Abstract

Replacement dolomites may continue to recrystallize in the presence of subsequent diagenetic and hydrothermal fluids. During recrystallization, they can change both their solid geochemistry and that of their included fluids (by releasing older fluids and entrapping younger fluids). Thus it may be possible to decipher a complex fluid history by analyzing both the composition of the dolomite (δ18O, 87Sr/86Sr) and its contained fluids (Cl/Br), as the former reflects temperature and/or fluid-rock interactions, whereas the latter reflects salinity source regions of the last fluid to affect the dolomite. Applying this approach to the Bonneterre Dolomite of southeast Missouri, which hosts the world-class Viburnum Trend Mississippi Valley-type (MVT) Pb-Zn district, reveals the presence of two geochemically distinct types of brines whose influence varied temporally during the Bonneterre’s history of dolomitization and ore deposition. A plot of Cl/Br molar ratios of included fluids versus 87Sr/86Sr ratios of the enclosing dolomite indicates distinct salinity sources for early (halite dissolution) and later (extreme evaporation of seawater) dolomitizing brines.

Comparison with fluids in ore sulfides may permit us to relate replacement dolomites to ore-forming events. Early-stage copper- and zinc-bearing sulfides (δ34S ~0‰) have fluids with Cl/Br molar ratios (150–1,050) reflecting a similar importance of halite-dissolving brine during ore deposition. Main-stage cuboctahedral galena (δ34S ~15‰) has fluids with Cl/Br molar ratios (<100–400) similar to extremely evaporated seawater brines responsible for late replacement dolomites. We suggest that resident brines, which gained salinity (in part through halite dissolution), sulfur, and metals from local sedimentary and igneous rocks, were important for early replacement dolomitization and Early-stage ore deposition. This fluid type was supplanted by later, Main-stage brines that acquired salinity (through seawater evaporation) and metals in a more distal, basinal setting. Late-stage, vug-filling cubic galena records a renewed importance of the halite-dissolving brine as the influence of the more distal basinal brine waned. Halogen-sulfur isotope systematics of sulfides indicate that fluid mixing likely was a dominant ore-depositing mechanism during all stages of ore mineralization in the Viburnum Trend.

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