The Illinois-Kentucky ore district hosts the largest deposits of fluorite in the USA. A long-standing hypothesis for the formation of the Illinois-Kentucky deposits is that the ore fluid was a sedimentary brine infused with F-rich fluids exsolved from Permian ultramafic magmas. This ore fluid may have had a F concentration between 680 and 4,300 ppm, three orders of magnitude greater than typical F concentrations in sedimentary brines, and a highly acidic pH between 0 and 1.4. The ore fluid likely ascended faults that intersected permeable limestone formations where neutralization of the ore fluid by reaction with limestone likely drove fluorite precipitation. The present study was undertaken to test the implications of such an acidic F-rich fluid for fluorite ore formation using reactive transport modeling. The models show that an acidic F-rich fluid could have ascended a silicified fault transecting 100s of meters of limestone without becoming neutralized. Upon exiting the fault into a limestone aquifer, the F-rich fluid would have reacted with the limestone and become neutralized within a few meters, producing intense localized limestone dissolution. The models indicate that the neutralization of acidic F-rich fluid is an efficient fluorite precipitation mechanism that can quickly produce a large fluorite-dominant mineral deposit and abundant host rock dissolution consistent with field observations. Conversely, an ore fluid that has a low F concentration and a higher pH more typical of sedimentary brines cannot produce a fluorite-dominant deposit as large as those observed in the field within a geologically reasonable amount of time.

伊利诺伊州-肯塔基州矿区拥有美国最大的萤石矿床。伊利诺伊州-肯塔基州矿床形成的一个长期假设是，矿石流体是一种沉积盐水，其中注入了从二叠纪超镁铁质岩浆中溶出的富氟流体。这种矿石流体的 F 浓度可能在 680 到 4,300 ppm 之间，比沉积盐水中的典型 F 浓度高三个数量级，并且具有 0 到 1.4 之间的高酸性 pH 值。矿石流体可能上升的断层与可渗透的石灰岩地层相交，在该处通过与石灰石反应中和矿石流体可能驱动萤石沉淀。本研究旨在使用反应输运模型测试这种酸性富 F 流体对萤石矿形成的影响。模型表明，富含 F 的酸性流体可能在没有被中和的情况下上升到横切 100 米石灰岩的硅化断层。在离开断层进入石灰岩含水层后，富氟流体将与石灰岩发生反应并在几米内被中和，从而产生强烈的局部石灰岩溶解。模型表明，酸性富氟流体的中和是一种有效的萤石沉淀机制，可以快速产生大量以萤石为主的矿床和丰富的母岩溶蚀，与现场观测结果一致。相反，具有低 F 浓度和较高 pH 值（更典型的沉积盐水）的矿石流体不能在地质上合理的时间内产生与现场观察到的一样大的以萤石为主的矿床。