ZN-CU-NI-CO-RICH ORES AT THE BONNETERRE DOLOMITE-LAMOTTE SANDSTONE CONTACT, VIBURNUM TREND MVT DISTRICT, MISSOURI: PRODUCTS OF MULTIPLE SULFUR SOURCES AND METAL-SPECIFIC FLUIDS

An unusual Zn-Cu-Ni-Co-rich orebody of the Brushy Creek mine occurs more than 30 m below the main ore-bearing horizon of the Viburnum Trend district. It is related to early zones of fracture-enhanced permeability above the Lamotte Sandstone that promoted extensive dissolution of the lower Bonneterre Dolomite. Ores are composed mostly of early Zn- and Cu-bearing sulfides overprinted by Pb-Zn mineralization. Distinct Ni- and Ni-Co-rich areas and variability of sphalerite generations across the orebody indicate multiple, temporally variable sites of ore fluid introduction. S isotope studies were undertaken to assess whether the ores reflect a single, regional fluid that evolved from Cu-Ni-Co-rich to Zn-rich to Pb-rich or are instead products of distinct ore-forming events unrelated to the main Pb-rich ores of the district.

The δ³⁴S values of ore minerals (early pyrite and chalcopyrite, -7 to +5‰; early sphalerite +6 to +15‰; main sphalerite, +7 to +17‰) are consistent with deposition from fluids that utilized isotopically distinct sulfide reservoirs. Low-δ³⁴S sulfur sources for early ores likely included sulfide in local brines within the Lamotte Sandstone and diagenetic sulfide minerals within the basal units of the Bonneterre Dolomite. A trend of increasing δ³⁴S values of ore sulfides (from -5 toward +17‰) with vertical distance above the Lamotte/Bonneterre contact indicates that as the ore fluid system worked its way upward, it breached less permeable units in the lower Bonneterre, allowing incorporation of high-δ³⁴S sulfide from brines present higher in the stratigraphic section (like that recorded in Pb-Zn ores of the main mineralized horizon of the mine, +13 to +18‰). Banded sphalerite in the upper portion of the orebody reveals repetition of δ³⁴S values from +12 toward -5‰ within successive zones, indicating that as metal-rich fluid pulses exhausted one sulfide source, they utilized other local sources.

The episodic nature of ore introduction in the lower section orebody points to a system in which fluid mixing would have been highly variable, both temporally and spatially. Reaction path models require mixing of multiple, metal-specific and sulfide-bearing fluids in order to form the orebody. The sequence and concentrations of metal sulfides are inconsistent with a single, evolving metal-bearing fluid.