Kevin L. Shelton Zoned saddle dolomite crystals Research
 

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Replacement Dolomites as Recorders of Multiple Fluids and Fluid Sources: Halogen-87Sr/86Sr-δ18O Systematics of the Bonneterre Dolomite (Cambrian) of Southeast Missouri, USA

Kevin L. Shelton (SheltonKL@missouri.edu)1, Aaron W. Johnson1, Ry E. Stone1, Benjamin C. Gill1 & Jay M. Gregg (greggjay@umr.edu)2

1Dept. Geological Sciences, Univ. of Missouri-Columbia, Columbia, MO 65211 USA
2Dept. of Geology and Geophysics, Univ. of Missouri-Rolla, Rolla, MO 65409 USA

Replacement dolomites may continue to recrystallize in the presence of a later fluid(s). During recrystallization, these dolomites 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 chemistry of the dolomite (δ18O, Sr, 87Sr/86Sr) and its contained fluids (Cl/Br), as the former reflects temperature and fluid/rock interactions, whereas the latter reflects salinity source regions (fluid sources).

The Bonneterre Dolomite (Cambrian) of southeast Missouri hosts the world-class Viburnum Trend MVT Pb-Zn district. Previous studies of the Bonneterre concluded that its basal replacement dolomite formed by reaction with multiple brines and that fluid mixing was a dominant depositional mechanism for its MVT ores. However, the nature of the dolomitizing and ore-forming fluids remains unresolved. Although there are systematic spatial patterns of Fe, Mn and Sr concentrations within the basal dolomite, there are no corresponding patterns of 87Sr/86Sr or δ18O values. It appears that a large volume of dolomitized rock (87Sr/86Sr = 0.7095-0.7150, 30-50 ppm Sr, δ18O ~ -8‰ PDB) was affected by similar Late dolomitizing fluids at nearly constant temperatures (~120°C). There are, however, areas characterized by higher Sr contents (150-700 ppm), higher δ18O values (-6 to -3‰) and lower 87Sr/86Sr values (0.7090-0.7105) that represent Early dolomites unaffected by Late dolomitizing fluids.

A plot of δ18O vs. 87Sr/86Sr values defines an L-shaped trajectory between an Early dolomite end-member (-3‰, 0.7090) and a Late dolomite (-8‰, 0.7150). A plot of Cl/Br ratios of included fluids vs. 87Sr/86Sr ratios of the enclosing dolomite forms a similar L-shaped trajectory, indicating distinct salinity sources for Early (Cl/Br = 650-1700, halite dissolution) and Late (Cl/Br = 250-500, extreme evaporation of seawater) dolomitizing brines. Comparison with fluids in ore sulfides may allow us to link replacement dolomites to ore-forming events. Early Cu-Zn-bearing sulfides (δ34S~3‰) have fluids with Cl/Br ratios (150-1100) reflecting a similar dominance of a halite-dissolving brine during ore-fluid mixing. Main-stage galena (δ34S~15‰) has fluids with Cl/Br ratios (100-250) remarkably similar to extremely evaporated seawater brines responsible for Late replacement dolomites. Thus, we are able to fingerprint geochemically two main brines whose influence varied both spatially and temporally during the Bonneterre's history of dolomitization and ore deposition: A resident brine that gained metals, S and salinity from local sedimentary and igneous rocks was responsible for Early replacement dolomitization and ore deposition, and was displaced by a Late brine that acquired metals, S and salinity in a basinal setting.


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