TRACING THE SOURCE OF FLUORINE IN THE FLUORITE MINERALIZATION OF THE SOUTHERN RIO GRANDE RIFT

The Rio Grande Rift (RGR) is rich in fluorite mineralization, especially associated with Mississippi valley type (MVT) barite-galena deposits. The high abundance of fluorine deposits is anomalous but is similar to other anomalous MVTs such as the Illinois-Kentucky fluorspar district. We are in the process of testing two hypotheses for the origin of fluorine in the RGR MVT deposits, a.k.a Rio Grande rift type (RGRT) deposits. These include release of gaseous HF from magmas associated with rifting, and remobilization of fluorine from the Precambrian granites of the basement rocks that underlie the Pennsylvanian limestones, which host much of the mineralization in the region.

In this study chlorine is being used as proxy for determining the source of the fluorine because chlorine, unlike fluorine, has more than one isotope and can be used as an isotopic tracer. Chlorine and fluorine exhibit chemically similar behavior, and therefore are likely to be derived from the same source. Samples of fluorite from seven RGRT deposits, as well as granites, basalts and limestones were collected throughout the southern RGR. Chlorine is isotopically distinct in the two hypothesized potential reservoirs (d³⁷Cl mantle~+4.7‰; crust 0‰), therefore it should be possible to distinguish between crustal and mantle sources of chlorine (Banks et al., 1999). Complimentary to chlorine isotopic studies, Sr, Nd and Pb isotopes and Br/Cl ratios are also being applied as additional tracers.

Preliminary results indicate that the fluorites have ⁸⁷Sr/⁸⁶Sri=0.7314-0.7353, distinctly more radiogenic than local basalts (~0.7036) and carbonates (0.7266-0.7293). The radiogenic character of the fluorites indicates that the Sr was derived largely from a granitic source, possibly through interaction of the mineralizing fluids with the pre-Cambrian basement granites.