PETROLEUM-RICH FLUID INCLUSIONS IN FLUORITE, COAHUILA, MEXICO: IMPLICATIONS FOR SOURCES OF OIL AND FLUORINE

Fluid inclusions in fluorite from the La Purisima mine, northeast Mexico, were analyzed using UV-fluorescence, petrographic and micro-thermometric techniques. Three different population of inclusions were identified: (1) hydrocarbon-rich (L1 + V); (2) liquid-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 hydrocarbon-rich inclusions show necking down and leakage. Frequently, these inclusions contain a solid phase composed of heavy hydrocarbons (solid bitumens?). At low temperature, the L1 + V inclusions were unaffected by freezing. UV-fluorescence micro-thermometry indicates the presence of light and heavy hydrocarbons. During heating runs, these oil-bearing inclusions have homogenization temperatures between 50º and 150º C. The L2 + V inclusions range in salinity from 10.7 to 11.1 wt. % NaCl equivalent with homogenization temperature ranging from 75º to 155º 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 that the Upper Jurassic rock sequence of the Chihuahua-Sabinas basin was 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.