Many of the mining districts of the Colorado Front Range mineral belt contain mesothermal sulfide ores that exhibit a zonal distribution. Present data indicate that in most of the zoned districts pitchblende and/or secondary uranium minerals are most abundant in a transition zone between central areas containing predominantly pyritic gold ores and peripheral areas containing dominantly lead-zinc-silver deposits. Copper in the form of chalcopyrite is also probably more abundant in the transition zone than in adjacent zones. Many mineralized areas in the Front Range mineral belt are roughly co-extensive with and probably related to groups of Tertiary intrusives. The solutions that deposited sulfide and pitchblende in these areas probably were derived from many different types of source magma, but some bostonite magmas were probably particularly important sources of uranium-bearing solutions. At most localities where the relative ages of pitchblende and associated sulfides are known, the pitchblende is early in the paragenetic sequence, and not of intermediate age as suggested by its zonal position. This discrepancy may be the result of overlapping zones of deposition related to adjacent centers of mineralization, to changing environments of deposition within zones during the period of mineralization, or to unknown factors. Uranium is also present in districts in which the dominant mineralization is markedly different from the common pyritic gold-base metal sulfide mineralization. In the Jamestown district uranium appears to be associated'.mainly with fluorspar;, in other districts that exhibit little or no systematic hypogene zoning) it is ubiquitous. If uranium is present in zoned districts, a large proportion of she deposits probably should be in the transition zones. The application of this concept may be a valuable guide in prospecting for
uranium.
