RS-21 Precambrian Tungsten and Copper-Zinc Skarn Deposits of South-Central Colorado


Descriptions of tungsten (scheelite-powellite) skarns, copper-tungsten (scheelite) skarns, and copper-zinc skarns, which represent the three main types of deposits occurring in metamorphic rocks in south-central Colorado. 115 pages. 18 figures. 10 tables. Digital PDF download. RS-21D

From the Abstract:

Three intergrading types of mineralized skarn deposits occur in the Precambrian metamorphic rocks of south-central Colorado (Park, Fremont, Chaffee and Custer Counties): tungsten (scheelite-powellite) skarns; copper-tungsten (scheelite) skarns; and copper-zinc skarns. The scheelite skarns are essentially confined to recrystallized and metasomatized calc-silicate gneiss layers and lenses, a minor metamorphic rock type. The other two types occur mainly in amphibolitic gneisses and biotitic and sillimanitic gneisses, all common rock types. The copper-tungsten deposits of the Cleora district, Chaffee County, comprise a distinct intermediate type characterized by fracture-controlled veins.

The deposits, partly of metamorphic and partly of metasomatic origin, are wall-rock-controlled (all three types) and fracture-fault-controlled (Cu-W, Cu-Zn). They all apparently were formed about 1700 m.y. ago and appear to be related genetically to intrusions of Boulder Creek granitoid plutons, syntectonically emplaced during the apogee of metamorphism of Idaho Springs rocks.
Many of the deposits have been prospected and a few of the copper-zinc deposits were developed during the latter part of the 19th and early part of the 20th century into significant mines (Betty, Isabel, Cotopaxi, Sedalia, Independence, Marion). Because of their geological restrictions, the tungsten deposits offer little hope for any substantial production, but some of the copper-zinc deposits are worth a modern re-examination.