RE-OS DATING OF ARSENOPYRITE: PANACEA OR PROBLEMATIC?

Dating of ore and ore-related minerals using Re-Os has included exploration of the Re-Os systematics in arsenopyrite (FeAsS). This mineral is of particular interest because it is commonly associated with Au deposits and may be directly associated with Au deposition. Re and Os concentrations in arsenopyrite fall in the ppb to ppt range, respectively, and extremely high Re/Os ratios permit single sample dating. Sulfides with this Re-Os isotopic character are known as LLHR (Low-Level, Highly-Radiogenic; Stein et al., 2000, Economic Geology). Early success in Re-Os dating of arsenopyrite, as a means to determine the age of Au mineralization at Bendigo-Australia (Arne et al., 2001, Economic Geology), Meguma-Nova Scotia (Morelli et al., 2005, Economic Geology), and Muruntau-Uzbekistan (Morelli et al., in press, Geology), gave good reason to believe that arsenopyrite was the panacea for dating Au deposits. These deposits, however, are all of Phanerozoic age, whereas much of the world's Au is found in Archean rocks.

We dated arsenopyrite from the Pular-Parsori Au deposits in the Archean Sakoli Belt of central India. The Au-bearing veins occur within shear zones showing evidence of reworking associated with the Central Indian Tectonic Zone, traceable across the Indian peninsula. The veins occupy the interfingering contact between a tuffaceous metarhyolite and schists. Re concentrations (3-23 ppb) are similar to those of previous studies. The Os isotopic compositions, however, indicate significant common Os, although some samples were strongly LLHR. Surprisingly, Re-Os ages scatter between 190-100 Ma, suggesting that Re-Os systematics were disturbed in the Jurassic-Lower Cretaceous, possibly in association with Cimmerian rifting prior to India's accretion to southern Asia.

Early Re-Os studies utilizing arsenopyrite showed great promise. Although we report an exception to this trend, it is important to keep in mind that there is no set list of minerals that are “good or bad” for Re-Os chronology. All samples have a unique geologic history that includes their initial crystallization and response at the crystallographic scale to subsequent thermal and hydrothermal events. Geologic history and all its associated uncertainty is a critical component when evaluating any isotopic data set.

Keywords: Re-Os, arsenopyrite, Pular-Parsori, India, Cimmerian