Re-Os Systematics of Löllingite 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)

Abstract

Abstract Lollingite 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 lollingite and coexisting arsenopyrite ± galena ± tetrahedrite that occur interstitial to garnet crystals. Lollingite formed first while gold-bearing lollingite, 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 lollingite, arsenopyrite, and mixtures of arsenopyrite ± lollingite and lollingite ± arsenopyrite. In a plot of 187 Re/ 188 Os versus 187 Os/ 188 Os, samples of lollingite and lollingite ± arsenopyrite have 187 Re/ 188 Os ratios between 6.87 and 7.40 and 187 Os/ 188 Os ratios between 0.8506 and 0.8651, whereas arsenopyrite and arsenopyrite ± lollingite samples have higher 187 Re/ 188 Os ratios (7.14 to 11.32) and more radiogenic 187 Os/ 188 Os ratios (0.8828 to 0.9654). Thirteen analyses of arsenopyrite and arsenopyrite ± lollingite define a Model 1 isochron with an age of 1574 ± 38 Ma (2σ; MSWD = 1.4, initial 187 Os/ 188 Os ratio of 0.666 ± 0.006), whereas the five lollingite and lollingite ± arsenopyrite samples define a Model 1 isochron with an age of 1707 ± 290 Ma (2σ; MSWD = 0.32, initial 187 Os/ 188 Os ratio of 0.652 ± 0.036) that is indistinguishable from the arsenopyrite age. Rhenium and Os contents are extremely high for lollingite 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 lollingite. 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.