Piccione, and 9 co-authors, reported in 2019 that 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 cross-cutting 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 the authors suggested that F-bearing fluids associated with extension may be responsible for initial mobilization. A later generation of fluids was able to dissolve fluorite, and the authors hypothesized 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.
The authors concluded that there is great potential for LA-ICP-MS U-Pb dating of hydrothermal minerals to illuminate the timing of fluid events and to date associated ore deposits. At Round Top laccolith, the authors show evidence for multiple fluid events. Early fluid events have elevated REE with MREE-enriched patterns in nacrite and fluorite and range in age from 6.2 to 3.2 Ma. Later fluids brecciated and dissolved fluorite and nacrite and precipitated highly HREE-enriched carbonates: an YHREE carbonate followed by calcite.
These minerals could not be directly dated but are bracketed by 2.8 Ma nacrite and a 1.7 Ma calcite that has low REE. The most important observation here is that the timing of mobilization greatly postdates igneous emplacement. Instead the timing is consistent with that of Rio Grande rifting. Fluorite deposits to the north in New Mexico are suggested to have a mantle source. Perhaps smaller amounts of fluorine fluxed through the Trans-Pecos area, mobilizing REE from various accessory minerals in the laccolith. It appears that an even more aggressive fluid followed that was capable of dissolving fluorides, the primary REE host in the laccolith, and brecciating the existing vein minerals and host rock.
The authors speculated that sulfuric acid–bearing fluid was in the circulation system, perhaps due to mixing of H2S-bearing brines with groundwater, similar to that hypothesized for Carlsbad Caverns. A HREE carbonate and calcite were precipitated by this succeeding fluid. This fluid was followed by fluids that could both dissolve and precipitate calcite. A late generation of calcite with low REE but high U and U-Pb is dated at 1.7 Ma. The authors have not found evidence for REE mobilization that is later than this calcite generation. This application of U-Pb dating of fluorite nacrite and elemental characterizing of a suite of vein minerals shows that this approach can be of use in future work understanding the geochemical processes that remobilize REE. More precise U-Pb geochronology on fluorite using LA-ICPMS could be of use in creating details chronologies of hydrothermal fluid events and ore mineralization.
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