Geology Cripple Creek now compare Rattlesnake
By D.M.Vardiman, T.R. Brown, E. Roy, M. Ward, I.P. Hutchinson
Cripple Creek & Victor Gold Mining Co. Anglogold (USA) Exploration, Inc.
The Cripple Creek Mining District is located within a Tertiary age alkaline volcanic/diatreme complex dated at approximately 33.4 Ma (Jensen, 2003). Surrounding country rocks are Precambrian granites and metamorphic units. Figure 3 is a physiographic map of the Western United States showing the location of the Cripple Creek district relative to the Walker Lane – Trans Pecos structural boundary and the subsequently developed Rio Grande Rift.
The extensional Rio Grande system is most obvious in the Cripple Creek area along northwest striking horst and graben structures, such as the Four Mile and Oil Creek faults (Wobus, 1976). Subtle northeast regional trends also exist and may be related to a preexisting fabric that was developed during the Proterozoic. Work completed by Klipfel (1992) documents evidence for a possible Precambrian rift environment in northern Colorado along a general northeast trend. This rift environment may have extended south into the Cripple Creek area and may have been the mechanism for creating the subtle northeast fabric.
DISTRICT GEOLOGY
The principal Precambrian basement lithologies in the Cripple Creek area as described by Hutchinson and Hedge (1968) are: Biotite gneiss (postulated to be a metamorphic volcanogenic rock 1.7±? Ga); granodiorite (1.7 ±? Ga); Cripple Creek quartz monzonite (Silver Plume age, 1.5 ±? Ga); Pikes Peak granite (1.0 ±? Ga). The biotite gneiss and granodiorite were intruded by the Pikes Peak batholith, a large regional subalkaline intrusive complex. The intersection of these four units formed an area of structural weakness, which facilitated the emplacement of the Tertiary volcanic complex.
The bulk of the Cripple Creek volcanic complex is composed of a highly variable breccia unit, Cripple Creek Breccia, containing heterolithic diatremal breccia, volcanoclastic sediments, and bedded tuffs. Base surge deposits as well as fossiliferous lacustrine sediments are also present in some localities of the district. The breccia units have been intruded by a series of phonolite dikes and sills primarily along major structural zones. These dikes can have associated explosion or diatremal textures in their upper extent.
Following the main volcanic event, the complex was again intruded by a series of plugs, flow domes, and small stocks with compositions ranging from tephriphonolite to phonolite. The last phase of volcanic activity was the development of late-stage breccia pipes and the emplacement of mafic to ultramafic dikes. The major igneous activity within the Cripple Creek complex may have lasted approximately 3-4 million years (Kelley, 1996).
Detailed mapping of the district has shown a complex sequence of intrusive, extrusive, and diatremal eruption events (Pontius & Head, 1996). The volcanic complex can be divided into three generalized geologic terrains. A flat lying sequence of volcanoclastic sediments and phonolite flow units characterizes the eastern side of the district. Complex flow dome features with cross cutting intrusives of varying composition characterize the Central area of the district. Extensive areas of diatremal volcanoclastic breccias with scattered intrusive and flow dome features characterize the Western side of the district. Diatremal breccia is thought to underlie much of the deeper parts of the district.
Geologic and geophysical data suggest that the volcanic complex has several vents. This interpretation is more analogous to structurally controlled eruptive centers such as those in the Taupo area of New Zealand (Hedenquist, 1986) or small caldera environments (Sander & Einaudi, 1990 and Anderson & Eaton, 1990).
TERTIARY LITHOLOGIES
The Tertiary alkaline rocks of the Cripple Creek volcanic complex are divided into six principal units, although many variations exist within each group. These units are described in detail by Birmingham (1990) and Kelley (1996). A summary of these principal volcanic units is given below.
Breccia
Breccia is the most common and widespread rock type in the Cripple Creek volcanic complex. This unit ranges from fine-grained, stratified volcanoclastic sediments to course multi-phase heterolithic eruption breccias. The breccias are commonly matrix supported and composed of clasts from all surrounding rock types including the Precambrian units. The breccia unit is generally vuggy with variable dolomite cement and weathers to a light brown color.
Phonolite
Phonolite occurs throughout the volcanic complex and as domes around its margin. This unit commonly occurs as dikes, sills, flows, and small stocks. The unit varies from aphanitic to porphyritic with alkali feldspar being the most common phenocryst. Phonolite ranges in color from white to a reddish gray, although in most localities the unit is light gray.
Plagioclase Phonolite
Plagioclase Phonolite is primarily confined to the central area of the volcanic complex (historically referred to as Latite Phonolite). This unit occurs as dikes, stocks, flows, and sills. The unit is commonly porphyritic with plagioclase and pyroxene phenocrysts. Plagioclase Phonolite ranges in color from light gray to dark bluish gray.
Tephriphonolite
Tephriphonolite occurs primarily as small stocks in the central part of the volcanic complex (historically referred to as Syenite). This unit occurs as equigranular porphyry with plagioclase and pyroxene as the primary phenocrysts. Tephriphonolite has a salt and pepper appearance and a medium to dark gray color.
Phonotephrite
Phonotephrite occurs as a flow dome feature in one area of the volcanic complex (historically referred to as Trachydolorite). This unit is generally aphanitic, although in some exposures it is weakly porphyritic with pyroxene phenocrysts. Phonotephrite is dark gray to black in color and has a characteristic flaggy fracture pattern.
Lamprophyre
Lamprophyres occur throughout the volcanic complex, primarily as steeply dipping dikes (historically referred to as alkali basalt). This unit occurs late in the volcanic sequence. The unit is commonly porphyritic with olivine, pyroxene, and biotite phenocrysts. Lamprophyre is greenish black in color and readily alters to greenish clays.
ALTERATION
Epithermal gold deposits associated with alkaline magmatism can be characterized by: a large volume of alkali and carbonate metasomatism, Tellurium minerals, high gold/silver ratios, low concentrations of sulfides and base metals, and minimal acidic alteration (low clays) (Jensen and Barton, 2000).
Completely unaltered rocks at Cripple Creek are extremely rare. The phonolitic alkaline rocks were altered from the original sodium/potassium mineralogy by several pulses of potassium metasomatism. Whole rock potassium oxide values in highly altered Cripple Creek breccia can be as high as 15% potassium oxide. This occurs dominantly as adularia or low temperature feldspars; sericite and potassium rich clays are subordinate constituents. The potassic alteration overprints an early high temperature dark micaceous alteration. This high-temperature alteration is often biotite stable and has a higher base metal association. Mafic minerals are replaced by secondary biotite and sulfides, dominantly pyrite within the volcanics. Within lamprophyres and mafic Precambrian host rocks, potassium-metasomatism is characterized by narrow zones of sericite and carbonate alteration (Jensen and Barton, 2000).
Minor acid alteration (quartz-dickite) is observed in restricted environments. Quartz veins are narrow and may include potassium feldspar-flourite and pyrite and sometimes roscoelite (V mica). Many of the high-grade gold-tellerium veins are associated with this assemblage (Lindgren and Ransome, 1906).
STRUCTURE
The diatreme at Cripple Creek occupies the site of regional scale structural intersections that coincide with the Proterozoic Yavapai-Matzatzal terrane and an overprinting Laramide structural architecture. The Cripple Creek diatreme was emplaced during the regional retreat of the Paleogene Volcanic Arc after 43 Ma in a dilatant part of the Laramide architecture. Subsequent major geodynamic changes to the regional stress field led to the development of the Rio Grande Rift and alkaline magmatism continued at Cripple Creek until 27 Ma. Continued evolution of the principal stress direction saw dextral shear with s1 orientated N30°W during the time of subsequent gold mineralization. The relationship of the large-scale geodynamic setting to gold mineralization can be traced consistently down-scale to mapable structural orientations and kinematics within the orebody.
The district is cut by several major structural zones, which are commonly occupied by late alkaline or lamprophyre dikes. The principal structural fabric within the district is subvertical and oriented N20W to N50W and N20E to N70E. These structural trends correspond closely to the regionally observed Precambrian fabric, Figure 5. The majority of these structural zones display only minor post volcanic dip slip movement with offsets commonly being less than 1 meter.
MINERALIZATION
The main mineralizing event at Cripple Creek can be dated in a relative sense by crosscutting field relationships as postdating the last lamprophyre intrusive event. Emplacement of the alkaline volcanic complex at approximately 32 (?) Ma and subsequent gold mineralization at 27.6 ± 0.09 (Re-Os) Ma is believed to be associated with tectonic development along the mid-continent Rio Grande Rift system in Colorado. Gold deposits within the district are sited within second and third order district structures and occur in all rock types, including the Precambrian granitic rocks outside the main diatreme boundary.
Gold mineralization within the Cripple Creek district occurs in two principal styles, although they are commonly overlapping. Mineralization occurs primarily as (1) broad zones of low-grade, gold-pyrite mineralization which is microfracture-controlled and disseminated and (2) as fracture zones containing high-grade, gold-silver tellurides. The low-grade mineralization commonly contains microcrystalline native gold attached to pyrite and was deposited in permeable host rocks and/or microfractures adjacent to the narrow, high-grade structures. In many cases, the high-grade gold mineralization occurs adjacent to the contacts between Cripple Creek Breccia and the sills, dikes, domes, plugs, and/or Precambrian contacts.
Both types of mineralization are associated with potassium metasomatism. High-grade mineralization occurs as either native gold with iron oxides after gold-silver tellurides and pyrite, or as the gold-silver tellurides (calaverite, krennerite, and sylvanite) in association with pyrite. Minor gangue mineralization consists various occurrences of quartz, fluorite, and carbonate. Recent isotope studies indicate that the majority of the hydrothermal fluids are magmatic in origin. The majority of the historical production came from these high-grade gold-silver telluride vein portions of the deposit. Thompson and others (1985) offer a detailed description of the telluride system and the latest stage epithermal deposits. The low-grade gold system has been described by Pontius (1992) and Burnett (1995).
There are two late-stage, prominent hydrothermal breccia pipes in the northwest corner of the district. Gold mineralization within these late features is almost entirely in the clasts indicting that much of the activity was post mineral (Seibel, 1991).
Although complete bracketing dates are not currently available, it appears that the mineralizing system had a relatively long life (~2 m.y.), which may explain the large gold budget of the district. The system’s abundant carbon dioxide and trace element geochemistry of the lamprophyre intrusives suggests that its source may be from a carbonate rich, mantle-derived melt.
The gold presently being mined is generally less than 20 microns in size and occurs in three principal forms: native gold associated with pyrite as embayments or replacements along the margins of the pyrite grains and in some cases intergrown with pyrite; as native gold associated with hydrous iron and manganese oxides after tellurides; as gold-silver tellurides primarily in quartz-flourite veins. Oxidation is strongest and deepest along major structural zones. In general, oxidation of the deposit has a nominal depth of 122 meters (400 feet).
Bibliography
Anderson. W.B. and Eaton, P.C., 1990, Gold mineralization at the Emperor Mine, Vatukoula, Fiji: Journal of Geochem. Explor., V.36 p. 267-296.
Birmingham, S.D., 1990, Petrology and Rb-Sr Isotope Geochemistry of Alkaline Rocks of the Cripple Creek Volcanic Field, Colorado: Contributions to Mineralogy and Petrology, 1990.
Burnett, W.J., 1995, Fluid chemistry and hydrothermal alteration of the Cresson disseminated gold deposit, Cripple Creek, Colorado (MS thesis): Fort Collins, Colo.State Univ., p. 166
Hutchinson, R.M. and Hedge, C. E., 1968, Depth-zone emplacement and geochronology of Precambrian plutons, Central Colorado Front Range: Geological Society of America Special Paper 115, P. 424-425.
Hedenquist, J.W., 1986, Geothermal systems of the Taupo Volcanic Zone, New Zealand: Their characteristics and relation to volcanism and mineralization: In Smith,I,E.M.,ed., Late Cenozoic Volcanism in New Zealand. Royal Soc. N.Z. Bull., no.23, p. 134-168.
Jensen, E.P.and Barton, M. D., 2000, Gold Deposits Related to Alkaline Magmatism, In Hegemann S.G., Brown P.E. (eds) Gold in 2000, Reviews in Econ. Geol.13:279-314.
Jensen, E.P., 2003, Magmatic and Hydrothermal Evolution of the Cripple Creek Gold Deposit, Colorado, and Comparisons with Regional and Global Magmatic-Hydrothermal Systems Associated with Alkaline Magmatism, PhD dissertation: University of Arizona, Tucson, Az.
Kelley, K., 1996, Origin and timing of magmatism and associated gold-telluride mineralization at Cripple Creek Colorado (PhD dissertation): Golden, Colorado School of Mines. 259 p.
Klipfel, 1992, Proterozoic Geology and Massive Sulfide Mineralization of Northern Colorado: Paper presented at the twenty eighth annual Intermountain Minerals conference, August, 1992, Vail, Colorado.
Koschmann, A.H., 1949, Structural control of the gold deposits of the Cripple Creek district, Colorado: U.S. Geol. Survey Bull. 955-B, p. 19-58.
Lindgren, W., and Ransome, F. L., 1906, Geology and gold deposits of the Cripple Creek district, Colorado: U.S. Geological Survey Professional Paper 54, 516 p.
Pontius, J.A., 1992, Gold Mineralization within the Cripple Creek Diatreme/Volcanic Complex, Cripple Creek Mining District, Colorado, Randol at MINExpo 92, October, 1992, 21 p.
Pontius, J.A. and Head, J. A., 1996, Cresson Mine: Case history of a rapidly evolving mining project: Mining Eng., Jan. 1996, p. 26-30.
Sander, M.V. and Einaudi, M.T., 1990, Epithermal Deposition of Gold during Transition from Propylitic to Potassic Alteration at Round Mountain, Nevada: Econ. Geol., V. 85, p. 285-311.
Seibel, G.E., 1991, Geology of the Victor Mine, Cripple Creek Mining District, Colorado: M.S. Thesis, Colorado State University, 128 p.
Wobus, R.A., Epis, R.C. and Scott, G.R., 1976, Reconnaissance Geologic Map of Cripple Creek-Pikes Peak Area, Teller, Fremont and El Paso Counties, Colorado: U.S. Geol. Survey Map MF-805.
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