The chemical and thermal evolution of the fluids in the Cave-in-Rock fluorspar district, Illinois; stable isotope systematics at the Deardorff Mine

Iowa State Univ., Dep. Earth Sci., Ames, IA, United States

The general paragenetic sequence for the Deardorff mine, and for much of the Cave-in-Rock district, is fluorite, sphalerite, galena, quartz, calcite, and barite, with fluorite spanning the sphalerite, galena, and quartz deposition. Liquid hydrocarbons are observed in fluid inclusions of most minerals but are most numerous in fluorite. Stable isotope data, along with previously published temperature and salinity data on fluid inclusions, place significant constraints on the origin of the fluids responsible for these minerals and on the nature of the hydrothermal system.Mineralization was dominated by two meteoric recharged formation fluids: a main-stage, saline (19 + or - 1 equiv wt % NaCl), high-temperature (145 degrees + or - 5 degrees C) fluid with a delta D (sub H ₂ O) value of -25 + or - 5 per mil and a late-stage, more dilute (1-9 equiv wt % NaCl), generally lower temperature (125 degrees + or - 25 degrees C) fluid with a delta D (sub H ₂ O) -40 + or - 10 per mil. The deuterium enrichment of the main-stage fluid most likely resulted from exchange between formation waters (delta D nearly equal -50ppm) and the basement rocks under low water/rock conditions. Data on secondary fluorite inclusions suggest that the dilute fluids were present in the upper stratigraphic levels of the system during main-stage mineralization. Mixing of dilute and saline fluids in the ore horizon apparently occurred during postore carbonate deposition but not during that portion of the main-stage sequence which occurs at the Deardorff mine. Mixing of two saline fluids appears to have occurred during quartz deposition. The ore and late-stage fluid delta ¹⁸ O (sub H ₂ O) values range from -1 to +2 per mil and 0 to 8 per mil, respectively, with both fluids showing a trend to larger values across the paragenetic sequence. This increase in delta ¹⁸ O (sub H ₂ O) values occurred in spite of a decrease in salinity and a general temperature decline of the late fluids; it probably resulted from continuous equilibration of the fluids with sedimentary carbonates.The delta ³⁴ S values of sulfides (4.0-8.9ppm for sphalerite) are among the lowest recorded for sulfides in Mississippi Valley-type deposits and probably reflect a significant contribution of H ₂ S from petroleum and possibly igneous or crustal sulfur from the basement. Main-stage galena and sphalerite are generally in sulfur isotope equilibrium at the temperatures of ore deposition. The delta ³⁴ S values of later barites range from about 57 to 103 per mil, with some crystals showing systematic increases from the core to the edge of the crystals. These values are completely out of equilibrium with those for earlier sulfides and are believed to be the largest recorded for naturally occurring sulfates. Sulfate delta ¹⁸ O values, however, range from only 19.6 to 20.8 per mil and were apparently in equilibrium with the hydrothermal fluids. These data suggest that the sulfate derived from a small fluid reservoir where it underwent thermal chemical reduction with organic matter. In a manner consistent with this involvement of organic matter in the late fluids, the delta ¹³ C (sub CO ₂ ) of the fluids decreased from about -3 to -20 per mil during the carbonate paragenesis.

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