Petroleum-Rich Fluid Inclusions in Fluorite, Purisima Mine, Coahuila, Mexico

Abstract

Mexico has been a world leader in fluorite production. This study demonstrates a genetic link between petroleum basinal fluids and fluorite deposits. Fluid inclusions in fluorite from the La Purisima mine, northeast Mexico, were analyzed using UV-fluorescence, petrographic, and micro-thermometric techniques. Three different populations of inclusions were identified: (1) petroleum-rich (L1 + V); (2) aqueous-rich (L2 + V), and (3) immiscible liquids (L1 + L2 + V), where L1 is liquid hydrocarbon, L2 is a brine, and V is vapor phase. The L2 + V inclusions are primary, whereas the L1 + V and the L1 + L2 + V are pseudo-secondary. Some of the L1 + V petroleum-rich inclusions show necking down and leakage. Frequently, these inclusions contain a solid phase composed of heavy hydrocarbons (solid bitumens?). At low temperature, the petroleum-rich L1 + V inclusions were unaffected by freezing, and it was only possible to measure ice melting temperature (Tm = -10.2°C) in one three-phase, petroleum-rich inclusion (L1 + L2 + V), which corresponds to a salinity of 14.15 wt% NaCl equivalent. UV-fluorescence micro-thermometry indicates the presence of light and heavy hydrocarbons in the oily inclusions. During heating runs, these oil-bearing inclusions have homogenization temperatures between 50° and 150°C. The aqueous-rich L2 + V inclusions range in salinity from 5.7 to 18.13 wt. % NaCl equivalent with homogenization temperature ranging from 75° to 155°C. These inclusions show white fluorescence under UV, indicative of a brine. The salinity and homogenization temperature of all the inclusions are similar to those of Mississippi Valley-type (MVT) deposits and petroleum basins. In this study, we interpreted the Upper Jurassic rock sequence of the Chihuahua-Sabinas basin to be the source of the oil-trapped inclusions, and very likely, of some of the fluorine. The alkaline magmatism of the Trans-Pecos Province was the source of mineralizing fluids, and hence, of some of the fluorine too. This event remobilized the petroleum brine and hydrocarbons trapped as fluid inclusions within fluorite.