Dissertation
RARE-EARTH FRACTIONATION AND MINERALIZATION DURING HYDROTHERMAL AND SUPERGENE MODIFICATIONS OF A CARBONATITE STOCKWORK IN THE BEAR LODGE ALKALINE COMPLEX, WYOMING
Doctor of Philosophy (PhD), Washington State University
01/2015
Handle:
https://hdl.handle.net/2376/100047
Abstract
The Bear Lodge alkaline complex (BLAC) hosts one of the largest potentially economic rare-earth resources in North America. Rare earth mineralization in the BLAC is hosted primarily by a central calciocarbonatite dike swarm and pervasive vein stockwork. New 40Ar/39Ar ages indicate carbonatitic magma likely crystallized between 51.15±0.04 Ma and 51.6±0.2 Ma as it intruded other Eocene alkaline silicate rocks including heterolithic diatreme breccias. Three-dimensional modeling of the REE resource has revealed both vertical and lateral zonations of total REE concentrations and HREE/LREE ratios. The vertical, near surface, increase in REE concentrations correlates with replacement of REE-carbonates (ancylite, carbocernaite, and burbankite) by Ca-REE fluorocarbonates (bastnäsite, parisite, and intermediate varieties), dissolution of matrix carbonate, development of Fe- and Mn-rich gossan during oxidation, and oxidation of Ce3+ to Ce4+ forming cerianite and negative Ce anomalies in secondary fluorocarbonates and phosphates. These vertical changes demonstrate the importance of supergene processes and oxidizing fluids during the waning stages of carbonatite alteration. Stable C and O isotope ratios of calcite and bulk REE pseudomorphs increase toward shallower levels, and further support the involvement of late-stage, low-T (<250°C) fluids characterized by higher F/CO2 ratios, lower CO2/H2O ratios, and an increasing meteoric water component. Oxidized carbonatite of the BLAC is analogous to a poorly developed, Al3+-poor variety of the laterite horizons observed in a number of carbonatite complexes in tropical regions. Peripheral REE-rich veins characterized by an assemblage of "supergene" monazite, jarosite, rutile, barite, and plumbopyrochlore are remarkably similar to lateritically weathered carbonatite of Araxá and Catalão I, Brazil. Outward increases in HREE/LREE ratios through the carbonatite stockwork are likely influenced by crystal-fractionation processes. Early crystallization of LREE-selective minerals (e.g., ancylite and Ca-REE fluorocarbonates) will increase HREE/LREE, (PO4)3-/(CO3)2-, and (PO4)3-/F- ratios in residual fluids, and in some instances, may promote crystallization of HREE-enriched phosphate minerals as fluids emanate outward. Such processes can produce dramatic changes in REE chemistry and mineralogy as revealed by a unique anatase+xenotime-(Y)+brockite+fluorite occurrence peripheral to the central carbonatites. With geochemical similarities to NYF-type pegmatites, the Cole HFSE(+HREE) occurrence may represent the extreme compositions attainable from fractionation processes in carbonatite-fluid systems.
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Details
- Title
- RARE-EARTH FRACTIONATION AND MINERALIZATION DURING HYDROTHERMAL AND SUPERGENE MODIFICATIONS OF A CARBONATITE STOCKWORK IN THE BEAR LODGE ALKALINE COMPLEX, WYOMING
- Creators
- Allen Andersen
- Contributors
- Peter B Larson (Advisor)John A Wolff (Committee Member)Owen K Neill (Committee Member)James G Clark (Committee Member)
- Awarding Institution
- Washington State University
- Academic Unit
- Environment, School of the (CAHNRS)
- Theses and Dissertations
- Doctor of Philosophy (PhD), Washington State University
- Number of pages
- 288
- Identifiers
- 99900581530301842
- Language
- English
- Resource Type
- Dissertation