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COUNTRY ROCK MONAZITE RESPONSE TO INTRUSION OF THE SEARCHLIGHT PLUTON, SOUTHERN NEVADA

COUNTRY ROCK MONAZITE RESPONSE TO INTRUSION OF THE SEARCHLIGHT PLUTON, SOUTHERN NEVADA. John C. Ayers, Scott Crombie, Calvin Miller, Yan Luo, Miranda Loflin Vanderbilt University . Objectives.

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COUNTRY ROCK MONAZITE RESPONSE TO INTRUSION OF THE SEARCHLIGHT PLUTON, SOUTHERN NEVADA

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  1. COUNTRY ROCK MONAZITE RESPONSE TO INTRUSION OF THE SEARCHLIGHTPLUTON, SOUTHERN NEVADA John C. Ayers, Scott Crombie, Calvin Miller, Yan Luo, Miranda Loflin Vanderbilt University

  2. Objectives • To study the response of wallrock monazite to contact metamorphism & magmatic fluid infiltration. • To identify the geologic process associated with monazite ages measured in-situ.

  3. Why study contact metamorphic aureoles • Have better geologic control than regional metamorphism (small scale, simple geometry) • Protolith compositions generally available • Transects allow evaluation of effects of continuous changes in metamorphic grade • Fluid fluxes and peak temperatures vary systematically in relation to the contact

  4. Searchlight Pluton Panoramic Big Granite Mtn. Ireteba Peaks Copper Mtn.

  5. Searchlight/Ireteba plutons • Located in the Eldorado Mountains of Southern Nevada • Tilted to expose deeper portions of the pluton

  6. Cu, Au, and Ag ore deposits in roof zone • Two lithologies in the wallrock of the SL pluton: • Ireteba granite • Proterozoic gneiss • Lithologies contain monazite and sericitization • Metamorphism at 250-400°C and ~0.15-0.4 GPa. • Focus on transects. IR20 IR1

  7. Comparison of country rock and Searchlight granite intrusion

  8. Ireteba Whole Rock Stable Isotopes SL

  9. IMP208Pb/232Th age d18OSMOW Ireteba Granite: Monazite Zoning & AnalysisSpots

  10. Ireteba granite monazite ages

  11. Ireteba Monazite Ages

  12. 208Pb/232Th age d18OSMOW Ireteba Granite: Sample IR1

  13. Ireteba Granite: Sample IR20

  14. Gneiss Whole Rock Stable Isotopes

  15. Proterozoic Gneiss Monazite LA-ICP-MS analysis pits and EMP analysis spots labeled with ages in Ma with 1σ errors.

  16. Gneiss EMP monazite ages

  17. Proterozoic Gneiss Monazite

  18. Gneiss wallrock zircon grains

  19. Gneiss Wallrock Monazite and Zircon

  20. Conclusions • Monazite in deep wallrocks (Ireteba granite) partially recrystallized/reset and developed patchy zoning in response to Searchlight intrusion at 16.5 Ma. • No good evidence that fluids were responsible for monazite recrystallization – perhaps it was strain? • Monazite in shallow wallrocks (Proterozoic gneiss) had preexisting patchy zoning and lost some Pb in response to Ireteba intrusion at 65 Ma, but were unaffected by Searchlight intrusion. • Monazite is absent from Proterozoic Gneiss in roof zone samples, suggesting that high fluid fluxes that formed hydrothermal ore deposits destroyed monazite. • Focusing of fluids in roof zone prevented development of contact metamorphic aureole and monazite recrystallization on pluton flanks.

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