Sulfur isotope signals in molybdenite - a persistent message from the past

Trace elements in ore deposits, gleaned from minerals or isolated inclusions, are used by economic geologists to speculate on source and to model ore-forming processes. As well, sulfur isotope data are often obtained for sulfide minerals, but the interpretation of these isotopic data for understanding ore genesis has been thin - often simplistically concluding a “mantle source” or “crustal source”. With starting parameters for ore-bound metals that may include magmatic-hydrothermal components from lower crust with its own initial biogenic and redox history, subseqeuntly compromised by metamorphic processes, just about any source can be invoked to explain a measured range of sulfur isotopic data. Added constraints are essential. The sulfur isotopic composition of pyrite in magmatic-hydrothermal systems ranges widely, from negative to positive per mil values, as is also the case for other ore-forming sulfides of Cu, Pb, and Zn. In contrast, from Paleoarchean to Recent, the sulfur isotopic composition of molybdenite (molybdenum disulfide) is eerily consistent and decisively positive (δ34S commonly +2 to +6 per mil). This requires either an unwavering source for sulfur in all magmatic-hydrothermal molybdenites, and/or a template for a process that has been perfectly reproduced throughout earth history. A sedimentary sulfur source would have to be extraordinarily unique through time to fit the bill. Molybdenite, a common mineral in magmatic-hydrothermal ore deposits, is clearly tied to the magmatic system, occurring as disseminations and/or generations of veins spatially associated with evolved intrusive phases. Thus, the metals have traveled a finite distance from their source, forcing reliance on geochemical tracers to elucidate controlling processes. Molybdenum isotope ratios in molybdenite from some individual deposits span a large range of values, comparable to the range observed in all rock types. Rather than revealing source, the Mo isotopes reflect Rayleigh fractionation processes during episodes of late-magmatic volatile release that create quartz-molybdenite veins [1]. Similar fractionation cannot occur in sulfur isotopes; instead, the constant sulfur isotope ratios reflect quantitative reduction of magmatic SO2 to reduced sulfur species in hydrothermal fluids. Here we postulate on the starting conditions and the oxidative-reductive hydrothermal journey taken by molybdenum and sulfur - seemingly along unique paths - prior to their precipitation as the disulfide molybdenite. While scientific research is often pointed toward exploring inconsistencies and perturbations in the geologic record, just as important is to discover and explain the remarkable consistencies maintained over geologic time. [1] Hannah, J.L., Stein, H.J., Wieser, M.E., de Laeter, J.R., and Varner, M. (2007) Mo isotope variations in molybdenite: Vapor transport and Rayleigh fractionation of Mo, Geology 35: 703-706.