Summary:
The Cripple Creek & Victor Deposit is widely recognized as an alkalic-type low sulfidation epithermal gold deposit.
Alteration
Epithermal gold deposits associated with alkaline magmatism can be characterized by: 1) a large volume of alkali and carbonate metasomatism, 2) tellurium (Te) minerals, high Au/Ag ratios, 3) low concentrations of sulfides and base metals, and 4) minimal acidic alteration (low clays) (Jensen and Barton 2000).
Unaltered volcanic rocks in the Cripple Creek Diatreme are extremely rare. The alteration history of the diatreme is complex and most recently summarized by Eric Jensen (2003). Original Na>K volcanic alkaline rocks were modified by multiple pulses of alkali metasomatism.
Though not economically significant, early widespread alkali/specularite alteration occurred during the late stages of diatreme formation. This alteration is overprinted by a later K-feldspar + pyrite + carbonate gold-bearing event, characterized mainly by adularia or low-temperature feldspars, with subordinate sericite and K-rich clays. Whole-rock K2O in highly altered volcanics can reach 15%. This alteration is closely associated with economic gold mineralization.
In lamprophyres and Precambrian mafic host rocks, K-metasomatism is further marked by narrow zones of sericite and carbonate alteration (Jensen and Barton 2000).
K-feldspar ± pyrite + carbonate alteration assemblages also overprint an earlier, high-temperature, biotite-stable, dark micaceous alteration with elevated base metals. Mafic minerals are replaced by secondary biotite and pyrite.
Minor acid alteration (quartz-dickite) occurs in restricted zones. Narrow quartz veins may host K-feldspar, fluorite, pyrite, and occasionally roscoelite. Many high-grade Au-Te veins are associated with this assemblage (Lindgren and Ransome 1906).
Structure
The district is crosscut by several major structures, often intruded by late-stage alkaline or lamprophyre dikes. The dominant structural fabric is sub-vertical, trending N20W-N50W and N20E-N70E, mirroring regional Precambrian trends. Though individual structures show minimal slip (<1 m), their orientation and kinematics strongly control ore localization and geometry.
Major veins in the Cripple Creek District display vertical continuity of more than 1,000 m below the present-day surface (Loughlin and Koschmann 1935; Thompson and others 1985; Kelley et al. 2020). A signification portion of the high-grade ore zones occur in veins that are controlled by the northwest and northeast trends that parallel regional structures and suture zones that were developed during the Mesoproterozoic deformation which experienced reactivation during the Laramide orogeny or events associated with the Rio Grande rift (Kelley and others 2020; Kelley and Ludington 2002).
Mineralization
The main gold mineralization event at the Cripple Creek & Victor (CC&V) deposit post-dates the final lamprophyre intrusions and is associated with tectonic activity linked to the development of the Rio Grande Rift. Alkaline volcanic emplacement began after 34 million years ago (Ma), with gold mineralization dated by rhenium-osmium (Re-Os) geochronology at 30.9 ± 0.09 Ma and 26.6 ± 0.09 Ma. Gold is hosted in all major rock types, including Precambrian granites located outside the diatreme, and is preferentially concentrated along second- and third-order structural features.
Mineralization is expressed in two overlapping styles: (1) low-grade disseminated and microfracture-controlled gold-pyrite-telluride mineralization, and (2) high-grade fracture-hosted gold-silver telluride veins. The low-grade mineralization is commonly microcrystalline and occurs in pyrite- and telluride-bearing zones of enhanced permeability near high-grade structures. In contrast, high-grade mineralization is typically localized along lithologic contacts, particularly between Cripple Creek Breccia and either intrusive or Precambrian host rocks, where fluid flow was focused due to contrasts in porosity and permeability or pre-existing brecciation (Kelley et al., 2020). Sheeted vein systems, typically 0.5 to 3 meters wide, are composed of networks of narrow fissures less than 50 millimeters wide (Kelley et al., 2020).
Isotopic analyses, including oxygen-18, deuterium, sulfur-34, and lead isotope ratios, indicate that the hydrothermal fluids responsible for mineralization were primarily magmatic in origin. These signatures suggest a deep, reduced fluid source derived from mantle or lower crustal levels, supporting a genetic link between gold deposition and the regional alkaline magmatism associated with rift-related extension.
Both mineralization styles are accompanied by alteration assemblages dominated by potassium feldspar, with variable pyrite and carbonate. High-grade gold typically occurs as native gold within iron oxides (after pyrite and tellurides) or as telluride minerals such as calaverite, krennerite, and sylvanite. Common gangue minerals include quartz, fluorite, and carbonate.
Historically, most production came from the high-grade telluride vein systems (Thompson et al., 1985), while the lower grade disseminated system was later detailed by Pontius (1992) and Burnett (1995). Two prominent late-stage hydrothermal breccia pipes, Globe Hill and Ironclad, occur in the northwestern portion of the district. Near-surface exposures of gold-bearing breccia clasts suggest that significant brecciation occurred after the main phase of mineralization (Seibel, 1991). Deep drilling in 2003 intersected gold-molybdenum mineralization dated at 26.6 Ma, which overprints earlier 30.9 Ma mineralization, implying a mineralizing system that was active for approximately 4.3 million years.
The large gold endowment at CC&V may reflect both the prolonged duration of mineralization and a mantle-derived, carbonate-rich, carbon dioxide-bearing melt source, as inferred from lamprophyre geochemistry. Oxidation profiles are deepest along major structures and typically extend to depths of approximately 122 meters (400 feet).