Day 3B Road Log: Gold King Mine Release and Environmental Concerns
— J.M. Blake and S. Timmons

The western San Juan Mountains near Silverton, Colorado have a long legacy of mining, including Sunnyside, Gold King, and the Red and Bonita Mines. The area stratigraphy consists of a Proterozoic crystalline basement overlain by Paleozoic, Mesozoic, and Eocene sedimentary rocks, and a 1- to 2-km-thick, Oligocene- to Miocene-age volcanic cover (Yager and Bove, 2007). Much of the surface geology is Tertiary (Oligocene) Silverton Volcanics that consist of a pyroxene andesite member, the Burns Member which contains porphyritic andesite to rhyolite flows, and the Henson Member which contains volcaniclastic sedimentary rocks (Yager and Bove, 2007). The Silverton and San Juan Calderas underwent hydrothermal alteration and are now highly mineralized (Bove et al., 2007). Historically, acidic and metal laden waters from hydrothermally altered deposits drained into nearby streams (Church et al., 2007a; Yager et al., 2016).

On August 5, 2015, an estimated three million gallons of sediment and metal-laden water (U.S. EPA, 2016a) were released from the Gold King Mine into Cement Creek, a tributary to the Animas River. This release flowed south in the Animas River into New Mexico, into the San Juan River, and finally into Lake Powell, Arizona. The U.S. Environmental Protection Agency (U.S. EPA, 2016a) estimated that close to 540 tons of metals, mostly iron and aluminum, entered the Animas River over the 9-hour period of the Gold King Mine release. This, however, was not the first time a large release has occurred from this mining district. In 1974 and 1978, large releases occurred that carried acid mine drainage down Cement Creek into the Animas River (U.S. Bureau of Reclamation, 2015).

This trip will include discussions of water and sediment geochemistry in the Animas River and Cement Creek, alluvial ferricrete deposits, several mine locations such as Topeka Gulch, Gold King and Red and Bonita Mines, as well as iron bog deposits, and remediation efforts at the former town site of Gladstone related to the Gold King Mine (Figs. 3.9, 3.10). In addition to discussing the geology and geochemistry at each stop, there will be discussion of the effects of the Gold King Mine release downstream into New Mexico and Utah (Fig. 3.10). For more information on the regional geology, the reader is referred to the Day 2 Road Log, Yager and Bove (2007), and Baars and Ellingson (1984).