Re-Os SYSTEMATICS OF LOELLINGITE AND ARSENOPYRITE IN GRANULITE-FACIES GARNET ROCKS: INSIGHTS INTO THE METAMORPHIC HISTORY AND THERMAL EVOLUTION OF THE BROKEN HILL BLOCK DURING THE EARLY MESOPROTEROZOIC (NEW SOUTH WALES, AUSTRALIA)

  • Author(s) / Creator(s)
  • Loellingite and arsenopyrite aggregates occur in spessartine-almandine garnet rocks (garnetite) metamorphosed to granulite facies, which are spatially associated with Pb-Zn-Ag mineralization in the giant Broken Hill deposit, southern Curnamona Province, New South Wales, Australia. Sulfarsenide and sulfide minerals comprise loellingite and coexisting arsenopyrite 6 galena 6 tetrahedrite that occur interstitial to garnet crystals. Loellingite formed first while gold-bearing loellingite, which occurs as rare relicts in arsenopyrite, was destroyed to produce arsenopyrite 6 detectable micro-inclusions of invisible gold. Standard mineral separation procedures produced pure separates of loellingite, arsenopyrite, and mixtures of arsenopyrite and loellingite and loellingite and arsenopyrite. In a plot of 187Re/188Os versus 187Os/188Os, samples of loellingite and loellingite and arsenopyrite have 187Re/188Os ratios between 6.87 and 7.40 and 187Os/188Os ratios between 0.8506 and 0.8651, whereas arsenopyrite and arsenopyrite and loellingite samples have higher 187Re/188Os ratios (7.14 to 11.32) and more radiogenic 187Os/188Os ratios (0.8828 to 0.9654). Thirteen analyses of arsenopyrite and arsenopyrite and loellingite define a Model 1 isochron with an age of 1574 +/- 38 Ma (2s; MSWD = 1.4, initial 187Os/188Os ratio of 0.666 +/- 0.006), whereas the five loellingite and loellingite and arsenopyrite samples define a Model 1 isochron with an age of 1707 +/- 290 Ma (2s; MSWD = 0.32, initial 187Os/188Os ratio of 0.652 6 0.036) that is indistinguishable from the arsenopyrite age. Rhenium and Os contents are extremely high for loellingite and arsenopyrite (Re = 120–475 ppb; Os = 65–345 ppb), likely as a result of concentration of Re and Os in these minerals during granulite-facies metamorphism from the inferred exhalite protolith. Petrographic observations combined with the Model 1 Re-Os ages and literature SHRIMP U-Pb ages of monazite in garnetite suggest that arsenopyrite formed on the retrograde path at the expense of loellingite. Cooling from peak Olarian P-T conditions (~800 8C at 1602 Ma) to at least 550C (first temperature of stability of arsenopyrite) at ca. 1574 Ma occurred at a rate of ~9C/Myr, which is similar to the rate of cooling determined for previously published SHRIMP U-Pb ages from successive monazite generations (McFarlane & Frost 2009). These results are consistent with the late phase of retrograde metamorphism that began between ca. 1590 and 1575 Ma.

  • Date created
    2019-01-09
  • Subjects / Keywords
  • Type of Item
    Article (Draft / Submitted)
  • DOI
    https://doi.org/10.7939/R3MS3KH9M
  • License
    Attribution-NonCommercial 4.0 International