Joshua Tree National Park
Phosphate minerals such as monazite and xenotime are the primary major source of rare earth elements (REE) on Earth. As the global demand for REE continues to increase, understanding the mechanisms by which these phases form and are concentrated into economically significant deposits is of vital national importance. This research focuses on determining how and why these geologically important minerals form through cases studies in eastern California. These deposits are some of the very few localities in the continental US that contains high concentrations of REE-bearing phosphate minerals. Despite their potential importance, there iscurrently no coherent petrogenetic model for REE-mineralization in this region. A detailed petrologic study will be performed using state of the art analytical techniques including electron microprobe, in-situ U-Th-Pb geochronology and in-situ Sm/Nd isotope geochemistry. The results of this study will enable 1) development of a petrogenetic model for REE-phosphate formation in eastern California and 2) a quantitative assessment of the nature of the REE resource. Results from this study will have broad impact, providing a fundamental understanding of the mechanisms by which REE-phases form and are concentrated as well as providing baseline data for the development of exploration models.
Funding:
USGS Mineral Resources External Research Program (MRERP). 03/01/12 - 02/28/14 “Evaluating mechanisms for rare earth phosphate mineralization in southern California” USGS announcement
Publications:
McKinney, S.T., Cottle, J.M., Lederer, G. 2015. Evaluating Rare Earth Element (REE) mineralization mechanisms in Proterozoic gneiss, Music Valley, California. GSA Bulletin. doi:10.1130/B31165.1 McKinney_etal_2015_GSAB
Funding:
USGS Mineral Resources External Research Program (MRERP). 03/01/12 - 02/28/14 “Evaluating mechanisms for rare earth phosphate mineralization in southern California” USGS announcement
Publications:
McKinney, S.T., Cottle, J.M., Lederer, G. 2015. Evaluating Rare Earth Element (REE) mineralization mechanisms in Proterozoic gneiss, Music Valley, California. GSA Bulletin. doi:10.1130/B31165.1 McKinney_etal_2015_GSAB
Mountain Pass, California
Rare earth element (REE) ore-bearing carbonatite dikes and a stock at Mountain Pass, California, are spatially associated with a suite of ultrapotassic plutonic rocks, and it has been proposed that the two are genetically related. This hypothesis is problematic, given that existing geochronological constraints indicate that the carbonatite is ∼15–25 Myr younger than the ultrapotassic rocks, requiring alternative models for the formation of the REE ore-bearing carbonatite during a separate event and/or via a different mechanism. New laser ablation split-stream inductively coupled plasma mass spectrometry (LASS-ICP-MS) petrochronological data from ultrapotassic intrusive rocks from Mountain Pass yield titanite and zircon U–Pb dates from 1429 ± 10 to 1385 ± 18 Ma, expanding the age range of the ultrapotassic rocks in the complex by ∼20 Myr. The ages of the youngest ultrapotassic rocks overlap monazite Th–Pb ages from a carbonatite dike and the main carbonatite ore body (1396 ± 16 and 1371 ± 10 Ma, respectively). The Hf isotope compositions of zircon in the ultrapotassic rocks are uniform, both within and between samples, with a weighted mean εHf i of 1·9 ± 0·2 (MSWD = 0·9), indicating derivation from a common, isotopically homogeneous source. In contrast, in situ Nd isotopic data for titanite in the ultrapotassic rocks are variable (εNd i = –3·5 to –12), suggesting variable contamination by an isotopically enriched source. The most primitive εNd i isotopic signatures, however, do overlap εNd i from monazite (εNd i = –2·8 ± 0·2) and bastnäsite (εNd i = –3·2 ± 0·3) in the ore-bearing carbonatite, suggesting derivation from a common source. The data presented here indicate that ultrapotassic magmatism occurred in up to three phases at Mountain Pass (∼1425, ∼1405, and ∼1380 Ma). The latter two stages were coeval with carbonatite magmatism, revealing previously unrecognized synchronicity in ultrapotassic and carbonatite magmatism at Mountain Pass. Despite this temporal overlap, major and trace element geochemical data are inconsistent with derivation of the carbonatite and ultrapotassic rocks by liquid immiscibility or fractional crystallization from common parental magma. Instead, we propose that the carbonatite was generated as a primary melt from the same source as the ultrapotassic rocks, and that although it is unique, the Mountain Pass ultrapotassic and carbonatite suite is broadly similar to other alkaline silicate–carbonatite occurrences in which the two rock types were generated as separate mantle melts.
Publications:
Poletti, J. Cottle, J.M., Hagen-Peter, G. 2016. Petrochronologic Constraints on the Origin of the Mountain Pass Carbonatite and Ultrapotassic intrusive Suites, California. Journal of Petrology. doi: 10.1093/petrology/egw050.
Publications:
Poletti, J. Cottle, J.M., Hagen-Peter, G. 2016. Petrochronologic Constraints on the Origin of the Mountain Pass Carbonatite and Ultrapotassic intrusive Suites, California. Journal of Petrology. doi: 10.1093/petrology/egw050.
Central City, Colorado
M.Sc. student, Carina Edelman, is working on mechanisms for rare earth element mineralization in the Central City region of Colorado.