Abstract
Two magmatic REE-rich occurrences, located near Jamestown, Colorado, and hosted in the Precambrian Longs Peak granite batholith, exhibit unusual textures that suggest formation by fluoride-silicate melt immiscibility. Both contain small (<2 mm diameter) globular F-, P-, and REE-rich segregations of fluorite and monazite-(Ce). In addition, the northern of the two localities preserves evidence of a second melt immiscibility event in the form of larger (up to several cm diameter) aplite-hosted globular segregations of fluorite and the REE minerals allanite-(Ce), monazite-(Ce), fluorbritholite-(Ce), törnebohmite-(Ce), and cerite-(Ce). The southern of the two localities lacks these cm-scale globular textures, but instead contains much larger aggregates of these same REE minerals, with up to >57 wt. % ΣREE2O3, yet no fluorite, as well as large aggregates of allanite-(Ce) and quartz, and an amphibole-bearing REE-rich rock containing allanite-(Ce), other REE minerals, quartz and minor apatite. A new Nd-Sm laser ablation age of 1.422(24) Ga on monazite-(Ce) and allanite-(Ce) from the southern locality implies the same age of formation of 1.420(25) Ga as for the northern locality, with equally similar initial εNd1.42Ga values of these REE minerals. A newly discovered third locality, containing primarily allanite-(Ce), minor monazite-(Ce), and thorite, without fluorite, extends the number, spatial distribution and total volume of these mineralogically unusual magmatic REE occurrences. We suggest that the REE were concentrated in these three localities by multiple stages of fluoride-silicate melt immiscibility. For the southern locality, slower cooling of a possibly larger magma volume, or in a deeper environment, allowed greater aggregation of the immiscibly separated REE-rich phases, as well as loss of the volatile element F, resulting in a greater availability of Ca accommodated by the crystallization of amphibole and minor apatite.
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Acknowledgements
MBR acknowledges Brian Walko for his support in locating the southern occurrence, and Yuan Yuan, Linus Raschke, and Simon Raschke for support in sampling and core drilling. We thank the Balarat Outdoor Education Center, Denver Public Schools, for providing site access to the northern locality. We thank Aaron Bell for support with the electron microprobe work and Lang Farmer for bulk Nd-Sm isotope analysis and valuable discussions. We acknowledge Kevin Mahan and Alexander Gysi for valuable discussions as well as two anonymous reviewers and the MD editor Karen Kelley for constructive suggestions. Financial support was provided by the Department of Physics, University of Colorado and the University of Electronic Sciences and Technology, Chengdu, through discretionary funds.
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Sample collection and preparation, data collection and analysis were performed by Charles Stern, Julien Allaz, Shea Burnham and Markus Raschke. LA-ICP-MS isotopic analysis were performed by Shea Burnham and Andrew Kylander-Clark. The first draft of the manuscript was written by Charles Stern and all authors commented on previous versions of the manuscript. All authors read and approved the final manuscript.
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Supplementary information
ESM 1
Compositions of bulk whole-rock samples of the different lithologies associated with the REE mineral occurrences near Jamestown, Colorado. (XLSX 28 kb)
ESM 2
Tables 1-4. Compositions determined by electron microprobe of some of the minerals occurring in the REE mineral occurrences near Jamestown, Colorado. (XLSX 47 kb)
ESM 3
Table 1-3. Electron microprobe setup for amphibole, allanite-(Ce) and monazite-(Ce). (XLSX 17 kb)
ESM 4
Nd-Sm isotopic data. (DOCX 24 kb)
ESM 5
Isochron (PDF 256 kb)
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Stern, C.R., Burnham, S., Kylander-Clark, A. et al. Petrogenesis of magmatic REE mineral occurrences near Jamestown, Colorado (U.S.A.). Miner Deposita (2024). https://doi.org/10.1007/s00126-024-01291-2
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DOI: https://doi.org/10.1007/s00126-024-01291-2