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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)
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- Author(s) / Creator(s)
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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
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- Subjects / Keywords
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- Type of Item
- Article (Draft / Submitted)