UNITED STATES DEPARTMENT OF THE INTERIOR GEOLOGICAL SURVEY Mineral Resources of the Comanche-Big South, Neota-Flat Top, and Never Summer Wilderness Study Areas, North-Central Colorado By Robert C. Pearson, William A. Braddock, and Vincent J. Flanigan, U.S. Geological Survey and Lowell L. Patten, U.S. Bureau of Mines Open-File Report 81-578 1981 This report is preliminary and has not been edited or reviewed for conformity with U.S. Geological Survey standards. CONTENTS Page Summary ................................................................ 1 Introduction.............................................................. 3 Previous studies..................................................... 7 Present Investigation............ ................................... 7 Acknowledgments...................................................... 8 Geology ................................................................ 8 Rocks................................................................ 10 Precambrian rocks............................................... 10 Mesozoic sedimentary rocks...................................... 12 Coalmont Formation.............................................. 12 Tertiary volcanic rocks......................................... 12 Tertiary intrusive rocks........................................ 14 White Rlver(?) Formation........................................ 15 Structure............................................................ 15 Interpretation of geophysical data........................................ 16 Introduction......................................................... 16 Regional geophysical setting......................................... 16 Aeromagnetlc data.................................................... 17 Gravity data......................................................... 18 Conclusions.......................................................... 18 Mineral resources......................................................... 21 Geochemical studies.................................................. 23 Comanche-Big South study area........................................ 23 Mining claims and mineral prospects............................. 23 Interpretation of geochemical data.............................. 32 Conclusions..................................................... 33 Neota-Flat Top study area............................................ 34 Interpretation of geochemical data.............................. 34 Conclusions..................................................... 34 Never Summer study area.............................................. 34 Mining claims, mineral prospects, and recent exploration........ 34 Interpretation of geochemical data.............................. 45 Conclusions..................................................... 53 References................................................................ 55 Illustrations Page Plate 1. Geologic map of the Comanche-Big South, Neota-Flat Top, and Never Summer wilderness study areas, north-central Colorado................................................. In pocket 2. Geochemical sample locality map of the Comanche-Big South, Neota-Flat Top, and Never Summer wilderness study areas, Colorado............................................... In pocket 3. Aeromagnetic map of the Comanche-Big South, Neota-Flat Top, and Never Summer wilderness study area, Colorado....... In pocket Figure 1. Index map showing location of Comanche-Big South, Neota- Flat Top, and Never Summer wilderness study areas, Colorado..................................................... 4 2. Photograph of Mount Cumulus, Never Summer wilderness study area, showing cirque wall and scree-covered slopes..... 6 3. Generalized geology in the vicinity of the Comanche-Big South, Neota-Flat Top, and Never Summer study areas, Colorado....... 9 4. Complete Bouguer gravity map of the Comanche-Big South, Neota-Flat Top, and Never Summer areas....................... 19 5. Residual gravity map of the Comanche-Big South, Neota-Flat Top, and Never Summer study areas................. 20 6. Mining claims and Bureau of Mines sample localities near the Comanche-Big South and Neota-Flat Top study areas........ 24 7. Mining claims and prospects near Spencer Heights, Comanche-Big South study area................................ 25 8. Plan of adit near Poudre Falls................................. 31 9. View of north slope of upper Jack Creek valley................. 35 10. Patented mining claims, Teller mining district, Jackson County, Colorado............................................. 37 11. Unpatented mining claims and Bureau of Mines sample localities, Never Summer study area and vicinity, Jackson County, Colorado............................................. 38 12. Plan of the Bear Paw adit Never Summer study area, Jackson County, Colorado..................................... 39 13. Plan of adit No. 1, South Fork Michigan River, Never Summer study area............................................ 41 14. Plan of adit No. 2, South Fork Michigan River, Never Summer study area...................................... 42 15. Distribution of vein minerals at selected localities in the Never Summer study area, and adjacent areas.............. 43 16. Plan of adit southeast of Lake Agnes........................... 44 17-22. Geochemical maps, Never Summer study area, for: 17. Silver......................................................... 46 18. Lead........................................................... 47 19. Molybdenum..................................................... 48 20. Arsenic........................................................ 49 21. Citrate-soluble heavy metals................................... 50 22. Zinc........................................................... 51 Tables Page Table 1. Sedimentary rock units, Never Summer study area............... 13 2. Gold and silver assay values in the Comanche-Big South, Neota-Flat Top, and Never Summer study areas................ 27 3. Spectrographic and radiometric analyses of selected samples from the Comanche-Big South, Neota-Flat Top, and Never Summer study areas.......................................... 30 4. Analyses of selected rock samples, from the Comanche- Big South study area, Colorado.............................. 58 5. Analyses of selected stream-sediment samples from the Comanche-Big South study area, Colorado..................... 59 6. Analyses of selected rock samples from the Neota-Flat Top study area, Colorado........................................ 60 7. Analyses of selected stream-sediment samples from the Neota-Flat Top study area, Colorado......................... 62 8. Analyses of selected rock samples from the Never Summer study area, Colorado........................................ 63 9. Analyses of selected stream-sediment samples from the Never Summer study area, Colorado........................... 70 10. Semiquantitative Spectrographic analyses of samples of the Mount Cumulus stock......................................... 73 iii STUDIES RELATED TO WILDERNESS Study Areas In accordance with the provisions of the Wilderness Act (Public Law 88-577, September 3, 1964) and the Joint Conference Report on Senate Bill 4, 88th Congress, the U.S. Geological Survey and the U.S. Bureau of Mines have been conducting mineral surveys of wilderness and primitive areas. Studies and reports of all primitive areas have been completed. Areas officially designated as "wilderness," "wild," or "canoe" when the Act was passed were incorporated into the National Wilderness Preservation System, and some of them are presently being studied. The Act provided that areas under consideration for wilderness designation should be studied for suitability for incorporation into the Wilderness System. The mineral surveys constituted one aspect of the suitability studies. This report discusses the results of a mineral survey of some national forest lands in the Comanche-Big South, Neota- Flat Top, and Never Summer Wilderness Study Areas, north-central Colorado that are being considered for wilderness designation. Mineral Resources of the Comanche-Big South, Neota-Flat Top, and Never Summer wilderness study areas, north-central Colorado By Robert C. Pearson, William A. Braddock, and Vincent J. Flanigan, U.S. Geological Survey, and Lowell L. Patten, U.S. Bureau of Mines SUMMARY 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