New Mexico Mineral Symposium — Abstracts

The San Juan Mountains encompass a large area of southwestern Colorado and part of northern New Mexico. The rugged mountain range is very scenic and has been called the "Alps of North America."

The mountains consist of a base of eroded Precambrian igneous and metamorphic rocks overlain by Mesozoic and Paleozoic continental and marine sediments. These sediments are overlain with thick accumulations of Late Cretaceous and Tertiary volcanics that are associated with Laramide tectonics. During mid-Tertiary times renewed eruptions from vent volcanoes formed the widespread San Juan Formation, which consists of lavas, breccias, and mudflows. Volcanic activity created widespread ash-flow tuff deposits and formed at least fifteen calderas. Large-scale subsidence following evacuation of magma chambers formed landslide formations from the collapse of the caldera walls. Later eruptions resulted in thick accumulations of the Burns and Henson Formations, the host rock for the veins in the Eureka mining district. Subsequent uplift and stress due to the renewed tectonic activity created major fractures that were filled by the hydrothermal solutions that formed the Smuggler-Union, Tomboy, and other rich gold-based metal veins.

The area discussed in the slide presentation is the Ouray-Silverton-Telluride triangle. This area is intensely mineralized and contains major Au-Ag-Pb-Cu-Zn deposits.

Gold was discovered in 1860 in the San Juan Mountains by some of the first prospectors to venture into the rugged range. Charles Baker is credited with making the original placer-gold discovery in Eureka Gulch, three miles north of Howardsville. The news of this discovery brought hundreds of prospectors into the area. However, the placer deposits did not "pan out," and the area was abandoned after 1861. Because the region was Inaccessible and the territory was controlled by the Ute Indians, mining and settlement were not established until the 1870's.

Lode gold was discovered in 1872 at the Little Giant mine in Arrastra Gulch. Discovery of the Belle Creole, Washington, and Sunnyside veins in the Lake Emma area followed. In 1873 a treaty was signed with the Ute Indians that opened up the San Juan Mountains for mining and settlement.

By 1876 Silverton was well established as a mining and railroad center serving the mines of the southern San Juan Mountains. Major gold mines in the area were the Gold Prince mine at Animas Forks, the Sunnyside mine at Eureka, and the Gold King mine at Gladstone. Telluride, on the western edge of the San Juan Mountains, was the main settlement serving the mines in this area. In 1875 John Fallon discovered gold in Marshall Basin and developed the Sheridan mine. Later the Sheridan, Mendota, Union, and Smuggler mines were consolidated by English bankers into the Smuggler-Union mine, one of the largest producers in the area. Other major discoveries that resulted in profitable operations were the Liberty Bell mine, discovered in 1876, and the Tomboy mine, discovered in 1880. Ouray, on the northern edge of the San Juan Mountains, was the main supply point for the mines of Yankee Boy and Imogene Basins, the Engineer Mountain area, and Poughkeepsie Gulch. The dominant gold producers were the American Nettie mine, discovered in 1889, and the Camp Bird mine, discovered by Tom Walsh in 1896.

By 1956 all major mining properties had been consolidated into one large mining operation, The Idarado Mining Company. Extensive development was completed, and the area was mined profitably until 1976 when operations ceased.
Native gold is the primary gold-bearing mineral in the region and occurs with base-metal sulfides and numerous, mostly quartz, gangue minerals. Petzite and krennerite occur rarely with the gold ore in some of the veins. The mines of the area have produced millions of dollars in precious and base metals and have supplied the mineral collector with thousands of fine specimens.