RE-OS SYSTEMATICS OF LöLLINGITE AND ARSENOPYRITE IN GRANULITE FACIES GARNET ROCKS: INSIGHTS INTO THE THERMAL EVOLUTION OF THE BROKEN HILL BLOCK DURING THE EARLY MESOPROTEROZOIC

Löllingite and euhedral arsenopyrite crystals are reported from granulite facies spessartine-almandine garnet rocks closely associated with the Pb- and Zn-sulfide orebodies at the Broken Hill deposit, Southern Curnamona Province (SCP), New South Wales, Australia [1,2]. Sulfide minerals comprise löllingite and coexisting arsenopyrite ± galena ± tetrahedrite interestitial to garnet crystals. Löllingite formed first whereas gold-bearing löllingite, now occurring as relicts in arsenopyrite, was destroyed to produce arsenopyrite ± gold microinclusions.

Standard mineral separation procedures produced pure separates of löllingite, arsenopyrite and arsenopyrite ± löllingite. Re-Os data of these mineral fractions show a very narrow range of ¹⁸⁷Re/¹⁸⁸Os (7 to 11) and ¹⁸⁷Os/¹⁸⁸Os ratios (0.8505 to 0.9650) but in ¹⁸⁷Os/¹⁸⁸Os vs. ¹⁸⁷Re/¹⁸⁸Os space define a Model 1 isochron (n = 14) with an age of 1534 ± 33 Ma (2σ; MSWD = 0.78, initial ¹⁸⁷Os/¹⁸⁸Os ratio of 0.672 ± 0.005). Os and Re contents are extremely high for all sulfide phases (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.

In the polydeformational and polymetamorphic history of the granulite facies rocks of the SCP, monazite grew from lower amphibolite facies at ca. 1657 Ma to granulite facies at ca. 1602 Ma [3]. The current age of sulfide mineralization implies a cooling rate of 3 to 4ºC/Ma in the SCP from ca. 780ºC (monazite precipitation) to ca. <550ºC and formation of arsenopyrite from the destruction of gold-bearing löllingite during late retrograde metamorphism [3, this study].

[1] Spry, P.G., Wonder, J.D. (1989) Can Miner 27, 275-292.

[2] Plimmer, I.R. (2006) Miner Petro 88, 443-478

[3]McFarlane, C.R.M., and Frost, B.R., (2009) J Metamorphic Geol 27, 3-17