Lode and Placer Gold
More or less three fourths of the gold that has been produced in Oregon has been obtained from the lode and placer deposits that are located in the Blue Mountains geomorphic province, and this area takes up a lot of the northeastern part of the state. This area has deposits that are located in an area that was named by Lindgren as the gold belt of the Blue Mountains. The belt of the Blue Mountains is around fifty miles wide and 100 miles long, and it stretches from John Day on the west to the Snake River on the east. The most important mining areas are located in Baker and Grant Counties, and in neighboring areas of Malheur and Union Counties. All of the lode deposits are in pre Tertiary rocks and are thought to be connected with cretaceous dioritic intrusions.
The geology in the southwest of Oregon is intricate and is not entirely comprehended. It is very closely connected with plate tectonics and crustal sub-duction. A great number of gold quartz veins have been and can still be found in greenstone of Triassic age, which develops in belts from the southwest to the northeast parts in the county of Josephine. Black slate, peridotite, and serpentine of Jurassic age every now and then hold gold quartz veins and are likely to match the greenstone belts. Granite, diorite, and gabbro intrusive bodies of Jurassic and Cretaceous age can also be seen in a lot of areas of the county, but are in the main lacking of mineralization apart from where they are in contact with ancient rocks. This county is distinguished due to the chromium, copper, and nickel production it has, not to mention the gold, and investigative work for nickel remains to be done even now.
The most productive lode gold mine in the county was the Greenback that produced around 175,000 ounces of gold from a continual quartz vein in greenstone. The Benton Mine was developed in gold quartz veins in greenstone in close proximity to the contact with intrusive diorite, and it produced 18,500 ounces of gold. The close by Gold Bug Mine produced 37,500 ounces and a number of other lode mines produced between 1,000 and 13,000 ounces of gold, principally in the years between 1893 and 1942.