A sulfate conundrum: Dissolved sulfates of deep-saline brines and carbonate-associated sulfates

作者：Labotka Dana M^*; Panno Samuel V; Locke Randall A

来源：Geochimica et Cosmochimica Acta, 2016, 190: 53-71.

DOI：10.1016/j.gca.2016.06.033

• Cambrian brines
• Illinois Basin
• Sulfate isotope geochemistry
• Carbonate-associated sulfate
• Mississippi Valley-type ore fluids

Sulfates in deeply circulating brines and carbonate-associated sulfates (CAS) within sedimentary units of the Cambrian strata in the Illinois Basin record a complex history. Dissolved sulfate within the Mt. Simon Sandstone brines exhibits average delta S-34(SO4) values of 35.4% and delta O-18(SO4) values of 14.6% and appears to be related to Cambrian seawater sulfate, either original seawater or sourced from evaporite deposits such as those in the Michigan Basin. Theoretical and empirical relationships based on stable oxygen isotope fractionation suggest that sulfate within the lower depths of the Mt. Simon brines has experienced a long period of isolation, possibly several tens of millions of years. Comparison with brines from other stratigraphic units shows the Mt. Simon brines are geochemically unique. Dissolved sulfate from brines within the Ironton-Galesville Sandstone averages 22.7% for delta S-34(SO4) values and 13.0% for delta O-18(SO4) values. The Ironton-Galesville brine has mixed with younger groundwater, possibly of Ordovician to Devonian age and younger. The Eau Claire Formation lies between the Mt. Simon and Ironton-Galesville Sandstones. The carbonate units of the Eau Claire and stratigraphically equivalent Bonneterre Formation contain CAS that appears isotopically related to the Late Pennsylvanian- Early Permian Mississippi Valley-type ore pulses that deposited large sulfide minerals in the Viburnum Trend/Old Lead Belt ore districts. The delta S-34(CAS) values range from 21.3% to 9.3%, and delta O-18(CAS) values range from + 1.4% to -2.6% and show a strong covariance (R-2 = 0.94). The largely wholesale replacement of Cambrian seawater sulfate signatures in these dolomites does not appear to have affected the sulfate signatures in the Mt. Simon brines even though these sulfide deposits are found in the stratigraphically equivalent Lamotte Sandstone to the southwest. On the basis of this and previous studies, greater fluid densities of the Mt. Simon brines may have prevented the less dense Mississippi Valley-type fluids from interacting with these deeply circulating brines. Progressive in situ quartz cementation that occurred in the Mt. Simon Sandstone contemporaneous to the ore pulses may also have precluded fluid migration. The Mt. Simon brines appear to be a mixture of evaporated connate Cambrian seawater, recirculating deepseated crystalline basement brines, and meteoric water.

• 出版日期2016-10-1