Geology and Ore Deposit of the Kômori Mining District, Japan

Abstract

The Kômori mine is located in Koyto prefecture, south-western Japan.
The Kômori mine is composed of pyrrhotite-copper deposits occurring in ultrabasic rocks, monthly producing about 8, 000 tons of crude ore containing 1.85% copper.
This region is geologically composed of Paleozoic formation, ultrabasic (including basic) rocks of Jurassic period and granite of Cretaceous period. The ultrabasic rocks consist of serpentinized peridotite. serpentine, pyroxenite and gabbro.
The ore deposits are classified into two types, i.e., the vein type and the impregnated type. The former occurs along the sheared zones in the serpentine near the contact with the gabbro or dioritic stocks, which intruded the serpentine. The impregnated type deposits exist in the above-mentioned gabbroic or dioritic stocks, especially in the marginal parts. It is noticeable that the impregnated deposits always occur close to the vein type deposits.
The granite is widely developed in the nothern part of this region, but it is inferred from the geologic. reconnaissance that the cupola of a cryptobatholithic granite exists underneath the deposits of the Kômori mine.
Ore minerals in the vein type deposit are pyrrhotite, chalcopyrite, sphalerite, cubanite, nickel-bearing mackinawite, cobalt-bearing pentlandite, molybdenite, star-shaped sphalerite, galena, native bismuth, bismuthinite, polybasite(?), arsenopyrite, magnetite, pyrite, etc., and gangue minerals are chlorite, talc, quartz and carbonate minerals; whereas ore minerals in the impregnated ores consist of pyrrhotite, chalcopyrite, sphalerite, ilmenite, etc., and gangue minerals are chlorite, quartz, sphene, talc, amphiboles, apatite, plagioclase and carbonate minerals.
The, pyrrhotites in the vein type deposit have usually strong magnetism, but the pyrrhotites in the impregnated type show two different degrees of magnetism, strong and weak.
The decrepitation method revealed that pyrrhotites in the vein ores contain only primary fluid inclusions which have decrepitation temperatures of over 350°C, whereas pyrrhotites in the impregnated ores. contain not only primary fluid inclusions with decrepitation temperatures of 300°-350°C but also secondary fluid inclusions which have decrepitation temperatures of 170°-200°C.
No difference in magnetic properties is recognized between the pyrrhotites with secondary inclusionss and those without.
With regard to the genesis of ore deposits of the Kômori mine, the writer believes that they were formed probably by the activities of granitic magma.