Genetic model for the Cripple Creek district: Constraints from 40Ar/39Ar geochronology, major and trace element geochemistry, and stable and radiogenic isotope data
Published:January 01, 1996
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Karen D. Kelley, Samuel B. Romberger, David W. Beaty, Lawrence W. Snee, Holly J. Stein, Tommy B. Thompson, 1996. "Genetic model for the Cripple Creek district: Constraints from 40Ar/39Ar geochronology, major and trace element geochemistry, and stable and radiogenic isotope data", Diamonds to Gold: I. State Line Kimberlite District, Colorado; II. Cresson Mine, Cripple Creek District, Colorado, Tommy B. Thompson
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Since its discovery in 1891, the Cripple Creek district has produced more than 653 tonnes of Au(21 million oz) from high-grade gold-telluride veins. About99.5tonnes (3.2mi1lion ounces) of gold have been added to the district resource as a result of recent exploration (past 5 years) which has delineated low-grade, near surface disseminated gold deposits that occur as broad zones in permeable rocks adjacent to major structures (pontius, 1992;Harris et sl., 1993). The ore deposits of the district are localized within and adjacent to an elliptical northwest-trending diatreme complex that covers about 18 km2, Although some of the deposits occur within Proterozoic rocks surrounding the complex, most of the ore bodies are spatially associated with Tertiary alkaline igneous rocks and breccias within the complex.
During the most productive mining years beginning in 1893, several comprehensive studies of the Cripple Creek district were conducted that included detailed descriptions of the mineralogy, texture, and field characteristics of each rock type and of the ores (Cross and Penrose, 1895;Lindgren and Ransome, 1906;Loughlin and Koschmann, 1935). Over the past 12 years, selected ore deposit studies have focused on the fluid inclusion characteristics, mineralogy, and alteration of the deposits (Thompson et el., 1985; Saunders, 1986; Nelson, 1989; Seibel, 1991; Burnett, 1995). The close spatial association between the deposits and alkaline igneous rocks, and relatively hot (average of 350±C), hypersaline (>40 wt. % NaCI equivalent) fluid inclusions in quartz from early stages of vein formation (Thompson et sl., 1985) have led many workers to suggest that the ore fluids were magmatically derived (Lindgren and Ransome, 1906; Thompson et sl., 1985; Pontius, 1992).
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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