Published:January 01, 1996
Figures & Tables
Diamonds to Gold: I. State Line Kimberlite District, Colorado; II. Cresson Mine, Cripple Creek District, Colorado
Driving north from Denver to Fort Collins, the road skirts the eastern edge of the Front Range of the Southern Rocky Mountains, primarily on lower Tertiary and then upper Cretaceous sedimentary formation of the Denver Basin. To the east lie the Great Plains. The basement rocks exposed in the Front Range are buried by up to 12,000 feet locally along the front. The ranges are comprised of Proterozoic crystalline rocks ranging in age from the 1800Ma Idaho Springs metasediments and metavolcanics, the 1700MaSilverplume granite and the 1400Ma Log Cabin and Sherman granites of the Sherman Batholith.
North from Fort Collins toward Laramie the route traverses hogbacks of uplifted Permian Pennsylvanian through lower Cretaceous sedimentary beds, locally traveling down the strike valleys. Road cuts provide excellent exposures of the Jurassic Morrison. Formation, and the Cretaceous Dakota and Ingleside Formations. Quarries developed in these formations produce sandstone for building materials and glass sand, limestone for agricultural and construction use and shales for cement manufacture. The oldest sedimentary rocks of the Permian-Pennsylvanian Fountain Formation unconformably overlie the paleo-erosion surface of the Proterozoic crystalline rocks. This route also follows the old Overland Trail stagecoach route.
In the Livermore area a large open plain named the Livermore Embayment is formed by a graben. The bounding faults, seen on the south, west and north, place the sedimentary units up against the Proterozoic rocks.
North of Livermore, the route enters the 1400Ma Sherman Batholith. The principal rock types are the coarse grained pink hornblende granite termed the Sherman Granite. The Sherman exhibits widely spaced joints and rounded weathering surfaces. The tightly jointed blocky white granite is termed the Log Cabin Granite.
The route bisects the Virginia Dale Ring Complex, a striking circular feature on aerial photographs mapped by D.H. Eggler (1967). The core is largely Sherman Batholith, while more mafic metamorphic units are common locally in the outer rings. The route follows a long curved valley formed in one of the outer rings.
The State Line Kimberlite District consists of Devonian age kimberlite intrusions into the Proterozoic crystalline rocks. Kimberlite emplacement is not restricted or controlled by any specific structural or age unit. M.E. McCallum of Colorado State University identified the first kimberlite in 1965, at the Sloan Ranch in the southern portion of the District. In collaboration with Eggler, several other occurrences were then identified. McCallum and Eggler continue their kimberlite, mantle and diamond