Wet Mountains Area, Colorado, USAi

Regional Level Types
Wet Mountains Area	Mining Area
Colorado	State
USA	Country

Antelope Creek District - fluorspar

Thorium (Th) and rare earth elements (REE) are noted in veins, syenite dikes, fracture zones, and carbonatite dikes (Armbrustmacher, 1988) associated with three Cambrian alkaline complexes (Olson and others, 1977) that intruded the surrounding Precambrian terrane. Thorium-REE−mineralized veins and fracture zones, which are distal to the three alkaline intrusive complexes, have the highest economic potential for thorium and rare earth elements resources. The thorium-REE veins and fracture zones are linear features, typically 1–2 m (3.3–6.6 ft) thick, but a few are as much as 15 m (49 ft) thick. Some individual thorium veins can be traced in outcrop for 1.5 km (0.9 mi) and some radioactive fracture zones as much as 13 km (8 mi). Most of these vein and fracture-zone deposits lie within a 57 km2 (22 mi2) tract of Precambrian gneiss and migmatite located south and southeast of a quartz syenite complex at Democrat Creek. Christman and others (1953, 1959) mapped nearly 400 veins in this area.

The thorium-REE veins and fracture zones are linear features, typically 1–2 m (3.3–6.6 ft) thick, but a few are as much as 15 m (49 ft) thick (fig. 11). Some individual thorium veins can be traced in outcrop for 1.5 km (0.9 mi) and some radioactive fracture zones for as much as 13 km (8 mi). Most of these vein- and fracture-zone deposits are distributed within a 57 km2 (22 mi2) tract of Precambrian gneiss and migmatite located south and southeast of a quartz syenite complex at Democrat Creek. Christman and others (1953, 1959) mapped nearly 400 veins in this area. The dominant minerals forming these veins are smoky and clear quartz, microcline, barite, iron oxides, carbonates, and accessory rutile and sulfide minerals. Waxy, red thorite is the primary thorium mineral.

Thorium-REE minerals in the Wet Mountains district are also deposited in carbonatite dikes and small plugs. The carbonatite dikes are especially associated with the McClure Mountain complex (Staatz and Conklin, 1966). The carbonatites take a variety of forms, such as composite dikes with two or more generations of carbonate side by side with lamprophyre (Heinrich and Salotti, 1975; Armbrustmacher and others, 1979); phreatic explosion breccia pipes satellite to the McClure Mountain complex (the Pinon Peak breccia pipes of Heinrich and Dahlem, 1967); and siliceous carbonate dikes associated with amethyst veining (the Amethyst carbonatites of Heinrich and Shappirio, 1966). Armbrustmacher (1979) separated the carbonatites into two groups: replacement carbonatites and primary magmatic carbonatites. Replacement carbonatites have microscopic textures that indicate the nearly complete pseudomorphous replacement of relict igneous dike minerals by carbonate minerals. The replacement carbonatite dikes have ThO2 contents of <0.1 percent (Armbrustmacher and Brownfield, 1978). In contrast, the primary magmatic carbonatite dikes do not display mineral replacement textures and are enriched in elements and minerals typical of magmatic carbonatites, such as thorium, niobium, and REE that reside in the minerals thorite, bastnasite, synchysite, ancylite, and monazite. Thorium concentrations in the primary magmatic carbonatite dikes commonly exceed 0.1 percent ThO2. However, thorium is more concentrated and is present in greater volume in the quartz–iron oxide–barite vein and fracture-zone deposits of the district in comparison with the carbonatites (Armbrustmacher, 1988).