Miharaite in bornite-rich copper ore from the Ulsan mine, Republic of Korea.

Abstract

Miharaite has been found from sulphide-sulphosalt ore rich in bornite occurring as a stringer vein, which cuts calcite-marble in diamond drill cores at the Ulsan mine, Republic of Korea by the present authors. The Ulsan miharaite occurs as microscopic grains, up to 500 μm long and 150 μm wide, but grains as large as 100 μm in maximum dimension are rather common. It occurs in bornite closely associated with wittichenite. In reflected light, it shows very weak bireflectance with reflection colour from pale-grey to white with greyish tinges. Between crossed polars, it shows moderate anisotropism, giving colour effects from greyish blue to yellowish brown. The optical properties of miharaite are quite similar to those of the associated wittichenite, however, the discrimination of these two minerals may readily be made by means of etch test; i.e., HNO3 (1:1) etches wittichenite slowly, gradually staining to brown colour, but does not miharaite at all. The reflectance, Rmax and Rmin (percent) in air are; λo=480 nm:32.0 and 31.3, 540 nm:33.0 and 32.2, 580 nm:33.5 and 32.2 and 660 nm: 35.1 and 32.4, respectively. Vickers hardness number (VHN): 195-230 kg/mm2 at a 25-g load. The average chemical composition of thirteen spot analyses by electron microprobe gives: Cu 27.62, Fe 6.05, Pb 23.10, Bi 23.54, S 20.66, Total 100.97 (all in weight percent), corresponding to the formular Cu4.00 Fe1.00Pb1.03Bi1.04S5.94 on the basis of total atoms =13, being very close to the ideal formula, Cu4FePbBiS6. The strongest reflections in X-ray diffraction patterns are 3.03 A (vs) (211), 3.00 A (s) (040), 2.19 A (w) (250, 411, 241) and 1.934 A (m) (431, 002). The Ulsan miharaite is considered to have been formed during the later stage of skarn formation by copper-rich polymetallic mineralization under relatively lower temperature, which has followed the main iron-tungsten mineralization.