Abstract

The Sierra Madre Occidental in northwestern Mexico hosts many of the largest fluorite deposits in the world, ranging from essentially monomineralic fluorite deposits to polymetallic deposits with abundant fluorite. The high-grade (>75% CaF 2 ) deposits occur as massive limestone replacements and veins in shales and middle Tertiary igneous rocks that are usually located at the rim of large ( approximately 30 km) calderas. Common to all the deposits is their association with fluorine-rich high silica rhyolites, which in some cases are considerably younger than the caldera in which they occur. The mineralizing solutions that formed the high-grade fluorite deposits were dilute (<0-3 equiv. wt % NaCl), heated (<200 degrees C) meteoric (?) waters that presumably leached fluorine from the associated fluorine-rich rhyolites. The source of the calcium for the solutions is less obvious, however, and knowledge of the source can be used to constrain the chemical characteristics of fluoride-bearing hydrothermal waters, precipitation mechanisms for the fluorite, and the geometry of the hydrothermal systems.In this study, strontium isotopes were used to determine the source(s) of strontium, and by inference calcium, of the four most important high-grade fluorite deposits of the Sierra Madre Occidental; Las Cuevas, El Realito, Navidad, and Rodeo. Las Cuevas and El Realito are massive limestone replacement deposits; Rodeo's mineralization occurs as veins in a quartz diorite and Navidad's as veins in lithic-rich dacitic ignimbrites. Fluorite from each deposit has distinct 87 Sr/ 86 Sr compositions (Las Cuevas, approximately 0.7060; El Realito, approximately 0.7074; Navidad, approximately 0.7068-0.7070; Rodeo, 0.7057-0.7058). In all the deposits the Sr isotope composition of the fluorite resembles that of the immediate wall rock. This similarity is particularly evident in the larger deposits, indicating that most of the strontium, and by inference calcium, in the fluorite was obtained at the site of deposition and that the fluorite from the largest deposits obtained more calcium from the wall rock than the smaller ones. This suggests that for the Mexican-type fluorite deposits: (1) addition of calcium to fluoride-bearing hydrothermal solution is an important mechanism of ore deposition, (2) the fluoride concentration of the hydrothermal solutions is mostly determined by the solubility product of fluorite, and (3) strontium isotopes might be used as a relative measure of the size of these deposits during exploration.

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