Gold-bearing Ores and Native Gold found in the Shakanai Kurokô Deposits in Japan

Abstract

The Shakanai mine in the northern part of Ôdate district, Akita Prefecture, is one of the largest Kurokô mines in Japan.
Recently, the character of gold and silver in black ores (Kurokô) has become one of practical problems in ore-dressing and refining. Nevertheless, the studies of native gold in these ores are scarce, if at all, less than those of vein type ores. By microscopic examination, the authors discovered native gold in the Shakanai ores. Thus, they studied the properties of native gold by using an electron probe microanalyser, a microphotometer and a microhardness tester.
The gold-bearing ores occur only within compact fine-grained black ores and, banded or colloform finegrained black ores. near vein-like ores rich in chalcopyrite.
Native gold is closely associated with galena. It's average grain size is 10 to 20 μ in diameter, though they are sometimes irregular-shaped. Some are dark yellow, while others are light yellow. The reflectivity ranges from high to low at wavelength 530 mμ. The etch test by CrO₃+HCl brings about a dark spotted texture or an appearance of sun-spot. The electron probe microanalyser examination revealed that the native gold is a impartial mixture of Au and Ag. The ratios, Au : Ag, however, grain by grain.
It is suggested that features of native gold in black ores may be useful for the interpretation of the varies process of black ore deposition.