File:Pophyritic phonolite (host rock next to Bluebird Dike, Cripple Creek Diatreme, Early Oligocene, 32 Ma; Cresson Pit, Cripple Creek Mining District, Colorado, USA) 3.jpg

English: Pophyritic phonolite in the Oligocene of Colorado, USA.

The Cripple Creek Gold District of central Colorado is famous for its unusual gold and silver mineralization. Precious metal mineralization occurs in the Cripple Creek Diatreme, the root zone of a deeply eroded volcano dating to the Early Oligocene (32 Ma). The dominant lithology at Cripple Creek is phonolite, a scarce, alkaline, intermediate, extrusive igneous rock. Cripple Creek gold can be found in its native state (Au), but it typically occurs in the form of gold telluride minerals: for example, calaverite - AuTe2, sylvanite - (Au,Ag)2Te4, petzite - Ag3AuTe2, krennerite - (Au,Ag)Te2, and nagyagite - Pb5Au(Sb,Bi)Te2S6). Silver also occurs in some Cripple Creek minerals, including sylvanite, petzite, krennerite, hessite - Ag2Te, tennantite - (Cu,Ag,Fe,Zn)12As4S13, acanthite - Ag2S, and argentian tetrahedrite - (Cu,Fe,Ag,Zn)12Sb4S13.

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The following are my personal notes from a 2007 presentation on Cripple Creek geology by mine representative Tim Brown:

The Cripple Creek District has four different pits being mined now, including the main Cresson Pit. The Cripple Creek & Victor Gold Mining Company is an AngloGold Ashanti joint venture operation. The heart of the mining district is a 32 Ma diatreme. It erupted along a major structural feature - through deep cracks in a big shear zone along a syenite-augen gneiss contact. Some Precambrian rocks occur in the diatreme. There are three eruptive sub-basins to this diatreme - one in the north, one in the south, and one in the southwest. The diatreme consists of volcanic breccias and phonolites and lamprophyre breccias. The lamprophyre breccia is relatively small, but the margins have produced 2.5 million ounces of gold.

Types of gold mineralization in the Cripple Creek District: 1) Lamprophyre breccia-hosted gold mineralization - 2.5 million ounces of gold (Cresson Mine). 2) Au-Te rich, sheeted vein system - 4.3 million ounces of gold (Portland-Independence Mine). 3) Au-hydrothermal breccias

Most Cripple Creek rocks are phonolites (earlier intrusions) and trachytes. All are on the alkaline side of extrusive rock classification. Later intrusions are mafic. Finally, there are ultramafic lamprophyres.

When gold is poured here, ~30% of each button is silver.

The Cripple Creek diatreme is 4.3 miles in size, in an east-west direction, and 4 miles across in a north-south direction. Some of the old underground workings along the gold telluride vein systems reached down to 3000’ depth.

Structural features in the diatreme are probably related to the Rio Grande Rift. This was an active volcano at one time. There are lots of north-south trending veins & northeast-southwest trending veins here. The district was discovered in 1891. It’s been producing gold for 110 years. Geochemistry of altered & mineralized rocks in the Cripple Creek District - can see a characteristic enrichment of a certain suite of elements (Au, Te, Ag, Sb, W), especially gold. A similar pattern is seen in alkaline districts elsewhere. Ca, Na, Cs were systematically depleted. Mo is systematically enriched in all the mineralized zones. Other base metals have an erratic enrichment-depletion pattern. High-grade gold ore rocks here are always associated with potassic alteration. But not all K-altered rocks have high-grade gold ore. There are NE trends, E-W trends, N-S trends, and NW trends in the Cripple Creek area. On a resistivity map, can readily see the Cripple Creek Diatreme boundary. Can also readily see the diatreme outline on a magnetic signature map, plus the syenite-augen gneiss boundary, trending northeast. Gravity maps show ENE trends and a strong N-S trend (gravity low), which extends under Precambrian rocks at the surface. Currently, gold mining at Cripple Creek is in surface pits. They may end up going underground to the diatreme pipe, below the already worked area. Have drilled 3500’ down - not much gold, but did find carbonized wood! The carbonized log is though to have been originally a tree caught up in an eruption by the Cripple Creek Volcano, and got incorporated in the diatreme breccia. 300,000 ounces of gold per year is produced here. They’ve just passed the 3 millionth ounce of gold for the district. Porphyritic phonolite bodies here have narrow rooted zones and they spread out above the root zones. The mining area mostly has near-vertical features. There are some flat-lying sills. There are syenite bodies in the diatreme - they are not exposed at the surface - they are pretty broken up. An aphanitic phonolite dike, the Bluebird Dike, is in the main Cresson Pit. There are 3 to 4 old, very long drain tunnels that kept the original subsurface mines free of water. Those mines tracked productive veins. The tunnels come out in the Squaw Gulch area. So, no water problems in the present mines. President Franklin Delano Roosevelt shut down the Cripple Creek District at one time - all nonessential mining was stopped during World War II. The lamprophyre breccia body bifurcates at depth - it has a pair of pant legs. Drill holes through the pant legs didn’t have much gold, but they are only small pinprick drill holes. Would like to redrill them. Current pit operations - expect to bottom out at 1000’-1200’ depth. The expected life of the current mine is to 2012. Old underground workings consistently go through the gradient between low & high resistivity areas. Au:Ag ratios in the area range from 1:1 to 10:1. The average grade of gold ore here is 2/3 of a ppm. The modern land surface is ~100 to 1000-2000 meters below the level of the original active volcanic surface.

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Geologic unit: float from host rock near the Bluebird Dike, Cripple Creek Diatreme, Early Oligocene, 32 Ma

Locality: Cresson Pit, north of the town of Victor, Cripple Creek Mining District, southern Teller County, central Colorado, USA