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University of Nevada Reno Geology and Geochemistry of the Broken University of Nevada Reno Geology and Geochemistry of the Broken Ridge Area, V Southern Wah Wah Mountains, Iron County, Utah A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science by Karen A. Duttweiler Hi April 1985 ■ H MINfci LIBRARY Thesis The thesis of Karen A. Duttweiler is approved: / ‘M i . University of Nevada Reno April 1985 ABSTRACT This study was undertaken to document the geologic, petrologic, and geochemical relationships of the tin-bearing rhyolitic lava flows and domes of the 12 m.y. old Steamboat Mountain Formation in the area of Broken Ridge. Early phases of volcanic activity produced a topaz- bearing rhyolite followed by eruption from many local centers of a crystal-poor rhyolite. The rhyolites contain high Si (>74%) and alkalies, and are enriched in Sn, Be, F, Nb, Li and Rb; Ca, Mg, Ti, P and Ba are low. These characteristics suggest that the rhyolites formed as highly differentiated magmas. Crystalline cassiterite and wood tin are abundant and widespread in heavy-mineral-concentrate samples. Some of the crystalline cassiterite occurs with specular hematite in veins within the rhyolite which formed either as a result of degassing during cooling of the rhyolite or from a later hydrothermal event. Reactivation along a 23 m.y. fault zone occurred after emplacement of the rhyolite that resulted in a series of high angle normal faults. Multiple hydrothermal events resulted in widespread alteration along the faults and concentration of Be, F, Sn, Nb, Mo, Cu, Zn, W, and Ba. i i i CONTENTS Page INTRODUCTION........................................................... ! Previous work..................................................... 3 Methods of study................................................. 3 Sample preparation............................................... 4 Sample analysis................................................. 5 Acknowledgements.................................................. 7 REGIONAL GEOLOGIC SETTING............................................ 9 Structural setting.............................................. 9 General stratigraphy............................................. 11 Oligocene volcanism........................................ 12 Miocene volcanism........................................... 13 Blawn Formation and associated mineralization........ 14 Steamboat Mountain Formation.......................... 17 GEOLOGY OF THE BROKEN RIDGE AREA..................................... 18 Stratigraphy..................................................... 18 Paleozoic sedimentary rocks (P, Paleozoic)................ 18 Needles Range Group; undivided (Tv, Oligocene).............. 18 Wah Wah Springs Tuff (Tw, Oligocene)................... 18 Lund Tuff (Tl, Oligocene)............................. 19 Isom Formation (Ti, Oligocene)............................. 19 Hornblende andesite (Tha, Miocene)......................... 19 Bauers Tuff Member of the Condor Canyon Formation (Tcb, Miocene)......................................... 19 Andesite (Ta, Miocene)...................................... 19 Silicic clastic rocks (Tt, Miocene)........................ 20 Vent breccia (Tbt, Miocene)................................ 22 Steamboat Mountain Formation (Tr, Miocene)................ 22 Glassy margin (Trv, Miocene).......................... 23 Green glass (Trg, Miocene)............................ 27 Gray rhyolite (Trs, Miocene).......................... 32 Rhyolite of Pine (Trp, Miocene)....................... 41 Alluvium and colluvium (Qac, quaternary).................. 43 Structural features............................................. 44 Rock alteration................................................. 46 Silicification............................................. 46 Argil lie alteration........................................ 48 Quartz-fluorite alteration................................. 49 Quartz-alunite alteration.................................. 50 GEOCHEMISTRY.......................................................... 52 Petrology of the Steamboat Mountain Formation.................. 52 Rock geochemistry............................................. 59 Bible Spring fault zone.................................... 66 Southern Broken Ridge Area................................. 74 Fourmile Wash.............................................. 75 Heavy-mineral-concentrate geochemistry.......................... 76 Distribution of barite and garnet.......................... 76 Distribution of Sn and cassiterite......................... 78 Tin mineralization.............................................. 80 DISCUSSION AND CONCLUSIONS........................................... 84 REFERENCES............................................................ 89 V APPENDIX A........................................................... 93 ILLUSTRATIONS Plate 1. Geologic map of the Broken Ridge area............... in pocket Figure 1. Map showing location of the study area................... 2 Figure 2. Map of southwest Utah and eastern Nevada showing faults and the extent of Miocene silicic rocks............................ 10 Figure 3. Map showing the locations of mines and prospect pits associated with the Blawn Formation............................. 15 Figure 4. Photomicrograph of the glassy margin (Trv)............... 26 Figure 5. Photomicrograph of the green glass (Trg)................. 31 Figure 6. Photograph of the gray rhyolite (Trs) showing steep flow-banding............................................................................................................ 34 Figure 7. Photograph of the gray rhyolite (Trs) with topaz crystals in groundmass........................................ 36 Figure 8. Microscopic views comparing topaz that is contained in groundmass and clear topaz contained in vugs................... 38 Figure 9. Photograph of the dome of Fourmile Wash.................. 39 Figure 10. Photograph showing the rhyolite of Pine (Trp) overlying the gray rhyolite (Trs)......................................... 42 Figure 11. Simplified geologic map showing areas of alteration..... 47 Figure 12. Photomicrographs of alunite in silicic clastic rocks..... 51 Figure 13. AFM and Na20-I<20-Ca0 ternary diagram for volcanic rocks in the southern Wah Wah Mountains.......................... 55 Figure 14. SiO2 variation diagrams of CaO, Na20 and 1^0 for volcanic rocks in the southern Wah Wah Mountains........... 56 vi Figure 15. Location map of rock samples from the Broken Ridge area............................................................. 60 Figure 16. Correlation diagrams of selected elements in volcanic rocks from the Broken Ridge area............................... 62 Figure 17. Histograms of Sn, Mo, and Be abundances in fresh versus altered rhyolite......................................... 64 Figure 18. Histograms of Nb, Ba and Cu abundances in fresh versus altered rhyolite................................................ 65 Figure 19. Distribution of Be and F in rock samples from the Broken Ridge area............................................... 67 Figure 20. Distribution of Sn and Nb in rock samples................ 68 Figure 21. Distribution of U and Th in rock samples................. 69 Figure 22. Distribution of Cu and Mo in rock samples................ 70 Figure 23. Distribution of Pb and Ba in rock samples................ 71 Figure 24. Heavy-mineral-concentrate sample location map with simplified geology.............................................. 77 Figure 25. Distribution of Sn and cassiterite in heavy-mineral concentrates..................................................... 79 Figure 26. Photomicrographs of cassiterite and hematite in vein in silicified rhyolite.......................................... 82 Figure 27. Schematic diagram showing the relationship between rhyolite tuffs and flows in the Broken Ridge area, and possible mineralization related to them......................... 86 TABLES Table 1. Limits of determination for the spectrographic analysis of stream sediments, heavy-mineral concentrates and rocks........ 6 vi i Table 2. Chemical methods used for selected rock samples............ 8 Table 3. Modal analysis of volcanic rocks in the Broken Ridge area............................................... 25 Table 4. Chemical composition of selected rhyolite samples from the Broken Ridge area........................................... 28 INTRODUCTION The geology and geochemistry of high-F or topaz-bearing rhyolites in Nevada and Utah have been of considerable interest in recent years in light of their structural and genetic implications and the moderate to high potential for Be, Sn, Mo, U, and F deposits that are commonly associated with them. Two such high-F rhyolites occur in the southern Wah Wah Mountains, a north-south trending mountain range in the eastern part of the Great Basin in southwest Utah (fig. 1). The presence of a 12 m.y. (million year) old topaz-bearing rhyolite and high Be, Sn, and Mo contents in stream sediment samples collected during a reconnaissance geochemical survey by Miller and others (1980) spurred interest in an area of the Mountain Spring Peak 7 1/2' quadrangle, Iron County, Utah. The area is located about 64 km northwest of Cedar City and 56 km southwest of Milford. Two major dirt roads give access to the area; the north-south Pine Valley road and the east-west county road or Jockey road. Other dirt roads requiring four- wheel drive vehicles make access to the area very good. Deposits of Be and Sn are commonly associated
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