Mineralium Deposita

, Volume 52, Issue 4, pp 463–470

Coarse muscovite veins and alteration deep in the Yerington batholith, Nevada: insights into fluid exsolution in the roots of porphyry copper systems

  • Simone E. Runyon
  • Matthew Steele-MacInnis
  • Eric Seedorff
  • Pilar Lecumberri-Sanchez
  • Frank K. Mazdab
Letter

DOI: 10.1007/s00126-017-0720-1

Cite this article as:
Runyon, S.E., Steele-MacInnis, M., Seedorff, E. et al. Miner Deposita (2017) 52: 463. doi:10.1007/s00126-017-0720-1
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Abstract

Veins and pervasive wall-rock alteration composed of coarse muscovite±quartz±pyrite are documented for the first time in a porphyritic granite at Luhr Hill in the Yerington District, Nevada. Coarse muscovite at Luhr Hill occurs at paleodepths of ~6–7 km in the roots of a porphyry copper system and crops out on the scale of tens to hundreds of meters, surrounded by rock that is unaltered or variably altered to sodic-calcic assemblages. Coarse muscovite veins exhibit a consistent orientation, subvertical and N-S striking, which structurally restores to subhorizontal at the time of formation. Along strike, coarse muscovite veins swell from distal, millimeter-thick muscovite-only veinlets to proximal, centimeter-thick quartz-sulfide-bearing muscovite veins. Crosscutting relationships between coarse muscovite veins, pegmatite dikes, and sodic-calcic veins indicate that muscovite veins are late-stage magmatic-hydrothermal features predating final solidification of the Luhr Hill porphyritic granite. Fluid inclusions in the muscovite-quartz veins are high-density aqueous inclusions of ~3–9 wt% NaCl eq. and <1 mol% CO2 that homogenize between ~150 and 200 °C, similar to fluid inclusions from greisen veins in Sn-W-Mo vein systems. Our results indicate that muscovite-forming fluids at Luhr Hill were mildly acidic, of low to moderate salinity and sulfur content and low CO2 content, and that muscovite in deep veins and alteration differs in texture, composition, and process of formation from sericite at shallower levels of the hydrothermal system. Although the definition of greisen is controversial, we suggest that coarse muscovite alteration is more similar to alteration in greisen-type Sn-W-Mo districts worldwide than to sericitic alteration at higher levels of porphyry copper systems. The fluids that form coarse muscovite veins and alteration in the roots of porphyry copper systems are distinct from fluids that formed copper ore or widespread, shallower, acidic alteration. We propose that this style of veins and alteration at Luhr Hill represents degassing of moderate volumes of overpressured hydrothermal fluid during late crystallization of deep levels of the Yerington batholith.

Keywords

Hydrothermal alteration Porphyry copper Yerington Greisen Muscovite 

Supplementary material

126_2017_720_MOESM1_ESM.docx (51 kb)
ESM 1(DOCX 51 kb)

Funding information

Funder NameGrant NumberFunding Note
Geological Society of America
    Society of Economic Geologists

      Copyright information

      © Springer-Verlag Berlin Heidelberg 2017

      Authors and Affiliations

      • Simone E. Runyon
        • 1
      • Matthew Steele-MacInnis
        • 1
      • Eric Seedorff
        • 1
      • Pilar Lecumberri-Sanchez
        • 1
      • Frank K. Mazdab
        • 1
      1. 1.Lowell Institute for Mineral Resources, Department of GeosciencesUniversity of ArizonaTucsonUSA

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