Geology and Geochemistry of Fluorite Ore Deposits and Associated Rocks in Northern Mexico.

Abstract

The Sierra Madre Occidental, which is one of the largest, continous continental silicic provinces in the world, hosts some of the most important fluorite deposits known. The deposits range from essentially monomineralic fluorite veins in igneous rocks and massive limestone replacement deposits at the contact with rhyolitic ignimbrites and agglomerates to lead-zinc sulfide deposits with 10-20 percent fluorite gangue. The fluorite deposits are associated with fluorine-rich igneous rocks which were emplaced in a relatively short time between 32 and 26 m.y. ago. This period corresponds to a time when Mexico was undergoing a transition from an arc to a back-arc tectonic regime. This tectonic regime was likely to have been associated with higher heat flows and crustal thinning. Initial fluorine enrichment of the magmas was probably produced by small amounts of partial melting of the crust or of crustal contamination of a mantle derived magma. Further enrichment of the magmas was probably the result of thermogravitational diffusion and crystal fractionation. Some of the largest fluorite deposits in the Sierra Madre Occidental are associated with major calderas. The high-silica, fluorine-rich magmatic activity, however, was not part of the major caldera forming volcanism but occurred later. The fluoride-bearing fluids that formed the fluorite deposits of the Sierra Madre Occidental were relatively cool (< 200(DEGREES)C), dilute meteoric waters. The ore-bearing solutions acquired their fluorine in shallow convection cells formed by the heat of the newly emplaced fluorine-rich igneous rocks. Fluorine adsorbed by ash and pumice, as well as fluorine in vitrophyres was leached and redeposited by the convecting groundwater. The amount of fluorine that the solutions in these systems carry is inversely proportional to their calcium concentration. This relation suggests that the limiting factor in transporting fluorine in natural post-volcanism systems is the solubility product of fluorite. Modern analogues of the mid-Tertiary fluorite depositing systems of the Sierra Madre Occidental are some fluorine-rich geothermal systems.