Zonal Arrangement in the Skarn of the Nakayama Deposit in the Nakatatsu Zinc Mine, with Special Reference to the Occurrence and Nature of Sphalerite

Abstract

The Nakatatsu mine, Fukui Prefecture, Japan, is one of the high temperature replacement ore deposits found in limestones of Palaeozoic age. It is one of most productive mines of lead and zinc in Japan.
In this paper some descriptions of the zonal arrangement of skarn, and occurrence and nature of sphalerite are given.
In the Nakatatsu mining district the limestone beds have many fine bands of shale and are intruded by many porphyrite-dykes.
The ore deposition began with skarnization of the limestone bed; in shale and porphyrites the skarnization did not take place to an appreciable extent. Well-defined, zoned skarns occur characteristically near the limestone side as follows:
Limestone; wollastonite-diopside skarn; diopside-garnet skarn; hedenbergite skarn.
Judging from field relations of the occurrence of skarn minerals, the formation of skarn may be divided into the following two stages,
1, Stage of the formation of wollastonite, diopside, and hedenbergite.
2, Stage of the formation of garnet.
Another characteristic feature in skarns of limestone origin is its beautifully banded structure which represents ghost textures of the host rock replaced by the skarns. The banded structure of these skarns is well shown by the arrangement of newly formed lime-silicate minerals.
After the skarnization, quartz-calcite-sulphide mineralization took place in the skarn masses.
From geological observations of the deposit, it is considered that there were no large openings to control sulphide mineralization before sulphide mineralization started and after skarnization. Therefore the path of sulphide-mineralizing solutions was controlled by the skarn rock itself. A part of the hedenbergite skarn zone became the main path of ascending solutions. Sulphide mineralization has occurred through small cracks in the skarn zones, and took place throughout the skarn mass.
The sulphide minerals of the deposit include sphalerite, galena, pyrrhotite and chalcopyrite.
Sphalerite, which is the main sulphide mineral, was examined, by X-ray powder, optic, and spectrographic methods. Its lattice constant was calculated and its Fe-content was estimated.
From the results of these studies, it was found that the Fe-content of sphalerite varies from 3.0% (lattice constant is 5, 413Å) to 9. 5% (5, 423Å). The Fe-content of the sphalerite becomes gradually higher from the outer zone sphalerite (wollastonite diopside zone) to the central zone sphalerite (hedenbergite zone). This variation of Fe-content in sphalerite corresponds well with that of the skarn zones.
The relation between skarnization and sulphide mineralization as described above for the Nakatatsu deposits is very interesting.