Radiometric ages have been measured on rocks of a crystalline terrane that includes ancient gneisses and migmatites, two granitic batholiths ( St . Kevin Granite and granite of Cross Creek), and various minor intrusive rocks . A whole- rock Rb-Sr isochron age on the St . Kevin Granite establishes it as 1390 ± 60 m.y. old. Mineral ages on the St . Kevin and numerous other rocks are either about the same as the St . Kevin whole- rock age or younger by as much as 200 m.y., even where the relative age is known to be older. Some minor Precambrian intrusive masses that are probably younger than St . Kevin Granite yield mica ages within analytical error of the St . Kevin age, indicating that these rocks can be younger than the granite by only a few tens of millions of years. The mica ages, both Rb-Sr and K-Ar, are thought to be minimal, but a K-Ar age of 2020 m.y. on horn-blende probably reflects excess argon. Mica ages from all rocks known geologically to be older than St . Kevin Granite are low and are interpreted as heating ages reflecting intrusion of the granite , in some cases modified further by heating during Laramide time. In this area, Precambrian intrusion and deformation had largely ended by 1200 or 1300 m.y. ago. Plutonism, represented here by the St . Kevin Granite and elsewhere by the Silver Plume and other granites, probably accounts for the numerous mineral ages of about 1300 m.y. previously reported from Colorado although weak regional metamorphism may also have been a factor.

Coloradao↗

USGS1959

Geology and thorium deposits of the Wet Mountains, Colorado; a progress report

No abstract available.

Colorado↗

USGS1955

Wet Mountain thorium investigations, 1952-1954, Fremont and Custer Counties, Colorado

Explore the source record for details and available documents.

Open-File Report↗

USGS1954

Wet Mountains, Colorado, thorium investigations, 1952-1954

A 22-square mile tract (McKinley Mountain Area) of pre-Cambrian rocks and veins containing thorium was mapped at the scale of 1:6,000. This tract lies on the west flank of the Wet Mountains, Custer and Fremont Counties, northeast of Westcliffe, Colo. The bedrock is a complexly interlayered sequence of gneisses of metasedimetary origin, migmatite and granitic gneisses that have been transected by an albite syenite stock and numerous northwest-trending dikes, veins, and fractures. Hornblende-plagioclase and biotite-quartz-plagioclase gneisses are the principal metasedimentary rocks; pyroxene-scapolite, garnet, sillimanite, and quartzite zones are locally present. The most prominent rock is a poorly foliated, microcline, alaskitic granite gneiss which occurs as layers ranging from more than 500 feet in thickness down to migmatitic “lit-par-lit” type of injections less than an inch thick. Of less wide distribution, but of similar occurrence, are quartz monzonite and leuco-granodiorite gneisses. Although the foliation of the rocks generally is steep and trends northeast over most of the area, several northeast-trending folds have been mapped in the northern half of the area; a vertically plunging fold occurs in the southwest. The albite syenite is nonfoliated and is about 595 million years old (late pre-Cambrian) by the Larsen zircon method of age determination. Many of the dikes are related to the stock. More than 800 radioactive occurrences were found along the northwest-trending veins. Almost all the radioactivity is due to thorium which in its purest form occurs as a hydrated thorite-like mineral. The veins also contain carbonate minerals, barite, quartz, red and yellow iron oxides, and minor sulfides; no genetic relation of these minerals to the thorium has been established. Although most of the deposits are only weakly radioactive, richer concentrations are scattered as pockets and lenses along the veins.

Colorado↗