Commentary Panel 7

The Formation Process of Placer Gold Deposits and the Origin of Placer Gold Mines

Gold in Nishimikawa was generated by volcanic activities beginning approximately 21 million years ago (pictured on the top left). About 15 million years ago, quartz veins containing gold were exposed to weather on the land surface. The weather carried gold, sand and gravel to the seabed, forming placer gold deposits (pictured on the bottom left). The gold remained on the seabed for another 10 to 12 million years, as a thick crust of dirt and micro-algae (diatom) fell over the deposited layers of placer gold.

 
About three million years ago, land began to rise out of the sea, and Sado Island was born. When this happened, the layers of earth in the seabed containing placer gold were also elevated above the sea level (pictured on the bottom right). Erosion caused by years wind and rain on Sado Island formed terraces, mountains, valleys, and Nishimikawa Placer Gold Mine. In other words, the formation of Nishimikawa Placer Gold Mine is deeply connected with the formation of Sado Island itself.

Pictures:

Picture on the left above: [About 21,000,000 years ago]

 Hot water containing melted gold and silver ran up through fractures from deep underground to the surface by volcanic activity. Later, gold and silver precipitated in fractures where hot underground water flowed, forming gold lode deposits.

   The words in the picture: 地下水⇒Underground Water   熱水⇒Hot Water

Picture on the left below: [About 15,000,000 years ago]

 The gold lodes that appeared on the ground surface were eroded by rain and running water, becoming granular gold powder (placer gold) and sinking to the seabed.

   The words in the picture: 金鉱脈鉱床⇒Gold Lode Deposits、火山岩⇒Volcanic Rock、砂金⇒Placer Gold、海⇒Ocean

Picture on the right below: [About 3,000,000 to 500,000 years ago]

As land began to rise from the ocean about three million years ago, the seabed strata containing placer gold also rose, forming Nishimikawa Placer Gold Mine.

   The words in the picture: 西三川砂金山⇒Nishimikawa Placer Gold Mine、砂金⇒Placer Gold