REACTIVE TRANSPORT MODELING OF THE ROLE OF A F-RICH, ACIDIC BRINE IN THE FORMATION OF FLUORITE- RICH ORE BODIES IN THE ILLINOIS-KENTUCKY MVT DISTRICT

The Illinois-Kentucky (IL-KY) Mississippi Valley-type (MVT) district contains the largest fluorite deposits in the United States. Unlike typical MVT deposits that are dominated by sphalerite, galena, or barite the IL-KY deposits are dominated by fluorite with minor amounts of other ore minerals. The predominant hypothesis regarding the formation of the IL-KY deposits is that the ore fluids were anomalously enriched in F that was derived from magmatic fluids exsolved during the emplacement of ultramafic bodies in the district. A longstanding conceptual model for IL-KY ore formation is that exsolved magmatic fluids mixed with sedimentary brine at depth to produce ore fluids with elevated F and low pH. The resultant acidic, F-rich brines then ascended through faults located throughout the district until reacting with shallower limestone host rocks to form fluorite ore bodies. This conceptual model is supported by recent fluid inclusion studies that indicate that ore fluids in the IL-KY district had F concentrations between about 680 and 4300 ppm. Given the high Ca concentrations of the ore fluids, such elevated F concentrations would have required the ore fluids to have had very low pH values in the range of 0 to 1.4. The current study was undertaken with the goal of evaluating the implications of such an acidic, F-rich fluid for the formation of IL-KY style ore bodies using numerical reactive transport modeling. The models indicate that an acidic, F-rich fluid can ascend 100’s of meters through a silicified fault that transects limestone units without being neutralized. Upon exiting the fault into a permeable limestone layer, the acidic, F-rich fluid is neutralized rapidly, dissolving limestone and precipitating high grade fluorite bodies. The models indicate that the reaction of an acidic, F-rich fluid with limestone is an efficient fluorite precipitation mechanism that produces results consistent with field observations of IL-KY deposits. Conversely, a brine having a F concentration typical of sedimentary brines and a pH more typical of MVT fluids fails to produce IL-KY style ore bodies in a geologically reasonable time period. These results support the viability of the F-rich ore fluid hypothesis of IL-KY ore formation.