Determination of fluorine concentrations in mineralizing fluids of the Hansonburg, New Mexico Ba-F-Pb district via SEM-EDS analysis of fluid inclusion decrepitates

Abstract

The Hansonburg, New Mexico district in the southwestern U.S.A. contains anomalously fluorite rich carbonate-hosted base metal sulfide deposits. A long-standing hypothesis for this fluorite enrichment is that the Hansonburg mineralizing fluids were correspondingly enriched in F relative to the typical sedimentary brines that formed carbonate-hosted base metal deposits.The purpose of the present study was to test this hypothesis by determining the F concentration in fluid inclusions hosted by drusy quartz that paragenetically overlaps fluorite. The fluid inclusions were thermally decrepitated creating evaporative solute mounds, which were then analyzed via scanning electron microscope-energy dispersive X-ray spectroscopy (SEM-EDS). Fluorine was detected in all evaporative solute mounds analyzed, equating to fluid inclusion F concentrations of 320 to 2500 ppm. These F concentrations are significantly greater than the F concentrations of tenth's to 10s of ppm F typical of sedimentary brines. Further, these high F concentrations indicate that the Hansonburg mineralizing fluid must have been very acidic during the time of fluorite mineralization with a pH of 1 to 2.4. This pH is much lower than the pH of 4.3 to 5.1 predicted in earlier studies and also much lower than the pH of 4 to 5.5 considered typical of carbonate-hosted ZnPb mineralizing fluids. High mineralizing fluid F concentrations in combination with low pH would have provided conditions favorable for the formation of the observed Hansonburg mineral assemblage, i.e. fluorite-barite rich and metal sulfide mineral poor.