FLUID INCLUSION HALOGEN-SULFUR ISOTOPE SYSTEMATICS IN THE VIBURNUM TREND MVT DISTRICT, MO: EVIDENCE OF FLUID MIXING THROUGHOUT CU-ZN-PB SULFIDE ORE DEPOSITION

Sulfur isotope compositions of ore minerals from the Viburnum Trend MVT district of southeast Missouri display large variations both temporally and spatially. Sulfides enriched in ³⁴S (d³⁴S ~ 15‰) indicate introduction of sulfate sulfur from basinal sources, whereas more ³⁴S-depleted sulfides (d³⁴S ~ 0‰) may reflect sulfur derived from local sources, i.e. underlying sandstone and/or granitic basement. Metal-specific sulfides (Cu, Zn, Pb) appear to have distinct sulfur isotope signatures. Several questions arise: (1) Do specific metal-depositing fluids with distinct d³⁴S values represent different fluid sources? or (2) Do they represent the same fluids that encountered different metal and sulfur source rocks? And (3) What do the answers to the first two questions imply about the mechanism(s) of ore deposition?

To address these questions, >50 sulfide mineral samples whose d³⁴S values exhibit the broadest range were selected for halogen analysis of their inclusion fluids. Minerals throughout the complete ore paragenesis for the district have the following values: Early bornite-chalcopyrite pods, massive chalcopyrite and sphalerite (d³⁴S=0 to 4‰, Cl/Br=100 to 1000); Main Stage octahedral galena (d³⁴S=10 to 16‰, Cl/Br=100 to 300); Late cubic galena (d³⁴S=3 to 14‰, Cl/Br=70 to 1000).

Early (chalcopyrite) and Main Stage (octahedral galena) sulfides exhibit similar d³⁴S-Cl/Br systematics. Although each stage has a distinct, relatively constant d³⁴S signature, the Cl/Br ratios of both stages require mixing of multiple fluids: one fluid generated by seawater evaporation and a second that derived salinity from dissolution of halite. Main Stage (octahedral galena) deposition appears to be dominated by the evaporated seawater end-member (in agreement with Crocetti and Holland, 1989). Late Stage cubic galena is unique. A d³⁴S-Cl/Br plot of its values shows an asymptotic relationship, indicating mixing of two fluids: an evaporated seawater fluid with a d³⁴S value of ~ 15‰ and a halite-dissolution brine with a d³⁴S value of ~ 2‰. The shape of the mixing curve may reflect different concentrations of sulfur relative to salinity in the end-member fluids. In summary, halogen-sulfur isotope systematics of sulfides indicate that fluid mixing likely was responsible for all stages of ore mineralization in the Viburnum Trend.