Abstract Cripple Creek is among the largest epithermal districts in the world, with more than 800 metric tons (t) Au (>26.4 Moz). The ores are associated spatially, temporally, and genetically with ~34 to 28 Ma alkaline igneous rocks that were emplaced into an 18-km 2 diatreme complex and surrounding Proterozoic rocks. Gold occurs in high-grade veins, as bulk tonnage relatively low-grade ores, and in hydrothermal breccias. Pervasive alteration in the form of potassic metasomatism is extensive and is intimately associated with gold mineralization. Based on dating of intrusions and molybdenite and gangue minerals (primarily using 40 Ar/ 39 Ar and Re-Os techniques), the region experienced a protracted but intermittent history of magmatism (over a period of at least 5 m.y.) and hydrothermal activity (intermittent over the final ~3 m.y. of magmatic activity). Key factors that likely played a role in the size and grade of the deposit were (1) the generation of alkaline magmas during a transition between subduction and extension that tapped a chemically enriched mantle source; (2) a long history of structural preparation, beginning in the Proterozoic, which created deep-seated structures to allow the magmas and ore fluids to reach shallow levels in the crust, and which produced a fracture network that increased permeability; and (3) an efficient hydrothermal system, including effective gold transport mechanisms, and multiple over-printed hydrothermal events.

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 km 3 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

The Wet Mountains–southern Front Range region is underlain by high-grade granitic gneiss, amphibolite, and schist of Early Proterozoic age. These rocks were intruded by granitic to granodioritic plutons during four episodes: one in the Early Proterozoic (1,660 to 1,700 Ma) and three in the Middle Proterozoic (1,485 to 1,440 Ma, 1,370 to 1,360 Ma, and about 1,060 Ma). We also report here a zircon age determination (536 ± 4 Ma) for syenite of the Cambrian McClure Mountain alkaline-mafic complex. The granitic gneiss was clearly formed before 1,700 Ma. Its protolith was probably pelitic to psammitic sedimentary rocks, in contrast to the volcanogenic rocks of this age farther west in the Gunnison and Salida areas of Colorado. The early Proterozoic plutons emplaced within the granitic gneiss are mostly somewhat younger than those emplaced within volcanogenic rocks to the west, although some are coeval. The middle Proterozoic rocks are representatives of the widespread “anorogenic granite-rhyolite suite” which is known in the St. Francois Mountains of Missouri and the subsurface of the midcontinent.