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    U .S. G

    eological Survey B ulletin 2194

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    149 28< 0.1 Recoil

    40Ar/39Ar AGE SPECTRA FOR JERRITT CANYON ILLITES

    Altered Roberts Mountains Formation

    Size (µm) 39Ar-recTg%

    U.S. Department of the Interior U.S. Geological Survey

    U.S. Geological Survey Bulletin 2194

    Argon Thermochronology of Mineral Deposits—A Review of Analytical Methods, Formulations, and Selected Applications

  • Argon Thermochronology of Mineral Deposits—

    A Review of Analytical Methods, Formulations, and

    Selected Applications

    By Lawrence W. Snee

    U.S. Geological Survey Bulletin 2194

    U.S. Department of the Interior U.S. Geological Survey

  • U.S. Department of the Interior Gale A. Norton, Secretary

    U.S. Geological Survey Charles G. Groat, Director

    Version 1.0, 2002

    This publication is only available online at: http://pubs.usgs.gov/bul/b2194/

    Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply endorsement by the U.S. Government

    Manuscript approved for publication August 23, 2002

    Published in the Central Region, Denver, Colorado

    Graphics by L.W. Snee

    Photocomposition by Norma J. Maes

    Edited by L.M. Carter

  • Contents

    Abstract ................................................................................................................................................. 1

    Introduction........................................................................................................................................... 1

    Acknowledgments .............................................................................................................................. 2

    Basis of the 40Ar/39Ar Technique ....................................................................................................... 2

    Formulations of the Method........................................................................................................ 2

    Standards ....................................................................................................................................... 4

    Thermal Release Experiments and Age Spectra .................................................................... 5

    Error Analysis ................................................................................................................................ 8

    39Ar/37Ar (Apparent K/Ca) Diagrams........................................................................................... 9

    Correlation Diagrams ................................................................................................................... 9

    Argon Loss ..................................................................................................................................... 10

    Extraneous Argon ......................................................................................................................... 11

    39Argon Recoil................................................................................................................................ 11

    Closure Temperature and Diffusion........................................................................................... 12

    Some Minerals Useful for 40Ar/39Ar Geochronology....................................................................... 14

    Amphiboles .................................................................................................................................... 14

    White Micas .................................................................................................................................. 16

    Brown Micas ................................................................................................................................. 18

    Illite.................................................................................................................................................. 18

    Alkali Feldspar ............................................................................................................................... 19

    Low-Temperature Potassium Feldspar.............................................................................. 20

    Sanidine and Anorthoclase ................................................................................................. 20

    Adularia................................................................................................................................... 20

    Plagioclase .................................................................................................................................... 20

    Whole Rocks.................................................................................................................................. 21

    Alunite and Jarosite ..................................................................................................................... 21

    Manganese Oxides....................................................................................................................... 22

    Others .............................................................................................................................................. 22

    Standard Analytical Techniques ....................................................................................................... 23

    Argon Thermochronology Applied to Mineral Deposits................................................................ 24

    General Strategy ........................................................................................................................... 24

    Some Examples ............................................................................................................................. 24

    Panasqueira, Portugal, Tin-Tungsten Deposit .................................................................. 24

    Cornubian Batholith and Associated Mineral Deposits, Southwest England ............. 26

    Red Mountain Intrusive System and Associated Urad-Henderson ..............................

    Molybdenum Deposits, Colorado ..................................................................................... 28

    Eastern Goldfields Province, Western Australia .............................................................. 31

    References Cited ................................................................................................................................. 32

    Figures

    1. Diagram showing typical 40Ar/39Ar age spectrum................................................................... 8

    2. Diagram showing typical 39ArK/37ArCa spectrum...................................................................... 9

    3. Diagrams of age and K/Ca spectra, and back-scattered electron image of

    Alaskan white mica sample ....................................................................................................... 10

    4. Hypothetical isochron diagram and hypothetical inverse correlation diagram................ 12

    5. Age and isochron diagrams and chemical plot for Brooks Range, Alaska,

    white micas ................................................................................................................................... 13

    III

  • 6. Graphs of examples of Turner’s theoretical argon-loss diagrams.............................. 14

    7. Graph of Harrison’s (1981) hornblende argon-loss behavior ....................................... 14

    8. Diagram showing 40Ar/39Ar age spectrum for a sample with excess 40Ar ................. 15

    9. Diagram showing 40Ar/39Ar age spectrum for a sample exhibiting 39Ar recoil .......... 15

    10. Graph showing example of a cooling curve for a mineralized area in Idaho............ 16

    11. Diagrams and drawing of complex argon systematics of a Precambrian

    amphibole from northern Pakistan ................................................................................... 16

    12. Back-scattered electron images of a white mica grain showing complexities of

    metamorphism and deformation that can affect argon systematics .......................... 17

    13. Diagram of a typical age spectrum for a biotite sample that contains

    some chlorite ........................................................................................................................ 18

    14. Diagram showing age spectra for size fractions of illite from

    Jerritt Canyon, Nev. ....................