Abstract

Löllingite 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 löllingite and coexisting arsenopyrite ± galena ± tetrahedrite that occur interstitial to garnet crystals. Löllingite formed first while gold-bearing löllingite, which occurs as rare relicts in arsenopyrite, was destroyed to produce arsenopyrite ± detectable micro-inclusions of invisible gold.

Standard mineral separation procedures produced pure separates of löllingite, arsenopyrite, and mixtures of arsenopyrite ± löllingite and löllingite ± arsenopyrite. In a plot of 187Re/188Os versus187Os/188Os, samples of löllingite and löllingite ± 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 ± löllingite 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 ± löllingite define a Model 1 isochron with an age of 1574 ± 38 Ma (2σ; MSWD = 1.4, initial 187Os/188Os ratio of 0.666 ± 0.006), whereas the five löllingite and löllingite ± arsenopyrite samples define a Model 1 isochron with an age of 1707 ± 290 Ma (2σ; MSWD = 0.32, initial 187Os/188Os ratio of 0.652 ± 0.036) that is indistinguishable from the arsenopyrite age. Rhenium and Os contents are extremely high for löllingite 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 löllingite.

Cooling from peak Olarian P-T conditions (∼800 °C at 1602 Ma) to at least 550 °C (first temperature of stability of arsenopyrite) at ca. 1574 Ma occurred at a rate of ∼9 °C/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.

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