POTENTIAL ROLE OF MAGMATIC GASES IN THE GENESIS OF ILLINOIS-KENTUCKY FLUORSPAR DEPOSITS - IMPLICATIONS FROM CHEMICAL-REACTION PATH MODELING

In this gaper, we present results of reaction path calculations using the chemical speciation and reaction path Programs SOLVEQ and CHILLER to model possible fluorite deposition mechanisms in the Illinois-Kentu cky fluorspar district. Input to the reaction path calculations were t he temperatures, salinities, and concentrations of major cation and an ions (Na, K, Cl, SO4, etc.) and dissolved gases (H2S, CO2, etc.) in th e hydrothermal fluids, based on published fluid inclusion data and unp ublished fluid inclusion gas compositions. Fluid pH values and concent rations of fluorine and various metals were then calculated assuming s aturation with observed hydrothermal minerals. We assumed saturation w ith fluorite, dolomite, quartz, muscovite (approximating illite), anhy drite, sphalerite, galena, and pyrite. Fluorite ore deposition mechani sms which we initially modeled include (1) simple cooling (from 150 de grees-140 degrees C, based on data and conclusions of Richardson and P inckney, 1984), (2) reactions of the hydrothermal fluids with limeston es (based upon the observed extensive replacement of limestones by flu orite), (3) combinations of cooling and limestone reaction, (4) isothe rmal or near-isothermal boiling of the fluids in response to pressure drops, and (5) mixing of two fluids with different Ca and F contents. Using the assumed initial fluid composition discussed above, the model ing predicts that none of these mechanisms accurately reproduces the o bserved fluorite-dominant mineralization stages and limestone replacem ent features. We then modeled how interactions of the hydrothermal flu ids at 300 degrees C with HF and CO2 (approximating a CO2- and HF-bear ing gas phase expelled from a crystallizing alkalic magma) might affec t fluid chemistries and results of the above deposition mechanisms. Th e influence of alkalic magmatism on Illinois-Kentucky fluorspar distri ct mineralization is probable based on the geologic relations of the d istrict. Titration of 0.5 g HF and 0.5 g CO2 into 1 kg of a fluorspar fluid composition produces extensive fluorite precipitation and acidic (pH 2.5 or lower), F-rich, Ca-poor fluids. Cooling of these fluids to 150 degrees C leads to further fluorite precipitation. Other possible magmatic gas compositions were modeled and yielded generally comparab le results. Paths modeling subsequent reactions of these modified flui ds at 150 degrees C with limestone (to produce the manto, Cave-In-Rock -style ores) predict extensive replacement of limestone initially by l arge amounts of fluorite; when the pH increases to above 4.5, dolomite and anhydrite also precipitate. When cooled from 150 degrees to 142 d egrees C, these fluids produce large amounts of fluorite with lesser q uartz. Isothermal or near-isothermal boiling reaction paths predict fo rmation of somewhat lesser amounts of fluorite. Reaction paths modelin g the mixing of the acidic, F-rich fluids with Ca-rich, F-poor fluids also predict copious fluorite precipitation with no associated mineral s. Thus, cooling or mixing of a fluid previously acidified by HF could produce open-space, vein-style fluorspar mineralization. These result s indicate that the fluids responsible for Illinois- Kentucky fluorspa r mineralization were most likely quite acidic (pH < 4) and rich in fl uorine in order to produce the fluorite-rich, dolomite-poor mineral as semblages and extensive dissolution of host limestones. A possible sou rce for the acid and fluorine may have been HF-rich gases which were e xpelled from alkalic magmas and then incorporated by migrating basinal brines. A possible sequence for Illinois-Kentucky fluorspar district mineralization which fits the available data is an initial major gas e xplosion to form the Hicks Dome cryptovolcano, followed by continued, less violent leakage of magmatic gases from alkaline igneous sources. These magmatic gases could have migrated upward not only in the immedi ate vicinity of Hicks Dome itself but also along major Illinois-Kentuc ky fluorspar district faults which cut crystalline basement; titration of the gases into basinal brines at depth led to preferential mineral ization of these structures. An analysis of the geologic setting of ot her fluorite deposits ana districts worldwide suggests that involvemen t of magmatic gases is probable for many of these districts as well.