Thermal Metamorphism of the Bedded Cupriferous Iron Sulfide Deposit at the Besshi Mine, Ehime Prefecture, Japan

Abstract

The Besshi deposit is a bedded cupriferous iron sulfide deposit in the Sanbagawa metamorphic terrain. The deposit is composed of the massive ores consisting mainly of sulfide minerals, and banded ores which are quartz-chlorite schists with a dissemination or banding of sulfide and oxide minerals.
In the upper levels of the deposit, the massive ore consists mainly of pyrite containing a small amount of chalcopyrite and sphalerite and occasionally minor bornite, hematite and magnetite. The banded texture parallel to the schistosity of the country rocks is often observed in the massive ore. This texture is due to a size variation of pyrite grains and a layered distribution of associated minerals. Pyrite is euhedral or subhedral, and contains about 0.14wt.% cobalt. The FeS content of sphalerite is 0.48-5.2 mole%. The banded ore consists of pyrite, chalcopyritc and a small amount of sphalerite, hematite, magnetite and rutile.
Between 17-22 levels, the massive ore has essentially the same constituents and texture as the upper levels, but it is accompanied by a small amount of monoclinic pyrrhotite. The pyrrhotite occurs along faults or fissures cutting the ore body, along the footwall and hanging wall, and at the peripheral part of the ore body. Pyrite contains 0.07-0.7wt.% cobalt. The FeS content of sphalerite is 13.8-18.3 mole%. The banded ore consists of pyrite, chalcopyrite, pyrrhotite and minor sphalerite, magnetite, ilmenite and rutile.
Below 23 level, the massive ore consists mainly of pyrrhotite, and contains chalcopyrite, sphalerite and pyrite and occasionally magnetite, cobaltite and mackinawite. Pyrite is often replaced by pyrrhotite or chalcopyrite, resulting in a small rounded shape or a "worm eaten" shape. Much of the pyrrhotite is of hexagonal type and. contains up to 0.31wt.% cobalt. The FeS content of sphalerite is 16.7-22.2 mole%. The banded ore consists of pyrrhotite, chalcopyrite and minor sphalerite, pyrite, magnetite, ilmenite and rutile. Biotite is occasionally observed in the banded ores and country rocks below 26 level.
Thus there is the zoning in mineral paragenesis with increasing depth. The ratio of pyrrhotite to pyrite, the FeS content of sphalerite and the Fe content of pyrrhotite increase downward. Pyrrhotite is regarded as a product of the reaction: pyrite→pyrrhotite+sulfur.
In the lower part of the deposit chalcopyrite might react with pyrrhotite or pyrite result-ing in "intermediate solid solution" of the Cu-Fe-S system with rising temperature and decreas-ing fugacity of S2. A schematic process can be described as follows:
pyrite
pyrrhotite
chalcopyrite
intermediate
solid solution
pyrrhotite
chalcopyrite
solid solution
mackinawite
clialcopyrite
(Temperature increasing)
(Temperature decreasing)
It is concluded that the reactions were probably caused by the thermal metamorphism by the Omogo acidic intrusives during the Ishizuchi igneous activity of Miocene or Pliocene age.