. On the other hand, the deformed serpentinite is in many places foliated.
The primary form of the chromite deposits in the mining area, is plate-like, and is controlled by the joint system of the massive serpentinite. The secondary structures of the deposits are controlled by the block movement of the massive serpentinite or folding of the foliated serpentinites.
In the Nittoh mine, two types of deformation of the chromite deposit are observed.
In this paper, the writers describe the structural characters of each ore mass with special reference to their relations to the intrusive tectonics of the chromite deposits.
Further studies, especially concerning the properties of ores will be published in the near future.