Pitchblende occurs locally along early Tertiary gold-, silver-, and sulfide-bearing quartz veins in the Central City district, Colorado, within the mineral belt of the Front Range. The veins cut a complex mass of Precambrian metamorphic and igneous rocks and early Tertiary intrusive porphyritic rocks.The veins are fissure fillings that formed at intermediate temperatures and pressures. They consist mainly of pyrite, sphalerite, and galena in a quartz gangue, but also contain tennantite, chalcopyrite, enargite, and pitchblende. The veins differ in quantitative mineralogy, and they can be classified as pyrite-type and galena-sphalerite-type veins. Vein filling took place during three stages of mineralization, from oldest to youngest: a uranium stage, a pyrite stage, and a base-metal stage. Major periods of fracturing and vein reopening took place between the vein-forming stages.The pyrite and base-metal stages of mineralization were of broad areal extent and produced a concentric zonal arrangement of the ores in the district. In contrast, the uranium stage of mineralization was local in extent and resulted in scattered clusters of uranium deposits, which show no definite spatial relation to the district-wide zoning pattern.Pitchblende is present in only a few veins. The ore bodies are localized in structurally controlled open spaces along faults. They occur locally along four of the six vein sets of the district, in the form of ore shoots or small lenses and pods that are separated by vein material essentially devoid of uranium. The shoots are small and measure at most a few tens of feet in height and length, and average less than a foot in width; few contain more than 50 tons of ore. Some of the shoots are systematically arranged within the veins but others are erratically distributed.

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