Bournonite from the Janggun mine, Republic of Korea - Contributions to the knowledge of ore-forming minerals in the janggun lead-zinc-silver ores. 1

In the Janggun lead-zinc-silver ores, bournonite occurs as anhedral grains up to 2mm long, closely associated with galena, minerals of the tetrahedrite-freibergite series, boulangerite, sphalerite, pyrite and rhodochrosite. The bournonite, though minor or trace in amount, tends to occur in the peripheries of the ore pipe. In reflected light, it is light greyish white in colour with greenish tints and exhibits weak bireflectance and distinct anisotropism without any internal reflections. Occasionally, lamellar twinning on {110} plane may be observed. Reflectance: Rmax=37.5, Rmin=33.8 percent at a light with wavelength of 580 nm, and VHN: 175-198kg/mm2 at a 50-g load. The chemical composition on the average from 15 spot analyses by electron microprobe for 5 grains is, Pb 43.3, Sb 24.4, Zn 0.0, Cu 12.9, S 19.7, sum 100.3 (all in weight percent); the corresponding empirical formula on the basis of S=3 is, Cu1.00 Pb1.03 Sb0.99 S3, which is close to the ideal formula CuPbSbS3. The strongest reflections on the X-ray diffraction patterns are; 3.90 A (8) (002), 2.97 A (4) (220), 2.73 A (10) (130), 2.67 A (4) (212), 2.59 A (5) (310), 1.766 A (6) (332), the patterns are identical with those of literature, and in harmony with the space group _??_-Pn 21m. This mineral is considered to have been formed at the latest stage of hydrothermal lead-zinc-silver mineralization. At the beginning of this paper, geologic environments of the ore deposits are briefly described.