Abstract

The Urad and Henderson stockwork molybdenite orebodies at Red Mountain, Clear Creek County, Colorado, are related to a rhyolitic subvolcanic center of Oligocene age referred to as the Red Mountain Complex. The two orebodies are well separated in space and are specifically related to different intrusive rocks of the complex. The Urad orebody was deposited first and at shallow depth. The Henderson formed deeper within the complex and is much the larger of the two. The Oligocene rocks intrude a batholith of Precambrian Silver Plume Granite near the western edge of the Colorado mineral belt and are within a mile of the north-north-east-trending Berthoud Pass fault. This fault is the major structural element of the area and has been identified by Tweto and Sims (1963) as a Precambrian fault reactivated in Tertiary time.The main part of the Red Mountain Complex crops out over an irregular area about 2,400 feet long and 800 to 1,200 feet wide. Dikes related to the main complex surround it and are present as much as 1 mile from the igneous center. Known rocks of the complex represent more than 15 stages and substages of igneous activity. All rocks are chemically and mineralogically similar and consist of essential quartz, K-feldspar, albite, and minor biotite. Clastic and fragmental textures are common features of the rocks exposed at surface, porphyries predominate at intermediate depths, and a granitic rock is present at the greatest depths reached in exploratory drilling. The major rock types of the complex exposed at the surface are columns that either grade into, or are cut off by, an underlying composite stock, the apex of which lies about 2,500 feet below the peak of Red Mountain. The stock, although much larger than the shallower columns, appears to be a steep-walled intrusive to explored depths. The space now occupied by the stock is thought to have been a magma chamber throughout the time the rocks exposed at the surface were emplaced; the magma chamber was their immediate source. The sequence of rocks exposed at surface is as follows: Tungsten Slide and East Knob units (relative ages unknown), Square quartz porphyry, rhyolite porphyry radial dikes, and the Red Mountain porphyry unit. Each unit consists of several members. Rock types which constitute the buried stock are, from oldest to youngest, the Urad porphyry, the Primos porphyry, and the Henderson granite. The Square quartz porphyry is the youngest known igneous rock predating the Urad orebody and is considered to have been the source intrusive for the Urad mineralized zone of which the Urad orebody is a part. The Urad mineralized zone is envisioned as a cylindrical body of mineralized rock. The orebody was localized within part of this zone by a series of moderately dipping arcuate fractures referred to as the "main fissure" system. The fissures are interpreted as cone sheet fractures caused by the emplacement of the Square quartz porphyry. About half of the orebody was contained in rocks of the Red Mountain Complex; the rest was in the Silver Plume Granite. The Urad mine, now worked out, produced about 14 million tons of ore. Intrusion of the Red Mountain porphyry, the principal rock of the complex at surface, destroyed much of the Square quartz porphyry, a large part of the Urad mineralized zone, and part of the Urad orebody. The magma from which the Red Mountain porphyry crystallized probably vented; a potential orebody may have been lost to the atmosphere during this event.The Henderson orebody is an umbrella-shaped stockwork entirely within the intrusive complex and contains more than 300 million tons of ore. The difference in the shape of the Urad mineralized zone and the Henderson orebody is thought to reflect the difference in the depth of formation. The Henderson deposit is genetically related to one or more phases of the Primos porphyry and to the Henderson granite and is a compound orebody. A master reservoir is postulated at depth beneath Red Mountain to explain the repetition of igneous and hydrothermal events and to provide the quantity of metal necessary for the formation of the ores. We believe that this magma derived its ore metal from crustal rocks anomalously rich in molybdenum during its rise toward the surface. The deposits at Red Mountain are remarkably similar in many respects to those at Climax, Colorado, but there are some notable differences. The abundance of Precambrian metamorphic rocks, possibly available for reaction with, or incorporation by, Oligocene magma at Climax, and the scarcity of these rocks at Red Mountain may account for the large amount of tungsten in the Climax deposit compared to that in the Urad and Henderson orebodies.

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