The geology of Oregon gold locations in the southwestern part of the state is complex and not fully understood, being closely associated with plate tectonics and crustal subduction. Numerous gold-quartz veins can be found in greenstone of the Triassic age (248 – 208 million years ago), which trends in belts from the southwest to the northwest parts of Josephine County. Black slate , peridotite, and serpentine of Jurassic age sometimes contain gold-quartz veins and tend to parallel the greenstone belts. Granite, diorite, and gabbro intrusive bodies can be found in many parts of the county, but are generally devoid of mineralization except where they are in contact with older rocks. Josephine County is noted for past chromium, copper and nickel production as well as gold, and exploratory work for nickel continues to this day.
The most productive lode gold mine was the Greenback Mine, which produced approx. 175,000 ounces of gold from a persistent quartz vein in greenstone. It was sunk to an incline of 1,000 feet on 12 levels. The Benton Mine was developed in gold-quartz veins in greenstone near the contact with intrusive diorite and produced 18,500 ounces of gold. The nearby Gold Bug Mine produced 37,500 ounces. Numerous other lode mines produced between 1,000 and 13,000 ounces of gold, primarily in the period between 1893 and 1942.
Josephine County is noted for rich pockets of gold close to the surface. Some of these pockets were fabulously rich, though they were mined out quickly. The best known pocket diggings was in the Briggs Mine near the California line, where masses of gold totaling 2,000 ounces were taken out in 1904. Slabs of gold up to 3 feet in length were reportedly recovered.
A number of nuggets the size of chicken eggs have been found in placer gravels and pocket deposits. A 17 pound nugget was found in 1859 on the East Fork of Althouse Creek below the Briggs Pocket. Another nugget weighing 15 pounds was found in the gravels near the Esterly hydraulic cut in the early 1860’s. Despite the abundance of nuggets in Josephine County, most gold recovered in placer mining operations is fine flakes.
Placer gold was discovered in 1850 and simple hand mining methods commenced in 1852. Before long, hydraulic methods were introduced and a number of deep cuts were made in the landscape. Placer gold can be found in stream channels, in bench gravels, and in terrace gravels up to 600 feet above the present stream levels. Old channels can be found in terrace gravels, some of which are rich. Gold is generally found at or near fractured or decomposed bedrock. Some of the bench and terrace gravels are cemented. Boulders are common in many stream gravels, and most gravels range from a few feet to over 50 feet in thickness.
Power shovels, dredges and dragline excavators were introduced around the turn of the century and were used up to 1952. Since 1960, individuals with portable suction dredges have found considerable gold in Josephine County and Jackson County to the east. Most placer work is done between February and September when streams contain sufficient water.