Vein fluorite U-Pb dating demonstrates post–6.2 Ma rare-earth element mobilization associated with Rio Grande rifting

Abstract

Numerous studies have documented rare-earth element (REE) mobility in hydrothermal and metamorphic fluids, but the processes and timing of REE mobility are rarely well constrained. The Round Top laccolith in the Trans-Pecos magmatic province of west Texas, a REE ore prospect, has crosscutting fractures filled with fluorite and calcite along with a variety of unusual minerals. Most notably among these is an yttrium and heavy rare-earth element (YHREE) carbonate mineral, which is hypothesized to be lokkaite based on elemental analyses. While the Round Top laccolith is dated to 36.2 ± 0.6 Ma based on K/Ar in biotite, U-Pb fluorite and nacrite ages presented here clearly show the mineralization in these veins is younger than 6.2 ± 0.4 Ma (the age of the oldest fluorite). This discrepancy in dates suggests that fluids interacted with the laccolith to mobilize REE more than 30 m.y. after igneous emplacement. The timing of observed REE mobilization overlaps with Rio Grande rift extension, and we suggest that F-bearing fluids associated with extension may be responsible for initial mobilization. A later generation of fluids was able to dissolve fluorite, and we hypothesize this later history involved sulfuric acid. Synchrotron spectroscopy and laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) U-Pb dating of minerals that record these fluids offer tremendous potential for a more fundamental understanding of processes that are important not only for REE but other ore deposits as well.

Uranium incorporation in fluorite and exploration of U-Pb dating

“…Uranium concentration in fluorite is highly variable (Piccione et al, 2019;Walter et al, 2018;Wolff et al, 2016), and is often insufficient for U-Pb dating (Piccione et al, 2019). In the Pierre-Perthuis fluorite ore, the geodic fluorite Flgeo, dated at 130 ± 15 Ma by Sm-Nd (Gigoux et al, 2015), could not have been tested for U-Pb geochronology because it contains no U.…”

“…To correct for DF, a matrix-match primary RM can be used if laser ablation generates similar crater aspect ratios between the reference material and unknown samples (Elisha et al, 2020;Guillong et al, 2020). Such a matrix-match RM is not yet available for fluorite U-Pb geochronology (Piccione et al, 2019). However, even with the use of such a RM, DF correction may turn out to be incorrect because of variable crater aspect ratios in fluorite.…”

“…To overcome these issues, a non-radiometric method based on the acquisition of chemical remnant magnetization in fluorite has been developed and employed to date fluorite deposits in areas where the geodynamic evolution and the apparent polar wander path are well documented (Kawasaki and Symons, 2008;Symons, 1994;Symons et al, 2017). Along with other elements, fluorite accepts the incorporation of uranium and lead in its crystal lattice in substitution for Ca 2+ (typically at low concentrations, from a few ppm to a few ‰, (Piccione et al, 2019)). As a result, fission-track (Grønlie et al, 1990) and (U-Th)/He (Evans et al, 2005;Wolff et al, 2015) thermochronology have been tested in fluorite.…”

“…In recent years, in situ U-Pb geochronology has been successfully applied to a growing number of mineral matrices, including calcite and dolomite (Burisch et al, 2017), hematite (Walter et al, 2018), opal (Nuriel et al, 2019), cassiterite (Moscati and Neymark, 2020), Mn-rich chrysocolla and pseudomalachite (Kahou et al, 2020), epidote (Peverelli et al, 2020), nacrite and fluorite (Piccione et al, 2019). Piccione et al, (2019) reported for the first time LA-ICP-MS U-Pb ages acquired on fluorite that are consistent with those of adjacent nacrite crystals, showing that fluorite U-Pb geochronology can constrain the timing of fluid flows, providing favorable U/Pb. However, the incorporation and retention of U and Pb in the crystal structure of CaF2 are very poorly documented, hence currently limiting the reliability of fluorite U-Pb dating.…”

The use of ASH-15 flowstone as a matrix-matched reference material for laser-ablation U-Pb geochronology of calcite

“…Recent advances in laser ablation techniques applied to multi-phase carbonates allow for accurate dating of a variety of sample types, including calcite cements (Li et al, 2014;Godeau et al, 2018;Anjiang et al, 2019;Holdsworth et al, 2019), hydrothermal veins (Coogan et al, 2016;MacDonald et al, 2019;Piccione et al, 2019), fault-related veins, breccia cement, and slickenfibers (Ring and Gerdes, 2016;Roberts and Walker, 2016;Nuriel et al, 2017;Hansman et al, 2018;Parrish et al, 2018;Nuriel et al, 2019), and speleothems (Woodhead and Petrus, 2019). With increasing attention on climatic, seismic, and environmental events in the geological record there is a growing need for dating techniques that can be accurately and easily implemented to samples at the sub-millimeter scale.…”