Source of Fluorine and Petrogenesis of the Rio Grande Rift-Type Barite-Fluorite-Galena Deposits

摘要

Abundant fluorite in the Rio Grande rift barite-fluorite-galena deposits is anomalous in comparison to typical Mississippi Valley-type (MVT) deposits. The source of fluorine in these deposits is controversial. We have tested two hypothesized sources for the origin of fluorine in these deposits, including release of gaseous HF from magmas associated with rifting and the leaching of fluorine from Proterozoic basement granites that underlie the deposits. In this study, chlorine isotopes were measured from fluorite fluid inclusions from eight Rio Grande rift deposits. Stable chlorine isotope ratios were used as a proxy for determining the possible source of the fluorine because chlorine, unlike fluorine, has more than one isotope and thus 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 if chlorine is associated with rift-related magmatism. As a complement to the chlorine isotope measurements, Br/Cl ratios were measured from the fluorite fluid inclusions and applied as additional tracers. In addition, Sr and Nd isotopes were measured from fluorites, regionally related granites, carbonates, and asthenospheric basalts to aid in understanding the petrogenesis of Rio Grande rift deposits. Sr and Nd isotope ratios of the fluorites (~(87)Sr/~(86)Sr = 0.71986-0.74078, ~(143)Nd/~(144)Nd = 0.51177-0.51234) are distinctly different from local basalts (~(87)Sr/~(86)Sr = 0.70304-0.70465, ~(143)Nd/~(144)Nd = 0.51277-0.51297) and Penn-sylvanian limestones (~(87)Sr/~(86)Sr = 0.70858-0.72926, ~(143)Nd/~(144)Nd = 0.51198-0.51245) but similar to the Proterozoic Bingham granite (~(87)Sr/~(86)Sr = 0.73285, ~(143)Nd/~(144)Nd = 0.51208). The relatively radiogenic Sr and unradi-ogenic Nd signatures of the fluorites indicate that the Sr and Nd were derived largely from a granitic source with some influence from a carbonate and/or asthenospheric source. This is consistent with previously published Pb isotope data on galena from Hansonburg mining district. delta~(37)Cl values from fluorite fluid inclusions range from -0.66 to +3.07 per mil relative to SMOC, and the Br/Cl ratio (by weight) for all the fluorite samples ranges between 0.00008 and 0.00050, except for San Diego Mountain, which has a relatively high Br/Cl ratio of 0.00242. The positive correlation between fluorite delta~(37)Cl and Br/Cl ratio is attributed to mixing of Cl from asthenospheric magmatic and evaporite sources. The calculated range of Cl derived from an asthenospheric source is 40 to 49 percent for the Mex-Tex mine sample and between 13 and 35 percent for the Sunshine no. 2 sample. These ranges in the asthenospheric component of Cl probably represent minimum possible values. Because F and Cl closely follow one another during magma degassing and exhibit similar chemical behavior, the presence of asthenospheric Cl is consistent with an asthenospheric magmatic source of F in the Rio Grande rift deposits.