Geochemistry of southwestern New Mexico fluorite occurrences implications for precious metals exploration in fluorite-bearing systems

Fluorite-bearing Cu-Au, Ag-Au, and Ag-Pb-Zn deposits comprise a geologicall y and genetically varied group of mineral deposits in southwestern New Mexi co. Additionally, several small to moderate tonnage former fluorite orebodi es and numerous non-economic fluorite occurrences are distributed throughou t much of New Mexico, western Texas, and northern Sonora, Mexico. We differ entiate fluorite-bearing deposit types utilizing variations in fluorite geo chemistry. Several types of data were collected by fluid inclusion microthe rmometry, delta D and delta(18)O analysis of inclusion fluids, and trace el ement analysis of fluorite separates. Fluid inclusion microthermometry indi cates that fluorite in most of the veins formed from low salinity (0-7 eq. wt.% NaCl) fluids at temperatures between 100 degrees and 220 degrees C. A small number of vein and replacement deposits hosted by limestone or granit e, including the Hansonburg Mississippi Valley Type (MVT) deposit, and the Zuni Mountains deposits, formed at similar temperatures, but from moderatel y saline (10-20 eq. wt.% NaCl) fluids. Magmatic fluids (>600 degrees C, 80 eq. wt.% NaCl), are trapped in fluorite veins in the Capitan pluton. delta D and delta(18)O values of inclusion fluids range, respectively, from -108 to -27 parts per thousand and from -9.9 to 7.3 parts per thousand relative to SMOW. Horizontal to subhorizontal trends in delta D versus delta(18)O sp ace suggest that variably-exchanged meteoric waters were responsible for fl uorite deposition in most deposits. Trace elements show a broad range in co ncentration from tenths to hundreds of parts per million. The sediment-host ed fluorites in this study tend to have low Sigma REE concentrations relati ve to fluorite hosted by igneous rocks. Limestone-hosted fluorites associat ed with felsic intrusive rocks have high Sigma REE concentrations relative to all other sediment-hosted fluorites. Signatures indicative of precious m etals mineralization are present in low-salinity epithermal systems contain ing Au and/or Ag. These are characterized by small and/or isolated ranges i n Sr concentrations, Sc/Eu ratios, (Tb/Yb)(n) ratios, and Eu anomalies. Two apparently barren fluorospar districts in southwestern New Mexico contain fluorite with signatures indicative of precious metals mineralization and, therefore, it is suggested that precious metals mineralization may also be present within these districts. (C) 2000 Elsevier Science B.V. All rights r eserved.