REACTIVE TRANSPORT MODELING OF ORE FORMATION IN THE ILLINOIS-KENTUCKY FLUORITE DISTRICT BY MIXING OF MAGMATIC, ACIDIC, F-RICH FLUID WITH SEDIMENTARY BRINE

The Illinois-Kentucky (IL-KY) district represents the largest occurrence of fluorite in the U.S. and one of the largest in the world. The district initially contained at least 8.4 million metric tons of fluorite (at an average grade of 30 to 40 %). A long-standing hypothesis is that the IL-KY deposits formed as a result of mixing of a resident sedimentary brine with F-rich fluids exsolved from Permian ultramafic magmas. The F-rich magmatic fluid was probably transmitted from depth mainly along vertical faults, and mixed with sedimentary brine at shallower depths where the faults intersected permeable limestone formations. Recent fluid inclusion research suggests that the F concentration of the magmatic fluid and sedimentary brine mixture, i.e. the resultant ore fluid, ranged from 680 to 4300 ppm, requiring a correspondingly low pH of 0 to 1.4, much lower than the typical MVT ore fluid pH range of 4 to 5.5. In the present study, reactive transport modeling was undertaken to investigate the implications of such an acidic fluid for water-rock reactions and ore formation in the IL-KY district. The models thus far indicate that an acidic, F-rich fluid can rise through a silicified fault without becoming significantly neutralized. Where the fault intersects a permeable limestone layer, the F-rich fluid exits the fault and is neutralized rapidly. Intense limestone dissolution occurs locally at the fault-limestone intersection, reaching 7 % limestone volume loss after 1,000 years. Fluorite precipitation is confined to within about 15 m of the fault, causing a maximum of 1.7 % volume gain of fluorite in the permeable limestone layer. The models predict precipitation of sulfide minerals in quantities that are more than an order of magnitude lower than the predicted fluorite abundances. The model results are generally consistent with field observations of solution collapse, the spatial distribution of ore, and the relative abundance of ore minerals.