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Abstract

The Cripple Creek district is renowned for epithermal gold telluride veins which have produced over 22 million ounces of gold from an intensely altered diatreme complex (total production + economic resources of >1000 tons). The district is also renowned for its association with a rare class of alkaline igneous rocks. The volcanism at Cripple Creek was part of a regionally extensive episode of Oligocene magmatism, including large volumes of calc-alkaline rocks and smaller, but widely distributed alkaline centers. Amongst the mid-Tertiary alkaline intrusive complexes, only Cripple Creek is associated with a giant (>500 ton) gold deposit. Further study of the magmatic and hydrothermal evolution of these systems will be necessary to explain this apparent disparity in gold enrichment.

Cripple Creek’s gold mineralization principally occurs as telluride minerals hosted by swarms of narrow veins. Most geological studies over the last century have focused on the high-grade veins and to a lesser degree, adjacent hydrothermal alteration, but metasomatism is now shown to be broadly developed and demonstrably accompanied many events throughout the evolution of the igneous complex. Alteration types ranged from minor early pyroxenestable varieties through various biotite-bearing assemblages into voluminous K-feldspar stable types. Hydrolytic (acid) styles of alteration are present but minor. Economic gold mineralization is intimately associated only with late, voluminous K-feldspar-pyrite alteration which affected >5 km3 of the explored portion (upper 1 km) of the complex. Although similar to other gold deposits related to alkaline magmatism, Cripple Creek differs markedly from other epithermal systems in terms of its large volume of K-feldspar added and paucity of quartz and acid alteration.

Keywords: alkaline, epithermal, Cripple-Creek-Colorado, phonolite, metasomatism, hydrothermal-alteration, tellurides, diatreme

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