Hishikari Gold Mine (Isa)

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The Hishikari Mine is located in the northern part of Kagoshima Prefecture. It has produced 165.7 tons （as of the end of March, 2008） of gold since production started. Hishikari Mine boasts high-grade gold containing an average of 40 grams of gold per 1 ton of ore （the average grade of the world's major gold mines is approximately 5 grams）, and, to this day, it still produces 7.5 tons of gold per year. Although there were many metal mines throughout Japan in the past, they closed down one after another. Today, the Hishikari Mine is the only domestic metal mine being operated on a commercial scale.

Ore Deposits of the Hishikari Mine

The ore deposits of the Hishikari Mine are known as "epithermal vein-type deposits". In the process of earth movements, known as "plate tectonics", in which the pacific plate slides under the Japanese archipelago, magma is generated. This magma comes up through cracks in the earth's crust, and erupts as volcanoes on the surface of the earth. When groundwater or water content of magma （hot water） is introduced into cracks of the surface of the earth during volcanic activity, substances that dissolved in hot water cool down and form ore veins. The gold deposits of the Hishikari Mine were formed in this way. They are believed to have formed approximately one million years ago. The gold deposits are very new from a geological standpoint and are characterized by their accompanying hot spring water with a temperature of 65 degrees C .

The Hishikari mine is the only gold mine in Japan in operation today. In the country’s long history of mining, there has never been a more productive gold mine, and never a mine yielding better quality gold. The reason? It all has to do with volcanic hot springs, and geological mechanisms perhaps unique to Japan.

The Hishikari mine is located in this valley in northern Kagoshima Prefecture. The hot water seeping into mineshafts is supplied to the nearby hot spring spa.

Sado Kinzan was Japan’s biggest gold mine for almost 400 years, beginning in the early 1600s. Another major discovery was made in Hokkaido in 1915, and the result was the Konomai gold mine. But these and other deposits had basically given out by the 1980s, and the mines shut down one after the other. Today, gold is mined in Japan only at Hishikari in Kagoshima in southern Kyushu.

Hishikari is now one of the best gold mines in the world. The average grade is 40 grams of gold to one ton of ore. A profit can be made at 2 grams per ton, so the gold at Hishikari was certainly a major find. Since digging began in 1985, the mine has produced seven to ten tons of gold per year—165 tons over the last 23 years. That is more than twice the amount produced at Sado and Konomai combined, making Hishikari the most important in Japan’s long history of gold mining.

The question is: how much remains in the ground there? Geologists suggest a minimum of almost 150 tons of gold. All of the gold produced from other mines in Japan throughout history comes to around 1,000 tons, and if we add the approximately 300 tons of gold from Hishikari, you can see just how rich its veins are.

But why was such a big gold deposit discovered only in the 1980s? And, could there be similar deposits elsewhere, waiting to be discovered?

Scientists want answers to these types of questions, so they are examining the mine’s geological structure, and trying to learn how the gold was deposited there. One scientist, Dr. Watanabe Yasushi of the National Institute of Advanced Industrial Science and Technology (AIST), explains: “The gold at Sado and Konomai formed more than 10 million years ago, but the deposits at Hishikari are only about 1 million years old, which is relatively young geologically speaking. The rock above had still not been eroded, so the veins were not exposed on the surface. That is why they were discovered only recently.

“The type of deposits at Hishikari give us a clue to use to discover gold somewhere else. You see, in a volcanic part of the world like Japan, molten magma rises up, and that magma may contain gold and silver. They make a hydrothermal fluid in the magma. The fluid rises higher and boils, and then, as it cools, the minerals in it settle and form veins in the rock. At the same time, the hydrothermal fluid in the upper part of the veins reacts with the surrounding rocks, making them metamorphic. That metamorphic rock could be a marker indicating the presence of gold.”

There is another clue, one that is easier to recognize: the water at hot springs. Watanabe says that recent research has shown that, “in hot spring locations where the water contains a lot of naturally occurring chlorine, and the pH level is neutral—in other words, the water is neither acidic nor alkaline—there’s a good chance that there could be some hydrothermal gold and silver deposits deep underground.” Of course, the 65 °C water bubbling up near the Hishikari gold mine meets those conditions.

Japan is known worldwide for its magnificent hot springs, so it could be that many more gold deposits are still lying underground.

Japan Oil, Gas and Metals National Corporation (JOGMEG) has explored for gold using the results of this research. The company has not discovered any veins at a grade high enough to make a profit because of the high cost of extracting minerals in Japan. The rich Hishikari mine is still an exception, and more effective exploration and extraction techniques are needed.

The search continues. Deep under the ocean off Japan, hydrothermal deposits have been found over the last few years, and scientists keep trying to discover more about the location and structure of their mineral veins.

www.smm.co.jp/E/business/metal/hishikari.html
web-japan.org/nipponia/nipponia45/en/feature/feature10....