By: Hiroyuki Ii, Faculty of Systems Engineering, Wakayama University, Japan. Visiting professor at the University of Waterloo in 2001-2002.
In A.D. 674 the first silver was produced from Tsushima Island between Kyushu Island and Korea Peninsula which is on the route connecting Korea and Japan (Fig. 1). The discovery depended on new smelter technology from China and Korea to extract silver from galena. Later a Japanese emperor interested in spreading Buddhism directed efforts to find gold, copper and mercury. Mining specialists from China and Korea worked on the project to find these minerals.
Figure 1: Map illustrating the position of Japanese gold deposits, mining areas and places mentioned in the text.
The first gold from Japanese placers was discovered in A.D.749 at the east of Tohoku area (Fig. 1). The discovery of this placer deposit helped to finance the construction of the famous huge statue of Buddha in Todaiji temple. The Buddha, made of gold-plated copper, was built in A.D.752 just after the discovery of the placer gold. It is a site that is frequented today by many foreign travelers.
Production of placer gold from this district continued until about 1300 and the total gold recovered reached an estimated 30 tonnes. In the Tyusonji religious complex the Konjikido wooden temple covered with gold was built in 1126 at the centre of a placer gold production area. The "Gold Island" story in the travels of Marco Polo is derived from Konjikido whose name means "golden-colored temple" in Japanese.
Granite is distributed in this area and some quartz veins can be found in the Paleozoic and Mesozoic sedimentary rocks surrounding and within the granite. This type of quartz vein which includes mesothermal gold is formed at 1 km to 4 km depth and 200º to 300ºC. The first placer gold discoveries in Japan were derived from mesothermal gold deposits. This is a similar setting to Precambrian gold deposits found in Timmins and Val DÕOr districts in eastern Canada where coarse gold grains are found in quartz veins associated with pyrite and arsenopyrite.
Most Japanese gold and silver ores are today extracted from epithermal deposits originally formed less than 1 km in depth and at temperatures of between 100 and 200ºC.
However, in the past, these deposits were not utilised. It was difficult to smelt metal from these ores because the gold grains are very fine and the ore contains a lot of sulphide minerals. In addition placer gold is not usually derived from such epithermal deposits because of the fine grain size.
The development of mined epithermal deposits started in 1550. Before the Tokugawa period (1603 ~1867), total gold production reached 230 tonnes. The Sado mine which closed in 1989 had a total output of 15.3 million tonnes of crude ore, containing 80 tonnes of gold and 2,330 tonnes of silver. It was the second largest gold mine in Japan. The mine was discovered in 1596 and supported the Tokugawa shogunate. The gold was used to supply currency for over 250 years. During the Tokugawa period alone, gold production in Japan was about 100 tonnes.
Large scale production of copper, lead, zinc, silver and gold extracted from Kuroko deposits or epithermal ores was made possible by European and American technological innovations. These involved water pumps, elevators, dynamite and new smelting methods and commenced in the Meiji period (1868~).
A Kuroko deposit is a kind of sediment formed on the sea floor accompanied with acidic volcanic activity. The deposit is composed of black ore (a fine-grained aggregate of barite, sphalerite and galena), yellow ore (an aggregate of chalcopyrite and pyrite) and a gypsum. Unfortunately today all Kuroko mines are closed. Until recently Kuroko deposits had only been discovered in Japan. The total production of copper, zinc, lead and silver from these deposits was very high. One of the biggest Kuroko mines, Kosaka mine, produced 12 million tonnes of crude ore with the following metal content (Cu -1.8%, Pb - 1.8%, Zn - 4.9%, Au - 0.8g/t, Ag - 93g/t).
Gold production from the Meiji period until today has reached 1,200 tonnes, with most of the gold coming from epithermal deposits. As mentioned above, the gold grains in many epithermal deposits are very minute, requiring a cyanide process for extraction, as well as the complex methods used to extract gold from a silica ore. Recently the Japanese government has tried to find more epithermal gold deposits. As a result, the Hishikari mine was discovered in 1981. This has an estimated total gold of 250 tonnes with an average grade 80 g/t. Although the Hishikari mine possesses a high-grade ore, the very fine grain-size of the gold (an average of 0.01mm) would have prevented processing in the past. After this discovery, similar gold veins were noticed in other districts of Japan.
Epithermal gold and silver deposits
Epithermal gold and silver deposits are formed by volcanic activity accompanying plate subduction. In Japan there are a many volcanic and hydrothermally active areas caused by plate subduction. Most epithermal gold deposits were formed by hot springs precipitating gold and ores are being formed in this way in Japan today.
Figure 2: Electrum grains (lightest color) within acanthite (light color) layer in silver black band composed of chlorite, pyrite, chalcopyrite, sphalerite and galena are 1 mm in diameter.
Epithermal ores fall into several groups. One of these is the Adularia-sericite type gold deposits, also known as Silver black. Many epithermal gold and silver ores are found in veins composed of quartz, clay minerals, adularia and calcite. They have a rhythmically layered structure and are found in most Japanese gold mines. Sometimes the layered ore includes "silver black", a black coloured band of mineral aggregates composed of electrum (gold and silver alloy), sulphides of silver, chalcopyrite, galena, sphalerite, tetrahedrite and pyrite and clay minerals. Gold and silver concentrations in silver black are quite high. Gold and silver values in the ore are strongly dependent on the quality and quantity of the silver black. Generally the silver content is over ten times higher than gold. Mineral composition in the silver black is very changeable. Figure 2 shows typical silver black from the No.2 veins of Seikoshi mine in central Japan and Figure 3 shows high-grade ore (Au content 1000 g/t) from Keisen No.3 vein 55 ml in Hishikari mine. Gangue mineral crystals found in cavities or "vugs" are often found and the Kushikino mine in Kyushu is famous for beautiful twinned calcite crystals (Fig. 4).
Another epithermal gold deposit is the so-called "Nansatsu-type" gold deposit. This is a massive rock, heavily silicified by acid hydrothermal solutions. Most of the elements in the original rock have been leached with the exception of silica. The original rocks were andesite lavas, tuffs, breccias and tuffaceous shales. Some silicified rocks contain 98 % or more of SiO2 which is mined for pure silica used to make glass and ceramics.
Figure 3: The paired thin brown dark bands in a quartz and adularia mixed layer are very small grain-sized aggregates of electrum from Keisen No.3 vein 55 ml in Hishikari mine.
Figure 4: Twinned calcite crystals from the Kushikino mine in Kyushu.
Some silicate rocks in the south of the Satsuma peninsula in southern Japan contain gold. This is called Nansatsu-type gold ore in Japan and contains native gold, pyrite, enargite, covellite and native sulphur with porous quartz. Figure 5 shows typical gold ore from the Akeshi Mine. The term Nansatsu means "south of Satsuma" in Japanese.
Figure 5: Gold ore from the Akeshi Mine. In the upper half of the sample, large vugs are filled with sulphur and the silicified rock is indicated by the light colour. In the lower half of sample, there is no sulphur and the rock is very porous. Covellite (a copper sulphide) or pyrite (iron sulphide) is concentrated at the boundary between two zones.
Gold is sometimes concentrated at the top of epithermal veins of copper, lead and zinc. Old Japanese mines started with gold production and mined copper, zinc or lead later. There were a lot of copper mines in Tohoku area, north of Honsyu Island and some copper mines contain gold at the surface. This gold ore is composed of chlorite, hematite and quartz. This is named "Narumi gold ore" for its color since "Narumi" means the mixing of red and green colors and is derived from Japanese traditional dyeing. Although visible gold in Narumi gold ore is very rare, Narumi gold ore in the San-ei mine in Yamagata prefecture recorded ca. 100 g/t of gold and visible (but minute) gold grains were noted by the author in the waste rock (Fig. 6).
Figure 6: Microscopic view of Narumi gold ore with native gold and chlorite in quartz from San-ei mine. Lateral width is 0.4 mm.