Mineral resources of the Comanche-Big South, Neota-Flat Top, and Never Summer Wilderness Study Areas, north-central Colorado
Year: 1981
Type: report
Abstract: A mineral survey was made of three areas in Roosevelt-Arapaho and Routt National Forests, Colorado, that have been proposed for inclusion in the National Wilderness Preservation System.The three areas, which are contiguous to Rocky Mountain National Park in north-central Colorado, are Comanche-Big South (68 mi ), Neota-Flat Top (12 mi ), and Never Summer (17 mi ).The mineral survey was accomplished in 1976 and 1977 as a cooperative investigation by the U.S. Bureau of Mines and the U.S. Geological Survey.The geology was mapped, 450 geochemical samples were collected and analyzed, and aeromagnetic and gravity maps were compiled and interpreted.Mining claims were plotted, and prospect pits, mine workings, and veins were mapped and sampled.The three study areas are alined in a northeast direction and extend from the Precambrian core of the northern Front Range southwest across the faulted west flank, where the Precambrian rocks have been thrust westward over strata of Mesozoic and Tertiary age.The Comanche-Big South area, in the central part of the Front Range, is composed of micaceous and hornblendic gneisses that have been intruded by granite to quartz diorite probably part of the 1.7-b.y.-old Rawah batholith and by 1.45-b.y.-old post-tectonic Silver Plume Quartz Monzonite and Sherman Granite.The gneissic rocks have been deformed by both folds and faults that trend mainly east-northeast to east.The gneisses continue to the southwest and are exposed in the southern part of the Neota-Flat Top area, most of which, however, is covered by andesitic to rhyolitic volcanic rocks that are as young as 28 m.y.old.Farther to the southwest, the Never Summer area is occupied largely by the upper plate of the Never Summer thrust, a thin sheet of Precambrian gneisses that overlies autochthonous sedimentary rocks.The western trace of the thrust lies just west of the study area, and windows eroded through the thrust expose Pierre Shale in the eastern part of the area.The thrust and the rocks above and below it have been arched and pierced by granodiorite and rhyolite-porphyry plutons, both about 28 m.y.old.The only evidence of possibly significant mineralization in the Comanche-Big South area is a uranium occurrence at the north edge of the area near Spencer Heights, where claims have been staked, a shaft dug, and numerous exploratory holes drilled since its discovery in the early 1950's.Although this activity resulted in insufficient ore-grade rock to justify opening a mine, the occurrence is significant in that it is localized in one of several major shear zones that cross the area.Each of these shear zones is a potential locus for uranium deposits, and thus each should be examined in more detail than was possible in this investigation.Except for common-variety materials, such as sand, gravel, building stone, feldspar, and quartz, no other mineral resources are known or suspected 4 in the Comanche-Big South area.The Neota-Flat Top area contains no mining claims, evidence of prospecting, or known mineralized rock.Geochemical analyses show that the rhyolitic volcanic rocks are abnormally high in molybdenum, tin, and zinc, but no evidence was found that these metals are concentrated in a recoverable form or grade.No mineral resources other than common-variety materials are known in the Neota-Flat Top area.The Never Summer area, in contrast to Comanche-Big South and Neota-Flat Top areas, is mineralized to a considerable extent, and the southern half is considered favorable for the occurrence of stockwork molybdenum deposits of the Climax type.Numerous small veins and innumerable mineralized fractures of the Teller mining district attest to a widespread mineralizing event believed related to the mid-Tertiary Mount Cumulus granite stock.The veins themselves probably contain only insignificant mineral resources.Geochemically, the veins and mineralized fractures are high in silver, arsenic, antimony, zinc, lead, molybdenum, fluorine, and barium.Most of these elements take the form of common sulfides, fluorite, and barite, but no silver mineral was identified.Stream-sediment geochemistry reinforces the conclusions based on rock analyses and on distribution of mineralized rocks.The most strongly mineralized part of the Never Summer area is the southeastern third, which is an extension of the main part of the Teller district in Jack Creek to the south.Specifically, the area around Mount Cindy is more iron stained and fractured than most and is anomalous in several metals.Another mineralized area is on the ridge at the three-way drainage divide separating South Fork Michigan River, Porcupine Creek, and Silver Creek, where the Precambrian gneisses have been silicified and intruded by a few small bodies of Tertiary porphyry.Samples of these silicified rocks are anomalous in silver, arsenic, and molybdenum.The Mount Cumulus stock is petrologically and geochemically similar to the porphyries associated with the Climax and Henderson molybdenum deposits.The rock is a rhyolite porphyry to equigranular granite that is finely fractured and weathers brown from oxidation of pyrite that was deposited in thin films along the fractures and to a lesser extent disseminated through the rock.Molybdenite is sparsely distributed along fractures and in miarolitic cavities.Fluorite is an abundant accessory, and ilmenorutile is common.Small intrusives of rhyolite, hornfelsing of the Tertiary sedimentary rocks, and geophysical evidence indicate that a stock similar to and perhaps continuous with the Mount Cumulus stock underlies at least the southeastern third of the Never Summer area, as well as adjacent areas, at a depth of less than 2 miles.Although coal and petroleum are being produced in North Park to the west and northwest of the Never Summer area from sedimentary units that may underlie the Never Summer thrust, it is doubtful that either commodity occurs within the Never Summer area.
DOI: 10.3133/ofr81578
Authors: R. C. Pearson, W. A. Braddock, V. J. Flanigan, L. L. Patten
Language: English
Subfield: Geochemistry and Petrology
Field: Earth and Planetary Sciences
Domain: Physical Sciences
Open Access status: bronze
OpenAlex ID: W1559929487