Lamprophyre breccia & lamprophyre dike (Cresson Pipe, Cripple Creek Diatreme, Early Oligocene, ~28.5 Ma; western side of Cresson Pit, Cripple Creek Mining District, Colorado, USA) 2 : 無料・フリー素材/写真
Lamprophyre breccia & lamprophyre dike (Cresson Pipe, Cripple Creek Diatreme, Early Oligocene, ~28.5 Ma; western side of Cresson Pit, Cripple Creek Mining District, Colorado, USA) 2 / James St. John
| ライセンス | クリエイティブ・コモンズ 表示 2.1 |
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| 説明 | Lamprophyre breccia and lamprophyre dike 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.The grayish-colored material seen here is an outcrop of a rare ultramafic igneous rock called lamprophyre breccia. This is exposed on the western margin of the Cresson Mine. Emplacement of this igneous body was one the last intrusive events in the Cripple Creek District, occurring at ~28.38 ± 0.21 Ma. The rest of the Cripple Creek Diatreme - phonolites, tephriphonolites, phonotephrites, and other rocks - was emplaced from 31.59 to 30.41 Ma.This lamprophyre breccia is part of the Cresson Pipe, which is about 600 by 900 feet in size (the older literature reports it to be about 100 to 150 meters in diameter). It's a small diatreme (Cresson) within a larger diatreme (Cripple Creek). The pipe is a heterolithic mix of rocks. Minerals include olivine, titanian pyroxene, pyrrhotite, phlogopite mica, and a matrix of analcime.The brecciated texture is from explosive devolatilization. Lots of gold mineralization occurred along this pipe, and mineralization increased from the top downward.In 1914, here at the Cresson Mine, miners encountered the Cresson Vug - a large cavity in altered breccia. The vug was carpeted with calaverite. Tens of thousands of ounces of gold were produced in a few weeks. Genuine samples of Cresson Vug calaverite are rare - they have quartz and lack fluorite (any specimen of calaverite-on-fluorite labeled as coming from the Cresson Vug is false or mistaken).A lamprophyre dike cuts through the lamprophyre breccia at this outcrop - see the subvertical feature at left of center.------------------------------------Description from Jensen (2007):The Cresson Pipe, a lamprophyre breccia, represents one of the more notable geologic features in the district. Lamprophyric rocks are the youngest known igneous rocks exposed in the district. Although they constitute only 1-2% of the volume of igneous rocks in the district, mineralized lamprophyres were mined in roughly 1/3 of the mines in the Cripple Creek district.Lamprophyric rocks derive their name from the greek root “lampros” and “porphyros” which combine to mean “glistening porphyry”; the name was first applied to mafic to ultra-mafic rocks rich in biotite phenocrysts that appear to glisten within the drab, fine-grained mafic groundmass. As commonly applied, the term lamprophyre is used as a field classification to describe mafic to ultramafic rocks with hydrous mafic phenocrysts, and if feldspars are present they are restricted to the groundmass of the rocks.The Cresson pipe is an elliptical, pipe-shaped body of breccia elongated ENE, with an average diameter of approximately 100 meters. The pipe plunges steeply to the south, tapering and becoming increasingly tabular at depth, and eventually bifurcating into two roots ~600m below the surface. The pipe is composed of heterolithic rock fragments, including a predominance of mafic and ultra-mafic clasts that are supported by a matrix that varies from a fine-grained, crystalline lamprophyric matrix rich in clinopyroxene and analcime (commonly altered to montmorillonite clays and carbonate), to a leucocratic matrix of carbonate-analcime-alkali feldspar±quartz. In portions of the pipe, the leucocratic matrixes contain rounded blebs of dark lamprophyric material. In thin section, the “blebs” are concentrically zoned, and exhibit fine-grained outer rims. These textures suggest that the “blebs” are not mechanically comminuted rock fragments, but were liquid at the time of incorporation into the leucocratic matrixes. These may reflect processes of liquid-liquid immiscibility, with the leucocratic matrixes representing lowdensity, hydrous phases that were exsolved from lamprophyric melts during crystallization.The Cresson pipe both cuts and is cut by lamprophyric dikes. Gold mineralization was strongly developed along the margins of the pipe, and appears to form an annulus around the pipe. Roughly 2.5 million onuces of gold were produced from the Cresson Mine, much of which was mined from the stopes along the pipe margins. It is interesting to note that fluorite was common in wall rocks within the annulus of mineralization that surrounds the pipe, but was notably absent from veins the cut into the lamprophyric rocks inside the pipe.Perhaps the most notorious discovery in the district was made inside the Cresson Mine and is referred to as the "Cresson Vug". This "vug" was a cavity 8 m x 4 m wide and 13 m high that was opened in the Cresson mine in November, 1914. The cavity walls were lined with gold tellurides, quartz, celestite and a white, soft mineral that was probably kaolinite or dickite. By the end of 1914, nearly 20,000 ounces of gold had been removed from the cavity. Good descriptions can be found in Patton and Wolf, 1915; Preliminary report on the Cresson gold strike at Cripple Creek, Colorado: Colorado School of Mines Quarterly, volume 9, pp. 1-15 and Smith et al.,1985; The Cresson Vug, Cripple Creek: The Mineralogical Record, volume 16, pp. 231- 238.------------------------------------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 brecciasMost 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.------------------------------------Geologic unit: Cresson Pipe, Cripple Creek Diatreme, late Early Oligocene, ~28.38 ± 0.21 MaLocality: western side of the Cresson Pit, north of the town of Victor, Cripple Creek Mining District, southern Teller County, central Colorado, USA------------------------------------Reference cited:Jensen (2007) - Geology of the Cripple Creek Gold-Telluride Deposit, Colorado: Descriptions and Locations of Field Trip Stops. 6 pp. [in Geological Society of America's data repository: www.geosociety.org/datarepository/2008/2008144.pdf ] |
| 撮影日 | 2007-10-27 16:44:30 |
| 撮影者 | James St. John |
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