CONCORDANT U-PB AND (U-TH)/HE FLUORITE AGES FROM THE TAXCO MINING DISTRICT, MEXICO

Mineralization of the La Azul fluorite deposits in the Taxco mining district, Mexico has been the subject of study due to their vicinity to economic ore deposits. This fluorspar deposit had been mined for some years, with calculated reserves of 1 Mt of fluorite. Previous reports have suggested that epithermal conditions produced the fluorite deposit in La Azul, which are characterized by uranium contents up to ~100 ppm, making these samples amenable for (U-Th)/He and U-Pb dating. Using this geochronometer, dates for the La Azul fluorites were reported as 32±2 Ma, coinciding with K/Ar ages found for sericite within the same fault system. Since sericite corresponds to the main mineralization event, it is inferred that the 32±2 Ma age represents the crystallization age of the fluorite. Additionally, this age is coeval with a rhyolite volcanic suite in the area that is thought to be the source of the F for the fluorite.

We used U-Pb on the same suite of fluorites from La Azul and found a mean age of 32.5±3.32 Ma, as well as a similar age range of 28.7 to 36.7 Ma, both of which almost perfectly overlap with the reported (U-Th)/He dates. This concordant age attests to the legitimacy of the (U-Th)/He ages previously reported, and demonstrates the ability of the U-Pb system to directly date fluorite with high U concentration.

These findings are important because fluorite is a relatively widespread mineral in many deposits, and can be used as a dating tool for mineralizing events. Moreover, the very first double dating of fluorite realized in this work demonstrates that both (U-Th)/He and U-Pb can be applied to this mineral if the U concentration is high enough. Accurate diffusion values of U, Pb and He in fluorite are unknown, but the available empirical data suggests that concordance of ages implies undisturbed samples that give the true age of fluorite precipitation, whereas samples with different ages will probably imply helium loss by diffusion that can be interpreted as a thermochronometer