Eureka Mining District, San Juan County, Colorado, USAi

Regional Level Types
Eureka Mining District	Mining District
San Juan County	County
Colorado	State
USA	Country

Ruben J. McNutt and George W. Howard, two of the county’s pioneers, were the driving force behind the Eureka Mining District. They formalized the district in 1873 and, while the original boundaries are unclear, they designated the region north of the Animas River, east of Cunningham Gulch, and west of the Continental Divide. The boundaries became defined in 1874 or 1875 when prospectors created the Mineral Creek district to the west, and the Mineral Point district to the north. Ultimately, the Eureka district assumed a complex shape. Silverton lay at the southwest corner, and from here, the west boundary passed northwest through the series of peaks that culminated as the Red Mountains. In particular, the west boundary passed through, south to north, Anvil Mountain, Ohio Peak, McMillan Peak, and Red Mountain No.3.

The district’s north boundary was inconsistent. It started at Red Mountain No.3, followed the ridge east to Red Mountain No.1, and wrapped around the head of Cement Creek to Hurricane Peak. The north boundary then followed the ridge northeast to Tuttle Mountain, east to Houghton Mountain, and crossed over the North Fork of the Animas to Cinnamon Pass. In essence, the entire north boundary followed the separation between south flowing and north-flowing drainages.

The east boundary followed a similar principal. That boundary was drawn along the Continental Divide from Cinnamon Pass south to Stony Pass. The Divide also served as the separation between San Juan and Hinsdale counties.

The south boundary was almost as complex as the north. Specifically, the boundary extended from Stony Pass west to Cunningham Gulch and then followed the gulch northward to Howardsville. Thus, the mountains on the west side of the gulch remained within the Animas district while those on the east side belonged to the Eureka district. The south boundary then followed the Animas River west to Silverton.

GEOLOGY: Mineralization is polymetallic veins (Mineral occurrence model information: Model code 85; USGS model code 22c; Deposit model name: Polymetallic veins; Mark3 model number 46). Controls for ore emplacement included faults/fractures. Local alteration includes silicification (silicic, sericitic, propylitic). Local rocks include intra-ash flow andesitic lavas. Regional Geologic structures include the North side of the Silverton caldera along a NE-trending graben (extends from the Silverton caldera to the Lake City caldera in Hinsdale County). Part of the district extends into the central downfaulted block of the Silverton caldera. Local geologic structures include fissures and veins. Crescentic faults outline the central blocks of the Silverton caldera.