REE fractionation, mineral speciation, and supergene enrichment of the Bear Lodge carbonatites, Wyoming, USA

Ore Geology Reviews 89 (2017) 780–807

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REE fractionation, mineral speciation, and supergene enrichment of the Bear Lodge carbonatites, Wyoming, USA Allen K. Andersen a,⇑, James G. Clark b, Peter B. Larson c, John J. Donovan d a

Department of Atmospheric and Geological Sciences, State University of New York at Oswego, Oswego, NY 13126, USA Applied Petrographics, 4909 NE 320 Ave., Camas, WA 98607, USA c School of the Environment, Washington State University, Pullman, WA 99164, USA d CAMCOR, University of Oregon, Eugene, OR 97403, USA b

a r t i c l e

i n f o

Article history: Received 10 December 2015 Received in revised form 19 June 2017 Accepted 24 June 2017 Available online 6 July 2017

a b s t r a c t The Eocene (ca. 55–38 Ma) Bear Lodge alkaline complex in the northern Black Hills region of northeastern Wyoming (USA) is host to stockwork-style carbonatite dikes and veins with high concentrations of rare earth elements (e.g., La: 4140–21000 ppm, Ce: 9220–35800 ppm, Nd: 4800–13900 ppm). The central carbonatite dike swarm is characterized by zones of variable REE content, with peripheral zones enriched in HREE including yttrium. The principle REE-bearing phases in unoxidized carbonatite are ancylite and carbocernaite, with subordinate monazite, fluorapatite, burbankite, and Ca-REE fluorocarbonates. In oxidized carbonatite, REE are hosted primarily by Ca-REE fluorocarbonates (bastnäsite, parisite, synchysite, and mixed varieties), with lesser REE phosphates (rhabdophane and monazite), fluorapatite, and cerianite. REE abundances were substantially upgraded (e.g., La: 54500–66800 ppm, Ce: 11500–92100 ppm, Nd: 4740–31200 ppm) in carbonatite that was altered by oxidizing hydrothermal and supergene processes. Vertical, near surface increases in REE concentrations correlate with replacement of REE(±Sr,Ca,Na,Ba) carbonate minerals by Ca-REE fluorocarbonate minerals, dissolution of matrix calcite, development of Fe- and Mn-rich gossan, crystallization of cerianite and accompanying negative Ce anomalies in secondary fluorocarbonates and phosphates, and increasing d18O values. These vertical changes demonstrate the importance of oxidizing meteoric water during the most recent modifications to the carbonatite stockwork. Scanning electron microscopy, energy dispersive spectroscopy, and electron probe microanalysis were used to investigate variations in mineral chemistry controlling the lateral complex-wide geochemical heterogeneity. HREE-enrichment in some peripheral zones can be attributed to an increase in the abundance of secondary REE phosphates (rhabdophane group, monazite, and fluorapatite), while HREEenrichment in other zones is a result of HREE substitution in the otherwise LREE-selective fluorocarbonate minerals. Microprobe analyses show that HREE substitution is most pronounced in Ca-rich fluorocarbonates (parisite, synchysite, and mixed syntaxial varieties). Peripheral, late-stage HREE-enrichment is attributed to: 1) fractionation during early crystallization of LREE selective minerals, such as ancylite, carbocernaite, and Ca-REE fluorocarbonates in the central Bull Hill dike swarm, 2) REE liberated during breakdown of primary calcite and apatite with higher HREE/LREE ratios, and 3) differential transport of 2
2

REE in fluids with higher PO3
4 /CO3 and F /CO3 ratios, leading to phosphate and pseudomorphic fluorocarbonate mineralization. Supergene weathering processes were important at the stratigraphically highest peripheral REE occurrence, which consists of fine, acicular monazite, jarosite, rutile/pseudorutile, barite, and plumbopyrochlore, an assemblage mineralogically similar to carbonatite laterites in tropical regions. Ó 2017 Elsevier B.V. All rights reserved.

1. Introduction Explored and considered as a potential resource of other commodities (principally Au, Cu, Th/U, and F) since the Black Hills Gold Rush of the 1870s, the Bear Lodge alkaline complex (BLAC) is the focus of renewed interest for rare earth elements. Oxidized ⇑ Corresponding author. E-mail address: [email protected] (A.K. Andersen). http://dx.doi.org/10.1016/j.oregeorev.2017.06.025 0169-1368/Ó 2017 Elsevier B.V. All rights reserved.

carbonatite-hosted REE mineralization at the center of the complex constitutes a combined inferred, measured, and indicated REE mineral resource of 49.8 million tons averaging 2.75 wt% total rare earth oxide (TREO) using a 1.5 wt% TREO cutoff grade (Dahlberg et al., 2014). Transition and sulfide-zone ore represents an additional inferred resource of 14.2 million tons at 2.41 wt% TREO. Significant mineralogical and geochemical changes occur both vertically and laterally through the network of anastomosing carbonatite dikes and veins with specific zones enriched in HREE

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(high HREE/LREE ratios) and those deemed critical to advancing technologies (Nd, Eu, Tb, Dy, and Y). This study originates from extensive exploration drilling between 2009 and 2014 which includes geochemical data and mineralogical observations from a subset of 242 drill holes, >188,000 total ft, and nearly 20,000 sample intervals (personal communication, John Ray, Rare Element Resources). The carbonatite stockwork at Bear Lodge provides an opportunity to examine both unoxidized and progressively more oxidized carbonatite equivalents, and the chemical and mineralogical modifications that result from hydrothermal and supergene processes. Using a variety of techniques including X-ray fluorescence (XRF), inductively coupled plasma mass spectrometry (ICP-MS), energy dispersive spectroscopy (EDS), electron probe microanalysis (EPMA), and cathodoluminescence (CL), we investigate the vertical and lateral changes in REE mineralogy and major and trace element chemistry, particularly REE, Ti, Nb, Sr, Ca, CO3, P, and F. This study builds upon the earlier work of Staatz (1983), Mariano (1978b, 1981), Jenner (1984), Felsman (2009), and complements the more recent investigation of Bear Lodge REE mineral parageneses by Moore et al. (2015), which focused primarily on the central ore body at Bull Hill and less altered/oxidized samples. The multi-stage origin (magmatic, hydrothermal, and supergene) of REE minerals in many carbonatite occurrences has lead to complex replacement textures and problems in the interpretation of their mode of formation (Wall and Mariano, 1996). Throughout different zones of the Bear Lodge carbonatite (BLC) stockwork secondary REE mineral assemblages, including pseudomorphs of polycrystalline REE minerals, share similarities to carbonatite complexes of the Kola Peninsula, the Maoniuping REE deposit, China, and lateritically weathered carbonatite at Araxá and Catalão I, Brazil and Mt. Weld, Australia (cf., Zaitsev et al., 1998; Xie et al., 2009; Mariano, 1989b; Lottermoser, 1990). Late-stage, HREE-enrichment is observed in a number of REE deposits/occurrences, many of which are related to carbonatite magmatism, including Lofdal, Namibia; Kalkfield and Ondurakorume complexes, Namibia; Juquiá, Brazil; Bayan Obo, China; Karonge, Burundi; and the Barra do Itapirapuã carbonatite, Brazil, among others (Wall et al., 2008; Bühn, 2008; Smith et al., 2000; Walter et al., 1995; Van Wambeke, 1977; Andrade et al., 1999). Other REE deposits, such as the Nechalacho layered suite at Thor Lake in the Northwest Territories of Canada and the Strange Lake deposit in Québec-Labrador, Canada, appear to have the reverse trend where late-stage hydrothermal fluids preferentially mobilize LREE to significantly greater distances from their magmatic source (Salvi and Williams-Jones, 1990; Sheard et al., 2012; Williams-Jones et al., 2012). The present paper focuses on the distribution of heavy and light REE between different mineral species (fluorocarbonates, phosphates, and cerianite) hosted in variably oxidized carbonatite and peripheral laterite-like veins across the BLC stockwork.

2. Geology of the Bear Lodge alkaline complex (BLAC) Located 25 km southeast of Devils Tower and Missouri Buttes, the BLAC is one of several Paleogene alkaline intrusive centers along a 70–80°W lineament transecting the northern part of the Black Hills uplift. Intrusive centers along this lineament represent the easternmost surface manifestations of immediate postLaramide magmatism (Duke, 2005). A general decrease in silica saturation and increase in alkalinity is reported from east to west across the belt, and K-Ar and 40Ar/39Ar age determinations (cf., McDowell, 1971; Staatz, 1983; Duke et al., 2002; Duke, 2005; Andersen et al., 2013) suggest magmatism progressed westward during the Paleogene.

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The BLAC is a bilobate dome consisting of an Eocene alkaline intrusive core flanked by outward dipping Paleozoic and Mesozoic sedimentary units, and pendants or rooted inliers of granite/gneiss of the Wyoming Archean Province (Fig. 1). The dome is aligned along a local north-northwest trend of igneous activity, an orientation mimicked by many of the smaller intrusive bodies. Domal uplift of the BLAC occurred as alkaline silicate magmas intruded Paleozoic sedimentary units as sills and other small, shallow intrusive bodies, significantly inflating the sedimentary section (Staatz, 1983; Dahlberg et al., 2014). The core of the subvolcanic complex comprises multiple hypabyssal intrusions of phonolite, tephriphonolite/phonotephrite (nepheline syenite and malignite), latite, syenite, trachyte, and volumetrically minor pseudoleucite porphyry (leucite-bearing phonolite and phonolitic leucitite), lamprophyre, and carbonatite (predominantly calcio-carbonatite/sövi te) (Jenner, 1984; Felsman, 2009; Olinger, 2012). Pipes and dome-shaped masses of intrusive heterolithic diatreme breccia occur throughout the BLAC, with the most prominent examples at Bull Hill, Carbon Hill, and Whitetail Ridge, all near the center of the complex (Moore et al., 2015). Carbonatite dikes and veins are emplaced along a NW-trend, intruding the Bull Hill, Whitetail, and Carbon diatremes (Fig. 2). Carbonatitic magma/fluid flooded the permeable diatreme matrix and invaded fractures in other adjacent silicate intrusions forming a stockwork of narrower veins well beyond the central dike swarm. Based on cross-cutting relationships, alteration patterns, and previous 40Ar/39Ar ages, carbonatite is thought to be the youngest intrusive phase within the BLAC, excluding perhaps lamprophyre (Duke et al., 2002; Duke, 2009). New 40Ar/39Ar analyses of carbonatite-hosted K-feldspar and biotite from the BLC give consistent ages between 51 and 52 Ma (Andersen et al., 2013). Carbonate and K-feldspar alteration (fenitization) is pervasive throughout the complex and is most evident in deeper drill core, when not masked by near surface oxidation. A majority of REE, Th, Au, and Cu mineralization is concentrated near diatreme breccia bodies where the northern and southern lobes merge, suggesting this was a center of carbonatitic and hydrothermal activity.

3. Analytical procedures and treatment of REE data 3.1. Whole rock geochemistry Whole-rock compositions were measured at Activation Laboratories Ltd. using a combination of techniques. Major, trace, and rare earth element concentrations were determined using a lithium metaborate/tetraborate fusion of pulverized rock samples with subsequent analysis by ICP-MS. Under certain circumstances, the presence of small amounts of phosphate in a sample are known to cause interferences that bias the results toward very low Nb2O5 values by this method; therefore Nb2O5, ZrO2, and Ta2O5 were reanalyzed by fusion XRF. A second trace element analysis included an aqua regia digestion followed by ICP-MS analysis for chalcophile elements known to volatilize (e.g., Sb, As, Mo, Ga, Zn) during the fusion process. Reduced iron was measured by titration, and sulfur species were determined by infrared methods. Fluorine was analyzed by ion selective electrode and CO2 by coulometry. Crystal fractionation along carbonatite vein margins may result in the underestimation of some minor and trace element (e.g., HFSE) concentrations, particularly when marginal material is excluded during sample selection. Thus, carbonatite samples are not a pristine representation of carbonatitic fluid composition. This bias is likely magnified in oxidized carbonatite, where much of the material that can still be retrieved through modern drilling methods is the HFSE- and P-enriched fenite margins, which remain adhered to wall rock after dissolution of the central carbonate-

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A.K. Andersen et al. / Ore Geology Reviews 89 (2017) 780–807 104°30’

104°25’

Neogene and younger sediments Paleogene carbonatite (extrapolated to surface)

Paleogene intrusive breccia (diatreme) Paleogene alkaline silicate intrusions Mesozoic sedimentary rocks Upper Paleozoic Lower Paleozoic

Fig. 2

Faults

44°30’

Archean granite/gneiss

Canada BLAC

US

Mexico Fig. 1. Simplified geologic map of the Bear Lodge alkaline complex (BLAC) showing the areas where carbonatite dikes and veins are most pervasive, extrapolated to the surface.

rich portions of veins, resulting in overestimation of HFSE concentrations. Potential bias was addressed by sampling oxidized surface samples in situ in addition to drill core samples. 3.2. Electron probe microanalysis (EPMA) and Scanning electron microscopy (SEM) Ca-REE fluorocarbonate compositions were determined by wavelength dispersive electron probe microanalysis (WD-EPMA) using a Cameca SX100 electron microprobe housed at the University of Oregon’s CAMCOR facility. Rare earth phosphate compositions were determined by WD-EPMA using a JEOL JXA-8500 F field emission electron microprobe at Washington State University’s GeoAnalytical Laboratory. The instrument was also used to generate many of the backscattered electron (BSE) images and X-ray element maps used in this study. Additional BSE-SEM images were obtained using FEI Quanta 200 F field emission SEMs housed at CAMCOR and Washington State University’s Franceschi Microscopy Center. The fibrous aggregates and platelets with narrow cross-section, and syntaxial intergrowths of Ca-REE fluorocarbonates and the hydrated/hydroxylated nature of REE phosphates introduce greater uncertainty compared to ideal samples required for high accuracy EPMA. Specific acquisition parameters and setup information for REE phosphate and fluoro-

carbonate analyses are presented in Appendix Table 1. Several analyses were performed for each sample to monitor reproducibility and the influence of syntaxy on mineral chemistry. Moderate beam currents and drift corrections (cf., Nielsen and Sigurdsson, 1981) were used to address analytical challenges, such as diffusive volatility of F and Na (Goldstein et al., 1984; Pyle et al., 2002) and variable electron microprobe oxide totals due to the presence of significant unanalyzed H2O or OH
. Our analyses included bastnäsite from the Mountain Pass REE deposit (California) and parisite from the Snowbird REE occurrence (Idaho), which gave the expected stoichiometry for fluorocarbonates from these localities and confirms that the appropriate peak overlap and drift corrections were made for the most accurate F measurements possible. The presence of other elements not included in the routine and submicron inclusions may also contribute to low totals and deviation from ideal stoichiometry. Various electron microprobe settings have been used to minimize crystal damage, volatility, and other potential sources of error, while still achieving precise measurement of REE. Recent studies which involve analysis of hydrated REE phosphates (e.g., Krenn and Finger, 2007; Berger et al., 2008; Göb et al., 2011) and Ca-REE fluorocarbonates (e.g., Ruberti et al., 2008; Guastoni et al., 2009; Moore et al., 2015) have used an accelerating voltage of 15–25 kV and a beam current between 3 and 50 nA. During our

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N

Carbon

Extent of Fig. 9

Whitetail Bull Hill Taylor

300 m

Taylor South

Fig. 2. Distribution of REE in the central Bear Lodge carbonatite stockwork based on geochemical data from 19,793 samples in 242 drill holes, and extrapolated to the surface. Areas of significant REE mineralization are divided into five areas: Bull Hill, Whitetail, Carbon, Taylor, and Taylor South. Red shaded areas contain >1.5 wt% total rare earth oxide. Image courtesy of Rare Element Resources, Ltd.

trials, completed using a range of beam conditions, the greatest accuracy and precision were achieved with an accelerating voltage of 20 kV and beam current of 20–50 nA. The small areas (<5 mm) of homogeneous composition require a focused beam and increased counting time, which also contributes to mineral degradation during analysis. A defocused beam (2–10 mm) was used where crystal dimensions permitted. These conditions also allow for lower detection limits (<0.1 wt%, Appendix 1) and analysis of REE at low concentration, while maintaining reasonable analysis time. Other studies have used lower beam currents to minimize damage and volatility, but sacrificed precision for REE. For this study on REE fractionation, we considered collection of the full REE suite important. The similar electronic structure of the REE leads to further difficulty during EPMA due to background interferences and peak overlaps in the L series X-ray spectra, both between the individual REE, most notably Pr, Eu, Gd, Ho, Er, Tm, and Lu, and between the REE and other elements that may be present in the analyte, such as F, Ti, Mn, and Ba (cf., Roeder, 1985; Laputina et al., 1999). To address these issues, automated peak overlap and modeled background corrections were made using ProbeForEPMA software (Donovan et al., 1993, 2007, 2011). Preferred X-ray analytical lines were chosen based on the recommendations of Roeder (1985), Williams (1996), and Pyle et al. (2002). A combination of natural and synthetic standards were used for calibration, including a set of synthetic REE orthophosphates. For this study, the boundary between LREE and HREE is placed between Gd and Tb. This placement is supported by the monoclinic to tetragonal structural transformation that occurs when the rare earth ion changes from coordination with nine oxygens to coordination with eight oxygens in GdPO4 and TbPO4, respectively (Ni et al., 1995; Boatner, 2002).

4. Results 4.1. Mineralogy Based on surface excavation and exploration drilling, the potentially economic REE resource at Bear Lodge is divided into five areas where REE are most highly concentrated: Bull Hill, Whitetail, Carbon, Taylor, and Taylor South (Fig. 2). Observed primarily in the central areas at Bull Hill and Whitetail, a vertical change from REE (±Sr,Ca,Na,Ba) carbonate minerals (ancylite, carbocernaite, and burbankite) to Ca-REE fluorocarbonate minerals (bastnäsite, parisite, synchysite, and mixed varieties) corresponds with an increase in sulfide oxidation, decrease in the volume of matrix carbonate, and increase in pore space. Fig.3a–h shows REE mineral replacement textures and general mineralogical changes with depth, as described in the following sections. 4.1.1. Unoxidized carbonatite Unoxidized carbonatite at Bear Lodge is heterogeneous, but predominantly calciocarbonatitic in composition with fine- to coarsegrained matrix calcite of variable Mn (1.0–8.0 wt%), Sr (0.5–1.5 wt %), and REE (50–1500 ppm) concentrations (Olinger, 2012; Moore et al., 2015; Hutchinson, 2016). Strontianite, dolomite, and ankerite constitute a lesser percentage of matrix carbonate minerals. The upper contact of unoxidized, sulfide zone carbonatite, which contains 40–75% matrix carbonate minerals, extends from depth upward to within approximately 230 m of the surface (Fig. 3a) (Dahlberg et al., 2014). Rare earth minerals occur as polycrystalline pseudomorphs containing any combination of ancylite, carboncernaite, burbankite, strontianite, Ca-REE fluorocarbonates, monazite, barite, celestine, apatite, and fluorite, but are chiefly composed of ancylite + strontianite ± barite ± carbocernaite (Fig. 3b–e). The

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present surface h

a

cerianite

oxide zone

~120 m

transion zone

Ca-REE fluorocarbonate pseudomorphs

g str rfc anc

f 1 cm

0.5 mm

monazite

Ca-REE fluorocarbonates

ancylite

burbankite/carbocernaite

~230 m

rhabdophane group REE phosphates

e str

cal

cal 0.5 mm

anc cal

d

bt 1 cm

ancylite+ strontianite+barite pseudomorphs 1.7 mm

anc

fl

b crb/ bbk?

sulfide zone (unoxidized carbonate) ~2200 m

c

primary burbankite?

cal

100 μm

Fig. 3. (a) Schematic diagram of REE mineral distribution with depth through the Bear Lodge carbonatite stockwork. (b) BSE image of polycrystalline pseudomorph with ancylite (anc) replacing early carbocernaite (crb) or burbankite (bbk) and trace fluorite (fl) in calcite (cal) matrix. (c) Pink ancylite + strontianite + barite pseudomorphs in calcite matrix with trace biotite in sulfide zone core sample. (d) Plane polarized transmitted light micrograph of hexagonal ancylite + strontianite + barite pseudomorphs in calcite matrix with trace biotite (bt). (e) Hexagonal pseudomorph consisting of ancylite (anc), strontianite, (str), and calcite (cal) in calcite matrix. Pseudomorphs of this type commonly contain barite. (f) Transition zone whole rock sample with yellow/cream colored Ca-REE fluorocarbonate pseudomorphs in Mn oxide + carbonate matrix. (g) BSE image with synchysite-dominant Ca-REE fluorocarbonates (rfc) replacing ancylite + strontianite mineralization in transition zone carbonatite. (h) Friable weathered oxide zone carbonatite equivalent consisting of Fe + Mn oxides and yellow/cream colored Ca-REE fluorocarbonate minerals.

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rutile/pseudorutile and in biotite-rich selvages of late-stage carbonatite veinlets, commonly in association with sulfides.

pseudomorphs have generally retained a hexagonal or pseudohexagonal cross-section, and the suspected relict primary phase, burbankite, has been observed in only a few samples (Moore et al., 2015; Olinger, 2012; this study). The reader is referred to Moore et al. (2015) for a more detailed description of REE mineral paragenesis and mineral chemistry of unoxidized Bear Lodge carbonatite. Accessory minerals in BLC intrusions are potassium feldspar, biotite/phlogopite, aegirine-augite, fluorapatite, ilmenite, pyrochlore/betafite, pyrrhotite, pyrite, sphalerite, and galena. Uranpyrochlore/betafite was first recognized in carbonatite veinlets cross-cutting trachyte and contemporaneous with sulfide mineralization (Mariano, 1978b). Our investigations show U-rich pyrochlore/betafite occurs as inclusions in ilmenite or

4.1.2. Oxide- and transition-zone carbonatite equivalents In the upper oxidized zone (0–120 m), carbonatite is altered to a friable combination of potassium feldspar, undifferentiated clay minerals, phlogopite/vermiculite, Fe, Mn, and Ti oxides, Ca-REE fluorocarbonates, REE phosphates (monazite and rhabdophane), apatite, and cerianite, with little or no matrix carbonate (Fig. 3h). The transition zone (120–230 m) is defined by partial sulfide oxidation and a decrease in matrix carbonate relative to sulfide zone carbonatite. Compared with material of the upper oxidized zone, matrix carbonates of transition zone carbonatite hold

b

a

10 μm

10 μm

c

d

ap fl LnPO4

ap

100 μm

20 μm

e

f

rhb rfc

SiO2

100 μm

1.0 mm

Fig. 4. BSE images of REE fluorocarbonates, phosphates, and cerianite in oxidized carbonatite. (a) Characteristic fibroradial Ca-REE fluorocarbonate aggregates. (b) Ca-REE fluorocarbonate cluster with fine botryoidal cerianite (bright white). (c) Fluorapatite (possibly hydroxylated) and fluorite mineralization. Fine grained REE phosphates (LnPO4) occur as an alteration product of fluorapatite. (d) Typical zoned REE phosphate cluster from Taylor. Bright areas are LREE-dominant, while darker domains contain Y > Ca > HREE > LREE. (e) Co-existing Ca-REE fluorocarbonates (rfc) and zoned rhabdophane (rhb) group REE phosphates in matrix of silicified Fe + Mn ± Ba ± Pb oxides. (f) Atoll-shaped cerianite (bright white) rimming colloform fluorocarbonates aggregates.

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a

b

10 μm

200 μm

Fig. 5. BSE images of REE and HFSE minerals in laterite-like monazite veins at Taylor South. (a) Fine acicular morphology of supergene monazite, the predominant REE mineral at Taylor South. (b) Pyrochlore (bright white) precipitated along rutile/pseudorutile cleavage planes.

1000000 Bull Hill (unoxidized, sulfide zone) Taylor South (monazite laterite) Taylor (oxidized, no carbonate) Whitetail (oxidized, no carbonate) Whitetail (transition) Bull Hill (oxidized, no carbonate) Bull Hill (transition)

100000

rock / primitive mantle

10000

1000

100

10

1

0.1 Cs Rb Ba Th

U Nb K

La Ce Pb Pr Sr P

Nd Zr Hf Sm Eu Ti Gd Tb Dy Y Ho Er Tm Yb Lu Sc

V

Fig. 6. Primitive mantle-normalized trace element patterns for whole rock carbonatite and REE mineralized vein samples (primitive mantle composition after McDonough and Sun, 1995).

1000000 Bull Hill (unoxidized, sulfide zone) Bull Hill (transition) Bull Hill (oxidized, no carbonate) Taylor South (monazite laterite)

100000

Taylor (oxidized, no carbonate) Whitetail (transition)

rock/chondrite

Whitetail (oxidized, no carbonate)

10000

1000

100

10 La

Ce

Pr

Nd

Sm

Eu

Gd

Tb

Dy

Y

Ho

Er

Tm

Yb

Lu

Fig. 7. Chondrite-normalized REE patterns for whole rock carbonatite and REE mineralized vein samples (chondrite composition after Taylor and McLennan, 1985). Nearly all transition and oxide zone carbonatite samples show some degree of HREE enrichment relative to unoxidized Bull Hill carbonatite. The laterite-like monazite veins at Taylor South have a REE distribution that is indistinguishable from other oxide and transition zone Bear Lodge carbonatites.

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Taylor (n=5) oxidized carbonatite normalized to average unoxidized Bear Lodge carbonatite

100

10

1

0.1

0.01 Cs Rb Ba Th

U

Nb K

La Ce Pb Pr

Sr

P

Nd Zr

Hf Sm Eu Ti

Gd Tb Dy

Y

Ho Er Tm Yb Lu Sc

V

Gd Tb Dy

Y

Ho Er Tm Yb Lu Sc

V

Gd Tb Dy

Y

Ho Er Tm Yb Lu Sc

V

Gd Tb Dy

Y

Ho Er Tm Yb Lu Sc

V

Taylor South (n=3) oxidized carbonatite normalized to average unoxidized Bear Lodge carbonatite

100

10

1

0.1

0.01 Cs Rb Ba Th

U

Nb K

La Ce Pb Pr

Sr

P

Nd Zr

Hf Sm Eu Ti

Whitetail (n=4) oxidized carbonatite normalized to average unoxidized Bear Lodge carbonatite

100

10

1

0.1

0.01

0.001 Cs Rb Ba Th

U

Nb K

La Ce Pb Pr

Sr

P

Nd Zr

Hf Sm Eu Ti

Bull Hill (n=4) oxidized carbonatite normalized to average unoxidized Bear Lodge carbonatite

100

10

1

0.1

0.01 Cs Rb Ba Th

U

Nb K

La Ce Pb Pr

Sr

P

Nd Zr

Hf Sm Eu Ti

Fig. 8. Trace element chemistry of altered samples normalized to the average of nine unoxidized Bull Hill carbonatite samples in Table 1a. Solid shading = oxide zone samples; Unfilled patterns = transition zone samples; unfilled Taylor pattern is a fluorite vein sample containing fine-grained REE phosphates and secondary apatite.

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together a more competent rock in which the precursor crystal forms of Ca-REE fluorocarbonate pseudomorphs are more recognizable (Fig. 3f). Ca-REE fluorocarbonates are the most abundant REE-bearing mineral and occur as clusters of radial platelets replacing the precursory ancylite + strontianite ± barite ± carbocernaite assemblage (Fig. 3g, Fig. 4a). Pseudomorphs which contain both fluorocarbonates and ancylite are most abundant within the transition zone. A change from pink colored ancylite-dominant pseudomorphs in white matrix calcite to yellow fluorocarbonatedominant pseudomorphs in black, Mn oxide-rich matrix is observed in core samples which transect the transition zone (Fig. 3c,f). Rare earth phosphates at Bull Hill, Whitetail, and Carbon are typically small, micron-sized crystals dispersed among Ca-REE fluorocarbonates, not easily analyzed by electron microprobe. Finegrained, botryoidal cerianite (CeO2) is abundant and associated with Ca-REE fluorocarbonates or Mn-Fe oxides (Fig. 4b). At Whitetail and Carbon, meter-scale portions of clay-altered diatreme breccia host high concentrations of REE, although not as physically adsorbed trivalent ions on clay surfaces as in weathered crust elution-deposited rare earth ores. Instead, REE are hosted by 1–10 lm fluorocarbonates, phosphates, cerianite, and rare crandallite-group minerals disseminated within the clay matrix. 4.1.3. Taylor oxide-zone Oxidized carbonatite equivalents at the Taylor prospect are significantly leached and contain almost no matrix carbonate. Secondary silicification is pervasive throughout the network of narrow veins and dikes. Taylor veins are mineralogically more heterogeneous than those from other areas of the deposit; some are dominated by fluorocarbonates similar to those at Bull Hill, while others are dominated by rhabdophane or secondary apatite and fluorite (Fig.4c). A notable difference at the Taylor prospect, is the greater abundance of rhabdophane group REE phosphates. Radial and sheaf-shaped clusters of rhabdophane occur within clusters of Ca-REE fluorocarbonates (Fig.4d,e). In terms of total phosphate abundance at Taylor, fluorapatite appears to be just as abundant, if not more so, than rhabdophane or monazite, although their proportions between individual veins are highly variable. Cerianite occurs as pseudomorphic replacement after earlier phases, late void-filling masses associated with Mn-Ba-Pb oxides, and in colloform fluorocarbonate aggregates (Fig.4f). At shallow levels, native gold is found within cubes of Fe oxide, pseudomorphous after pyrite. 4.1.4. Taylor South laterite-like monazite veins REE mineralization at Taylor South is significantly different from carbonatite or oxidized carbonatite at all other localities. Unlike Bull Hill or Taylor, just 0.5 km to the northeast, there are no carbonate or fluorocarbonate minerals present. The best exposures appear to match veins #10 and #19 of Staatz (1983), surrounded by areas of pervasive argillic alteration and silicification in the adjacent alkaline silicate host rocks. The yellow-green veins with highly irregular contacts consist predominantly of finegrained monazite (15–20% modal), jarosite (10–15% modal), and quartz or chalcedony (40–55% modal) with subordinate rutile/pseu dorutile/ilmenorutile (7% modal), barite (5% modal), potassium feldspar and zoned pyrochlore. Monazite at Taylor South occurs as 5–20 mm radial aggregates of acicular needles which terminate at points  1 mm (Fig. 5a). Zoned, partially metamict pyrochlore is associated with rutile/pseudorutile and commonly occurs along rutile cleavage traces (Fig. 5b). 4.2. Whole rock geochemistry Compared with carbonatites worldwide, BLC contain some of the highest concentrations of REE, Sr, and other elements which

Carbon

N >0.009

Whitetail

Ratio of: Tb2O3→Lu2O3(+Y2O3) 0.006-0.0075 ———————————————————— — TREO

Bull Hill

0.0075-0.009

<0.0045

Taylor

0.0045-0.006

300 m

a

Carbon

N Values in wt.% P2O5

Whitetail <0.4 0.6-0.8

0.4-0.6 >1.0

Taylor

b

300 m

Bull Hill

0.8-1.0

Fig. 9. Level plans at 6000 ft (1828.8 m) elevation through 3D model of REE resource at the BLAC based on 3867 drill hole assays with >1.5 wt% total rare earth oxide (TREO). (a) Level plan shows lateral variations in proportion of REE, Tb and heavier. (b) Lateral variation of P2O5 concentration. Note the correlation between HREE-enrichment and P2O5 at Taylor, but lack of correlation at Whitetail/Carbon. Images courtesy of Rare Element Resources, Ltd.

are incompatible and typically occur at trace levels in other rocks of igneous or metasomatic origin. At the BLAC, hydrothermally modified, oxidized carbonatite exhibits REE-enrichment relative to precursory unoxidized equivalents, reaching values of more than 20% TREO. Most notable are the concentrations of La (4140–21000 ppm), Ce (9220–35800 ppm), and Nd (4800–13900 ppm), which achieve even higher concentrations in oxidized equivalents: La (54500–66800 ppm), Ce (11500–92100 ppm), Nd (4740–31200 ppm) (Tables 1a and 1b). Europium is also noticeably higher (unoxidized: 164–311 ppm; oxide zone: 280–1270 ppm) compared to many other carbonatites, where Eu2+ is partitioned into early crystallized silicate phases in reduced magmas. Relative to primative mantle composition (McDonough and Sun, 1995), HFSE exclusive of the REEs (Th, U, Nb, Zr, Hf, Ti, Sc) are generally enriched in BLC (Fig. 6). A few samples have Zr, Ti, and Sc concentrations slightly less than primative mantle values, while Th and U concentrations are 3–5 orders of magnitude higher. Like other carbonatites, BLC are enriched in LREE relative to HREE, with steep chondrite-normalized REE profiles decreasing from La through Lu (Fig. 7). This remains ture for hydrothermally altered varieties,

789

A.K. Andersen et al. / Ore Geology Reviews 89 (2017) 780–807 Table 1a Whole rock compositions of unoxidized Bull Hill carbonatite and oxidized equivalents. Bull Hill (sulfide zone/unoxidized carbonatite) R25177

R25178

R25179

R31502

R33971

Oxide (wt.%) SiO2 Al2O3 Fe2Oa3 FeO MnO MgO CaO Na2O K2O TiO2 P2O5 CO2 Sb SO4 F LOI Totalc

R39334

2.16 0.60 – 8.10 3.20 0.96 31.96 0.08 0.43 0.14 0.23 32.20 0.27 8.80 0.18 28.28 85.39

5.09 1.48 – 11.40 3.01 0.59 33.29 0.10 1.26 0.20 0.06 30.90 0.06 11.80 0.07 26.07 94.48

1.25 0.38 – 10.50 2.85 0.50 30.75 0.11 0.32 0.10 0.04 32.50 0.32 7.40 0.11 29.52 84.15

7.28 2.35 – 9.10 3.00 0.87 24.12 0.16 1.74 0.80 0.28 26.30 0.54 15.90 0.19 21.41 87.74

Li Be B Sc V Cr Co Ni Cu Zn Ga Ge As Se Mo Ag Cd In Sn Sb Te Rb Cs Sr Ba Pb Y La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Zrd Nb Hf Ta W Re Au

37 5 <1 13 74 0.5 5.6 8.2 26.2 5570 153 24 69 14.1 1.55 8.4 3.55 0.75 <1 0.24 4.58 24 0.82 79380 8140 1330 166 17500 25300 2470 8570 1160 218 413 25.7 52.8 4.7 9 1.28 8.4 1.19 21.0 8.4 0.7 < 0.05 3 0.017 <5

28.9 1 <1 10 45 0.8 8.6 7.5 55.3 3820 97 19 48 17.2 1.62 17.1 4.35 1.07 1 0.02 8.48 31 0.45 45520 7402 1980 193 8710 15500 1750 6860 1110 213 415 29.4 67.1 5.9 11.3 1.7 12.1 1.68 5.0 55.9 0.5 < 0.05 <1 0.159 <5

22.7 3 <1 14 36 < 0.5 6.5 7 16.6 620 169 28 70 13.5 1.1 9.2 0.19 0.46 <1 0.07 4.27 17 0.53 94710 7084 1670 170 20600 30100 2940 10100 1290 236 432 29.7 68.5 5.1 8.7 1.06 9 1.43 8.0 21.5 0.6 < 0.05 2 0.017 <5

16.9 4 <1 14 85 16.4 2.6 5.7 8.68 3520 142 20 65 28.2 84.8 6.1 0.09 0.45 3 0.34 4.56 31 0.44 67840 9391 1210 416 17000 24300 2280 7620 1090 256 574 56.7 173 16 21.7 2.35 14.6 1.85 25.0 153.8 1.6 < 0.05 7 0.024 <5

18.24 7.66 4.89 2.11 11.43 – 0.80 9.60 2.60 3.27 2.65 2.53 24.67 31.41 0.09 0.14 3.65 2.02 1.32 1.89 0.34 0.48 20.60 27.30 0.03 0.62 0.90 5.10 0.29 0.34 23.16 24.21 95.06 91.37 Element (ppm), 96.8 166 4 2 <1 <1 6 6 198 197 6.8 7.8 8.9 6.5 6.8 7.2 25.1 52.2 4350 4110 102 93 15 15 49 38 12.2 17.8 5.82 5.7 20.1 32.9 35.5 53.5 0.39 0.63 3 2 0.05 0.19 6.13 10.7 200 130 6.95 3.73 7881 24560 5463 6020 2680 3160 213 207 4350 4140 9220 9360 1160 1230 4800 5300 834 1010 164 204 338 411 22.8 28.3 63.8 73.2 6.9 6.7 13.8 12 1.74 1.53 12.8 10.9 1.67 1.45 22.0 4.0 356.5 307.6 0.9 0.5 < 0.05 < 0.05 33 1 0.002 0.224 9 29

Hg Tl Bi Th

< 10 2.32 63.5 294

< 10 2.43 93.8 270

< 10 2.26 73.5 424

< 10 3.88 54.6 1320

< 10 8.8 111 191

< 10 4.98 155 199

R43643

589864

589890

7.77 3.28 3.45 2.36 1.28 1.11 2.04 1.75 0.56 12.50 9.10 10.10 1.68 2.38 1.44 0.79 1.10 0.53 22.35 32.83 26.63 0.11 0.07 0.11 1.81 0.75 0.50 0.24 0.15 0.24 0.26 0.14 0.81 23.70 30.20 26.40 0.78 0.48 0.56 25.30 17.10 19.90 0.35 0.18 0.81 15.75 24.87 20.70 94.10 95.46 87.44 unless otherwise noted 46.3 41.3 30.8 8 4 10 <1 <1 <1 12 17 18 73 146 71 14.4 14.3 11.5 15.2 2.5 2.9 5.3 7.7 5.2 66.2 20.5 10.9 2300 3250 4160 143 79 233 21 15 39 63 141 135 17.1 21.2 17.2 2.19 209 28.4 20.2 12.5 9.4 1.63 0.25 0.59 0.4 1.74 0.22 3 4 <1 0.13 1.6 1.06 19.7 5.32 5.55 44 30 13 1 0.75 0.28 64240 30410 70170 5218 8917 8066 1080 1910 2070 144 232 205 15800 5850 21000 22900 11500 35800 2200 1360 3810 7590 5250 13900 963 1060 1780 174 236 311 339 517 570 21.5 37.4 37.7 54.2 98.2 81.5 4.2 8.3 5.5 7.7 15 9.7 1.03 2.06 1.25 6.7 15.9 8.2 0.91 2.27 1.15 29.0 4.0 11.0 48.9 21.0 29.5 0.8 0.7 0.7 < 0.05 < 0.05 < 0.05 3 <1 4 0.35 0.034 0.043 41 > 109 10000 < 10 < 10 < 10 3.5 12.8 6.74 90.7 74.7 83.8 219 955 329

Bull Hill (transition) R25653

Bull Hill (oxidized carbonatite) R25340

R40895

R42467

8.96 1.40 11.73 < 0.1 1.42 0.44 29.48 0.05 1.07 0.60 0.17 26.60 0.03 0.60 0.73 28.65 85.33

12.81 2.60 9.65 16.00 3.78 1.44 2.14 0.05 2.07 0.97 0.47 5.31 0.02 1.90 1.77 13.81 69.48

12.63 2.97 23.17 15.00 8.11 1.46 1.13 0.05 2.40 0.19 0.29 2.68 – 2.50 0.58 12.42 82.90

10.27 2.99 7.44 14.70 23.42 1.70 0.94 0.11 2.35 1.35 1.44 2.68 – 0.40 1.25 12.84 81.20

32.6 13 <1 22 215 2.8 4 6.6 5.74 3060 228 33 244 80.8 182 7.5 < 0.01 0.4 5 1.95 5.81 22 0.72 14030 8835 2960 1713 30700 39600 3630 12200 1740 421 1080 138 566 65.2 105 9.78 49.6 5.69 31.0 132.8 3.6 < 0.05 10 0.013 <5

130 28 20 61 633 6.3 6.1 25.1 16 7250 561 85 380 167 467 9 11.3 1.13 28 1.47 14.4 80 1.59 1426 25170 >5000 4868 66800 92100 9060 31200 4830 1120 2570 286 1180 151 312 34.1 172 20.7 422.0 300.6 7.7 < 0.05 14 0.063 <5

160 8 3 23 235 6.4 1.5 1.6 54.9 5790 260 45 208 34.8 61.6 39.2 2.68 1.7 6 0.25 42.5 86 1.1 8489 25870 >5000 316 22900 40700 4460 17200 2750 528 982 54.1 125 8.9 13.4 1.77 14.9 2.1 11.0 132.8 1.1 < 0.05 <1 0.007 61

187 36 4 26 271 7.7 56.9 39.6 30.4 9770 325 50 282 25.8 154 22 6.06 0.82 5 1.22 12.6 121 10.2 3023 11240 >5000 333 31400 50500 5160 17600 2100 371 664 41.7 112 9.8 17.4 2.47 17 2.47 155.5 412.4 1.5 < 0.05 14 0.001 14

< 10 16.6 116 2750

100 29 289 5750

10 9.74 209 973

130 68.7 138 560

(continued on next page)

790

A.K. Andersen et al. / Ore Geology Reviews 89 (2017) 780–807

Table 1a (continued) Bull Hill (sulfide zone/unoxidized carbonatite)

U P REO+Y2O3 (%)e P (HREE+Y/ REE+Y) x100f (Ce/Ce*)g (La/Yb)CN

R25177

R25178

R25179

R31502

R33971

R39334

R43643

589864

589890

Bull Hill (transition) R25653

Bull Hill (oxidized carbonatite) R25340

R40895

R42467

6.6

57.9

34.9

45.5

83.6

137

49.2

21.9

13.9

88.8

268

106

220

5.59

3.49

6.60

5.38

2.12

2.20

5.02

2.62

7.75

9.20

21.47

9.01

10.83

0.48

0.92

0.44

1.30

1.59

1.55

0.48

1.57

0.45

2.88

3.27

0.60

0.49

0.80 1407.8

0.89 486.4

0.81 1546.7

0.81 786.8

0.95 229.6

0.97 256.7

0.81 1593.6

0.93 248.6

0.88 1730.6

0.75 418.3

0.78 262.4

0.90 1038.6

0.86 1248.1

Table 1b Whole rock compositions of altered and oxidized carbonatites peripheral to Bull Hill. Taylor South (carbonatite laterite)

Taylor (oxidized carbonatite)

R25164

R25165

R25166

R25167

Oxide (wt.%) SiO2 Al2O3 Fe2O3a FeO MnO MgO CaO Na2O K2O TiO2 P2O5 CO2 Sb SO4 F LOI Totalc

R25168

51.29 0.35 7.05 1.10 0.02 0.25 1.75 0.05 0.96 0.63 7.61 na 0.19 5.00 0.16 8.90 85.31

54.02 0.49 7.31 < 0.1 0.49 0.16 2.45 0.06 0.42 6.53 5.93 na 0.12 2.70 0.20 7.98 88.86

39.99 2.85 11.80 0.80 0.03 0.51 1.07 0.12 2.09 0.14 7.10 na 0.08 5.90 0.32 9.34 82.14

Li Be B Sc V Cr Co Ni Cu Zn Ga Ge As Se Mo Ag Cd In Sn Sb Te Rb Cs Sr Ba Pb Y La Ce Pr Nd Sm Eu Gd Tb

10.9 24 1 36 301 10 5.7 9.2 18.4 140 309 46 316 32.7 23.1 13.1 0.26 0.19 7 0.53 4.63 4 0.15 1871 19400 4620 735 39500 59000 5560 17800 2110 381 664 50

11 19 2 29 592 9.4 10.1 19.5 19.8 250 232 39 285 33.4 47.3 13.6 0.27 0.6 7 2.23 20.3 5 0.12 1243 14900 4710 830 22700 40200 4260 15000 1980 378 710 60.8

310 44 3 33 526 1.7 1.7 6.7 105 200 328 61 399 91.1 434 95.5 < 0.01 0.36 2 10.6 5.71 32 0.15 2697 41050 4460 2718 34500 55500 6360 23900 3700 756 1540 154

37.41 52.13 1.55 1.03 12.44 16.52 – – 5.20 8.12 0.18 0.20 12.02 0.68 0.24 0.05 1.10 0.71 0.37 0.18 10.37 0.82 0.35 0.68 0.02 0.03 0.50 0.60 0.91 0.29 7.63 9.22 89.95 90.58 Element (ppm), unless 17.4 16.6 7 10 3 1 34 24 292 1693 5.5 10.5 4.7 3.9 4 142 31.8 113 2400 3340 158 150 29 29 458 543 123 43.4 338 546 23.6 10.7 0.36 0.83 1.69 1.79 17 7 2.55 6.6 9.42 13.7 48 42 5.18 4.77 2269 772 14400 17780 2880 1960 3446 645 13000 10500 24600 24600 2720 2750 10100 10200 1980 1560 560 344 1540 844 205 90.6

R25169

R25170

15.06 7.47 10.53 2.75 6.24 15.21 – 14.60 5.22 27.17 0.15 0.16 34.38 4.29 0.27 0.16 1.45 2.35 0.39 0.54 3.49 4.55 0.21 0.21 0.02 0.12 0.40 < 0.3 18.00 0.37 – 11.01 95.59 90.75 otherwise noted 7 12.8 14 15 8 2 23 57 1507 378 5.6 9.2 4.4 7.4 40.7 6.5 49.9 35.8 650 2610 41 110 11 18 72 568 118 53.2 168 705 1.5 11.5 2.98 0.56 1.86 2.24 6 20 4.53 23.9 0.99 21.6 20 30 0.23 0.14 2210 4737 5312 18700 273 1420 3719 1195 1110 5450 3090 11500 528 1310 2530 4740 772 992 257 280 968 773 197 93.9

Whitetail (oxidized carbonatite)

Whitetail (transition)

R32227

R39919

R41397

R41455

R41862

14.15 4.16 5.47 15.80 8.47 0.14 12.23 0.34 3.34 2.30 9.32 1.38 0.05 1.20 1.16 8.92 87.05

27.81 9.78 8.86 15.30 7.06 0.32 0.46 0.15 6.22 1.11 1.36 1.67 0.01 0.60 0.64 7.59 87.26

34.10 8.87 5.53 15.50 4.22 0.27 0.41 0.15 7.32 0.76 0.12 1.37 0.02 1.00 0.29 7.92 86.47

6.74 1.62 12.17 < 0.1 1.64 0.25 38.80 0.02 0.91 0.35 0.07 29.10 0.00 0.80 0.29 32.58 96.24

8.54 1.55 11.32 < 0.1 1.68 0.14 36.57 0.09 1.15 1.16 2.19 26.60 0.03 0.60 0.41 29.23 94.66

12.3 18 3 39 670 9.8 6.6 5.3 51.6 3840 172 29 524 192 490 8.7 0.26 2.36 36 41.1 12.5 48 0.84 4273 23760 2890 4982 12900 23200 2500 9370 2430 786 2500 361

27.3 9 5 12 193 8.2 35.1 16.5 566 4570 216 63 337 177 70.8 26.6 21.3 0.45 10 0.47 5.07 106 0.5 1274 4618 1930 6798 20000 24500 4800 20900 4800 1270 3540 420

18.5 8 6 43 239 8.9 14.9 5.2 39.1 4730 227 40 682 117 317 19.4 46.6 2.7 7 4.55 4.12 143 7.54 494 19420 1380 1911 11100 24300 3020 13100 2540 673 1810 230

9.4 22 <1 16 139 5.6 5.8 9.7 12 2460 83 16 265 64.9 118 8.8 5.55 0.63 3 2.77 3.35 25 9.59 1055 11940 2690 1050 4510 9960 1230 5100 1030 281 783 104

8.8 12 <1 8 129 7.3 8.5 10.2 65.7 2610 65 16 332 71.3 162 2.6 16.9 0.39 5 4.28 7.33 22 1.08 1544 10530 2810 989 4040 9080 1180 5160 1190 311 865 97.5

791

A.K. Andersen et al. / Ore Geology Reviews 89 (2017) 780–807 Table 1b (continued) Taylor South (carbonatite laterite)

Dy Ho Er Tm Yb Lu Zrd Nb Hf Ta W Re Au Hg Tl Bi Th U P REO+Y2O3 (%)e P (HREE+Y/ REE+Y)x100f g (Ce/Ce*) (La/Yb)CN

Taylor (oxidized carbonatite)

Whitetail (oxidized carbonatite)

Whitetail (transition)

R25164

R25165

R25166

R25167

R25168

R25169

R25170

R32227

R39919

R41397

R41455

R41862

164 18.3 41.4 5.18 33.7 4.68 199.9 1034.6 2.1 < 0.05 17 < 0.001 <5 < 10 6.07 111 493 426

198 24.3 47.6 6.4 41.4 5.46 148.1 1712.7 1.8 < 0.05 13 < 0.001 <5 10 12.4 176 762 252

667 85.8 149 15.8 78.4 8.41 333.1 132.8 5.6 < 0.05 6 < 0.001 <5 10 2.82 130 3630 278

873 113 249 27 145 16.1 133.3 160.8 5.1 < 0.05 6 0.009 76 < 10 12.5 55.6 4020 61.6

310 28.1 49.5 4.81 27.5 3.49 133.3 160.8 1.9 < 0.05 3 < 0.001 180 50 31 41.4 3040 47.3

1030 128 231 20.5 89.8 9.71 27.0 18.1 7 < 0.05 18 0.005 <5 < 10 6.51 2.57 2910 64

376 43.9 70.9 6.97 35.1 3.9 4.0 363.5 2.5 < 0.05 7 0.001 6770 120 21.4 28.9 1660 75.5

1560 183 300 28.7 133 13.2 192.5 706.0 9.3 < 0.05 57 0.015 1340 < 10 33.6 48.1 6960 87.5

1590 197 346 38.2 206 25.7 133.3 90.9 10.2 < 0.05 9 0.03 175 < 10 19.7 99.2 442 439

847 90 146 14.9 92.5 11.5 251.7 608.2 5.8 < 0.05 6 0.012 1270 < 10 23.8 83.2 4560 90.9

396 44.2 65.5 5.56 31.3 3.57 111.0 195.7 2.6 < 0.05 10 0.002 1420 50 9.54 134 2030 41.7

346 37.1 57.5 6.06 35.2 4.01 37.0 307.6 2.2 < 0.05 13 0.001 587 30 8.8 230 1190 75.3

12.61 0.83 0.83 792.1

8.64 1.40 0.90 370.5

13.01 2.98 0.83 297.4

5.96 8.52 0.93 60.6

5.20 2.23 1.06 258.0

1.47 36.96 0.94 8.4

2.69 6.79 0.98 104.9

6.12 12.34 0.91 65.5

8.94 10.76 0.57 65.6

5.99 5.58 0.97 81.1

2.46 6.91 0.98 97.4

2.34 6.72 0.97 77.6

na: not analyzed; –: below detection. a Fe2O3 is calculated as the difference between Fe from FeO analyzed by titration and total Fe determined by ICP-MS. b Reduced sulfur, S, is calculated as the difference between SO4 calculated from total sulfur and SO4 analyzed directly by IR. c Total including LOI, excluding CO2 and trace elements. d Zirconium and niobium were determined by fusion XRF for samples with high phosphorous (Taylor and Taylor South) and are initially reported as ZrO2 and Nb2O5. Results are presented here as ppm Zr and Nb. e Percent total rare earth oxides including Y2O3. f Heavy rare earth element enrichment factor (percent of total REE, Tb and heavier, including Y). g Ce anomaly magnitude: ratio compares measured Ce concentration with expected Ce concentration based on the chondrite normalized values of La and Pr. Samples with a smooth chondrite normalized profile from La though Pr and no Ce fractionation will have a ratio close to unity.

although the proportion of HREE varies significantly and increases at peripheral localities (Whitetail, Taylor, Taylor South). Unoxidized carbonatites from Bull Hill have (La/Yb)CN ratios from 230 to 1730, while oxidized carbonatites at Whitetail and Taylor have (La/Yb)CN ratios under 300 and flatter chondrite-normalized REE patterns (Tables 1a and 1b; Fig. 7) To examine the effects of alteration on the major and trace element chemistry, oxide and transition zone samples were normalized to an average unoxidized Bull Hill carbonatite composition (Fig. 8). The most pronounced change is Ca and CO2 depletion as a result of calcite dissolution. Initial average CaO and CO2 concentrations in unoxidized carbonatite are 28.67 and 27.79 wt%, respectively, while oxide zone samples typically have less than 3.0 wt% of each. In most igneous rocks, Sr is considered a trace element, but at concentrations that are commonly between 2.4 and 9.4 wt%, it is a major element in BLC. Like Ca, major Sr depletion (494–8489 ppm) is observed in oxidized samples. Oxide and transition zone carbonatite from all localities exhibits HREE- and HFSE-enrichment. The enrichments are most pronounced at peripheral occurrences, Whitetail and Taylor, but are almost non-existent for two of the four central Bull Hill samples (Fig. 8). Three-dimensional geochemical models generated from drill core assays confirm that HREE/LREE ratios increase laterally and that the peripheral zones, Whitetail, Taylor, and portions of Carbon are HREE-enriched relative to the main carbonatite dike swarm at Bull Hill (Fig. 9a). Phosphorus and vanadium concentrations are highest in oxide and transition zone carbonatite at Taylor and Taylor South, and lowest at Whitetail (Tables 1a and 1b; Figs. 8, 9b) A single F-rich (18 wt% F), LREE-depleted Taylor vein sample (R25169, Table 1b) consists of

abundant fluorite, clays, apatite, and fine-grained REE phosphates. Carbonatites of the BLAC are enriched in chalcophile and siderophile elements (Cu, Zn, Pb, Bi, As, Ag, Mo, and Au), owing to the presence of sulfide minerals in all carbonatite intrusive bodies. 4.3. Mineral chemistry from EPMA and EDS 4.3.1. Ca-REE fluorocarbonates Ca-REE fluorocarbonates can be described as combinations of end member bastnäsite [Ln(CO3)F] (B layers) and synchysite [CaLn(CO3)2F] (S layers), with Ln representing trivalent rare earth elements, typically dominated by La3+ and Ce3+. Other minerals of the Ca-REE fluorocarbonate group can be regarded as ordered mixtures of B and S unit-layers stacked along the c direction, for example, parisite [CaLn2(CO3)3F2] as BS and röntgenite [Ca2Ln3(CO3)5F3] as BS2 (Donnay, 1953; Donnay and Donnay, 1953; Van Landuyt and Amelinckx, 1975; Wu et al., 1998; Meng et al., 2002). Ca-REE fluorocarbonates regularly occur as thin reticulated platelets, which appear as needles in thin section (Fig.4a, b), and syntaxial intergrowths where two or more members of the group grow in the same crystallographic orientation (Donnay and Donnay, 1953; Ni et al., 1993; Wall, 2000). For this study, the CO3 content of fluorocarbonates is calculated based on the molecular proportion of Ca and the Ca:Ln ratio. For some analyses, mineral formulae are calculated as mixed-layer BmSn-type compounds, rather than one of the four IMA-recognized Ca-REE fluorocarbonate minerals (bastnäsite, parisite, röntgenite, synchysite). Representative mean analyses shown in Table 2 reveal chemical variation in Ca-REE fluorocarbonates between different areas of the

792

Table 2 Summary of Ca-REE fluorocarbonate analyses from Bear Lodge. Bull Hill fluorocarbonates B2S B55B-943

B3S B58-752

B3S B55C-943

Taylor fluorocarbonates parisite-(La) B55C-943

bastnäsite-(Ce) B19-364

bastnäsite-(Ce) T90RCB

B2S T90RC

2r

n=9

2r

n=5

2r

n=6

2r

n=13

2r

n=6

2r

n=12

2r

bdl bdl 6.99 bdl bdl 0.19 0.05 0.04 25.68 22.88 2.59 8.35 0.75 0.10 0.21 bdl bdl 0.22 0.23 0.20 21.82 4.65 1.96 94.97

– – 1.57 – – 0.05 0.11 0.04 6.34 11.88 0.66 1.95 0.09 0.09 0.13 – – 0.16 0.21 0.08 1.94 1.25 0.53

0.57 bdl 3.98 bdl bdl 0.19 0.02 0.06 39.29 8.89 4.61 13.68 1.32 0.17 0.39 bdl bdl 0.15 0.13 0.20 22.53 4.03 1.69 100.21

0.49 – 0.85 – – 0.08 0.07 0.04 9.92 8.63 0.53 2.42 0.24 0.10 0.16 – – 0.27 0.18 0.15 0.91 1.02 0.43

bdl bdl 4.02 bdl bdl 1.18 0.01 0.19 21.91 25.43 2.70 9.59 1.47 0.24 0.66 bdl bdl 1.71 0.08 bdl 20.84 5.17 2.18 95.19

– – 1.79 – – 0.16 0.06 0.05 3.86 1.99 0.34 2.76 0.42 0.07 0.11 – – 0.94 0.07 – 0.72 1.08 0.46

bdl bdl 10.16 bdl bdl 0.49 0.09 0.05 25.53 21.67 2.89 8.13 0.69 0.05 0.17 bdl bdl 0.30 0.14 0.07 24.07 4.71 1.98 99.20

– – 2.15 – – 0.37 0.10 0.04 3.28 5.16 0.46 0.81 0.12 0.04 0.10 – – 0.12 0.16 0.08 0.38 0.80 0.34

bdl bdl 1.94 0.03 bdl 0.73 0.06 0.17 16.28 29.78 3.41 11.97 1.55 0.23 0.60 bdl bdl 0.34 0.47 0.01 18.41 4.89 2.06 90.85

– – 0.24 0.20 – 0.36 0.12 0.17 1.42 2.03 0.33 1.26 0.47 0.14 0.39 – – 0.20 0.83 0.06 0.80 1.17 0.49

bdl bdl 2.98 bdl bdl 0.27 0.10 1.35 15.98 25.13 3.81 13.25 2.21 0.52 1.85 0.07 0.36 0.69 0.28 0.70 19.11 6.42 2.70 95.08

– – 0.32 – – 0.06 0.10 0.14 1.15 3.38 0.58 1.48 0.25 0.05 0.53 0.08 0.12 0.85 0.04 0.52 0.73 1.18 0.50

bdl bdl 4.89 bdl bdl 1.23 0.15 1.90 15.20 23.67 2.91 10.23 1.88 0.46 2.02 0.07 0.63 4.38 0.55 0.23 21.21 5.31 2.24 96.92

– – 1.05 – – 1.05 0.10 0.82 4.52 9.19 0.86 3.44 0.81 0.27 0.95 0.14 0.24 2.86 0.48 0.22 2.18 1.66 0.70

Si Al Ca Mn Fe Sr Ba Y La Ce Pr Nd Sm Eu Gd Tb Dy Th P S CO3a F mol % CaCO3 Ca:Ln ratio P REE+Y P P ( HREE+Y/ REE+Y)*100 (Ce/Ce*)N

– – 1.006 – – 0.015 0.003 0.003 1.275 1.121 0.127 0.401 0.035 0.005 0.009 – – 0.007 0.026 0.020 3.886 1.986 25.85 0.34 2.98 0.10 0.56

– – 0.202 – – 0.004 0.006 0.003 0.344 0.542 0.038 0.101 0.005 0.004 0.005 – – 0.005 0.022 0.009 0.110 0.657

0.092 – 0.694 – – 0.018 0.002 0.005 2.357 0.526 0.273 0.795 0.074 0.010 0.021 – – 0.005 0.018 0.024 4.311 2.070 16.10 0.17 4.06 0.13 0.14

0.077 – 0.160 – – 0.008 0.005 0.003 0.602 0.497 0.035 0.156 0.016 0.006 0.010 – – 0.010 0.025 0.018 0.478 0.536

– – 0.199 – – 0.020 0.003 0.002 0.108 0.179 0.015 0.025 0.004 0.001 0.003 – – 0.002 0.012 0.005 0.084 0.238

– – 0.083 0.001 – 0.017 0.001 0.004 0.239 0.434 0.049 0.170 0.021 0.003 0.008 – – 0.003 0.016 0.000 0.986 0.616 8.43 0.09 0.93 0.38 0.90

– – 0.009 0.007 – 0.008 0.002 0.004 0.014 0.029 0.004 0.015 0.006 0.002 0.005 – – 0.002 0.027 0.002 0.084 0.152

– – 0.122 – – 0.006 0.001 0.028 0.226 0.353 0.053 0.181 0.029 0.007 0.024 0.001 0.004 0.006 0.009 0.020 0.974 0.779 12.55 0.14 0.91 3.63 0.74

– – 0.013 – – 0.001 0.002 0.004 0.014 0.040 0.009 0.019 0.004 0.001 0.007 0.001 0.002 0.007 0.002 0.014 0.023 0.145

– – 0.904 – – 0.124 0.010 0.173 0.968 1.500 0.183 0.630 0.112 0.027 0.115 0.004 0.035 0.171 0.080 0.030 4.655 2.910 19.41 0.24 3.75 5.67 0.82

– – 0.176 – – 0.113 0.007 0.067 0.272 0.599 0.047 0.190 0.043 0.015 0.050 0.008 0.012 0.106 0.072 0.030 0.321 1.037

Formula calculated to X(F,CO3) contentb – – – – – – 0.756 0.324 0.993 – – – – – – 0.120 0.016 0.026 0.001 0.004 0.003 0.018 0.004 0.002 1.421 0.279 0.860 1.637 0.157 0.725 0.173 0.018 0.096 0.601 0.159 0.265 0.089 0.023 0.022 0.015 0.004 0.002 0.038 0.005 0.005 – – – – – – 0.068 0.036 0.006 0.011 0.010 0.011 0.000 0.000 0.005 4.902 0.090 2.964 2.879 0.700 1.359 15.41 33.47 0.19 0.50 3.99 1.98 0.44 0.12 0.67 0.50

A.K. Andersen et al. / Ore Geology Reviews 89 (2017) 780–807

n=11 SiO2 Al2O3 CaO MnO Fe2O3 SrO BaO Y2O3 La2O3 Ce2O3 Pr2O3 Nd2O3 Sm2O3 Eu2O3 Gd2O3 Tb2O3 Dy2O3 ThO2 P2O5 SO3 CO2a F –O=F TOTAL

Carbon fluorocarbonates B2S 11P79A

Ce-depleted ? 11P79A-14

Whitetail fluorocarbonates

röntgenite-(Ce) C57A-65

parisite-(Ce) C57A-65

bastnäsite-(Ce) W20-270A

bastnäsite-(Ce) W20-270B

parisite-(Ce) W63-509

2r

n=4

2r

n=7

2r

n=16

2r

n=16

2r

n=16

2r

n=13

2r

SiO2 Al2O3 CaO MnO Fe2O3 SrO BaO Y2O3 La2O3 Ce2O3 Pr2O3 Nd2O3 Sm2O3 Eu2O3 Gd2O3 Tb2O3 Dy2O3 ThO2 P2O5 SO3 CO2a F –O=F TOTAL

bdl bdl 5.91 bdl bdl 0.44 0.16 0.40 25.37 24.05 3.53 11.87 1.21 0.23 0.45 bdl bdl 0.27 0.40 0.05 23.22 5.11 2.15 102.67

– – 0.58 – – 0.15 0.10 0.14 2.86 4.20 0.48 0.85 0.18 0.15 0.23 – – 0.13 0.13 0.05 0.49 0.63 0.26

bdl bdl 5.50 bdl bdl 0.62 0.07 0.78 24.81 2.50 5.62 19.39 2.13 0.42 0.78 bdl 0.06 0.20 0.19 0.19 19.86 6.27 2.64 89.38

– – 0.54 – – 0.13 0.02 0.36 0.31 1.00 0.25 0.55 0.16 0.04 0.20 – 0.08 0.10 0.34 0.07 0.74 0.73 0.31

bdl bdl 10.81 0.06 bdl 1.38 0.13 3.72 8.86 14.80 2.87 11.95 2.30 0.56 1.42 0.09 0.57 1.51 0.12 0.16 22.34 4.43 1.86 88.08

– – 1.05 0.07 – 0.84 0.12 0.72 1.36 3.86 0.42 1.76 0.48 0.18 0.34 0.05 0.11 0.30 0.06 0.11 1.98 0.77 0.32

bdl bdl 7.48 0.05 bdl 1.92 0.15 3.20 9.38 16.46 2.95 12.22 2.25 0.54 1.25 0.07 0.45 1.41 0.12 0.13 20.39 4.28 1.80 84.68

– – 1.45 0.06 – 0.74 0.07 0.40 1.17 3.19 0.26 1.45 0.36 0.10 0.23 0.04 0.08 0.28 0.07 0.05 1.95 0.85 0.36

0.33 0.08 2.79 0.60 bdl 0.76 0.09 2.17 13.15 26.21 3.41 14.15 2.50 0.59 1.77 0.06 0.46 2.23 0.11 0.23 20.20 5.05 2.13 96.94

0.38 0.16 0.66 1.14 – 0.19 0.12 1.33 0.77 2.63 0.30 1.58 0.55 0.21 0.57 0.07 0.36 1.71 0.03 0.07 0.54 0.85 0.36

0.31 0.04 2.95 0.12 bdl 0.76 0.04 2.23 13.10 26.27 3.42 14.20 2.59 0.63 1.87 0.08 0.45 2.44 0.09 0.25 20.16 5.15 2.17 97.14

0.30 0.05 0.78 0.32 – 0.29 0.07 1.74 0.76 3.99 0.36 1.45 0.51 0.26 0.93 0.12 0.32 2.34 0.05 0.12 0.36 0.79 0.33

bdl bdl 8.76 0.10 bdl 1.18 0.11 2.64 11.34 14.05 3.36 13.49 2.39 0.56 2.08 0.13 0.75 6.05 0.21 0.13 22.57 6.05 2.55 95.94

– – 3.21 0.44 – 0.27 0.12 1.35 1.13 6.56 0.34 1.40 0.27 0.16 0.51 0.11 0.32 1.16 0.11 0.08 1.63 1.92 0.81

Si Al Ca Mn Fe Sr Ba Y La Ce Pr Nd Sm Eu Gd Tb Dy Th P S CO3a F mol % CaCO3 Ca:Ln ratio P REE+Y P P ( HREE+Y/ REE+Y)*100 (Ce/Ce*)N

– – 0.799 – – 0.032 0.008 0.027 1.180 1.111 0.162 0.535 0.053 0.010 0.019 – – 0.008 0.042 0.005 3.807 2.040 20.99 0.26 3.10 0.87 0.53

– – 0.071 – – 0.011 0.005 0.009 0.133 0.194 0.024 0.041 0.007 0.006 0.009 – – 0.004 0.013 0.004 0.041 0.272

– – 0.869 – – 0.053 0.004 0.061 1.350 0.135 0.302 1.022 0.108 0.021 0.038 – 0.003 0.007 0.023 0.021 3.861 2.924 22.53 0.29 3.04 2.08 0.05

– – 0.056 – – 0.009 0.001 0.025 0.043 0.052 0.014 0.057 0.005 0.002 0.008 – 0.004 0.003 0.042 0.008 0.136 0.363

– – 0.153 0.005 – 0.048 0.003 0.025 0.041 0.087 0.006 0.017 0.007 0.003 0.006 0.001 0.003 0.005 0.006 0.004 0.053 0.269

0.012 0.004 0.109 0.018 – 0.016 0.001 0.042 0.176 0.348 0.045 0.183 0.031 0.007 0.021 0.001 0.005 0.018 0.003 0.006 0.934 0.579 11.67 0.13 0.86 5.56 0.90

0.014 0.007 0.026 0.035 – 0.004 0.002 0.025 0.010 0.038 0.005 0.022 0.007 0.002 0.007 0.001 0.004 0.014 0.001 0.002 0.110 0.098

0.011 0.002 0.115 0.004 – 0.016 0.001 0.043 0.176 0.350 0.045 0.184 0.032 0.008 0.022 0.001 0.005 0.020 0.003 0.007 0.946 0.591 12.14 0.13 0.87 5.68 0.91

0.011 0.002 0.030 0.010 – 0.006 0.001 0.033 0.010 0.057 0.005 0.020 0.006 0.003 0.011 0.001 0.004 0.019 0.002 0.003 0.067 0.090

– – 0.910 0.008 – 0.066 0.004 0.136 0.407 0.503 0.119 0.469 0.080 0.019 0.067 0.004 0.023 0.134 0.017 0.010 2.880 1.867 31.53 0.50 1.83 8.93 0.53

– – 0.268 0.037 – 0.013 0.005 0.061 0.045 0.255 0.012 0.044 0.007 0.005 0.016 0.003 0.009 0.026 0.009 0.006 0.109 0.636

Formula calculated to X(F,CO3) contentb – – – – – – 1.899 0.116 0.864 0.009 0.011 0.005 – – – 0.130 0.075 0.120 0.009 0.008 0.006 0.324 0.042 0.184 0.535 0.049 0.373 0.890 0.261 0.649 0.172 0.014 0.116 0.699 0.074 0.470 0.130 0.023 0.084 0.031 0.008 0.020 0.077 0.016 0.045 0.005 0.003 0.002 0.030 0.004 0.016 0.056 0.008 0.034 0.017 0.008 0.011 0.019 0.015 0.011 4.907 0.020 2.933 2.293 0.247 1.461 38.70 29.43 0.66 0.44 2.89 1.96 12.42 10.30 0.69 0.73

793

bdl: below detection limit. Individual fluorocarbonate analyses used to calculate averages are compiled in Appendix Table 2. a CO2/CO3 calculated by charge balance. b Formula calculated to F+CO3 content of specific fluorocarbonate species; i.e., X=2 for bastnäsite, X=5 for parisite, X=7 for B2S.

A.K. Andersen et al. / Ore Geology Reviews 89 (2017) 780–807

n=4

794

A.K. Andersen et al. / Ore Geology Reviews 89 (2017) 780–807 1000000 1000000

a

b

mineral/chondrite

mineral/chondrite

100000

Bull Hill 10000

B2 S-(Ce) B2S-(Ce) 1000

Whitetail

100000

10000

B3 S-(La/Ce) B3S-(La/Ce)

parisite-(Ce/La) parisite-(Ce)

parisite-(La/Ce) parisite-(La/Ce)

bastnäsite-(Ce) bastnaesite-(Ce)

bastnäsite-(Ce) bastnaesite-(Ce)

1000

100 La

Ce

Pr

Nd

Sm

Eu

Gd

Tb

Dy

Y

La

Ce

Pr

Nd

Sm

Eu

Gd

Tb

Dy

1000000

1000000

c

d

Carbon

100000

10000

Taylor

100000 mineral/chondrite

mineral/chondrite

Y

parisite-(Ce) parisite-(Ce)

10000

röntgenite-(Ce) rontgenite-(Ce) bastnäsite-(Ce) bastnaesite-(Ce)

Ce-depletedfluorocarbonates fluorocarbonates Ce-depleted

B2 S-(Ce) B2S-(Ce)

B2 S-(Ce/La) B2S (Ce/La dominant) 1000

1000

La

Ce

Pr

Nd

Sm

Eu

Gd

Tb

Dy

Y

La

Ce

Pr

Nd

Sm

Eu

Gd

Tb

Dy

Y

Fig. 10. (a-d) Chondrite-normalized REE profiles of Ca-REE fluorocarbonates from oxide- and transition-zone carbonatite at REE-rich zones, Bull Hill, Whitetail, Carbon, and Taylor. Mixed bastnäsite-synchysite compounds, B2S and B3S have the general formulas CaLn3(CO3)4F3 and CaLn4(CO3)5F4, and are calculated based on 7 CO3 + F and 9 CO3 + F, respectively.

carbonatite stockwork and variations in the proportion of Ca and the major REE cation (La, Ce, Nd). Mineral formula presented in Table 2 show that the molecular proportion of CaCO3 rarely exceeds 33% and Ca:Ln ratios rarely exceed 0.50; therefore, röntgenite, synchysite, and BmSn-type compounds with n > m are much less abundant than bastnäsite, parisite, and all BmSn-type compounds with m  n in between. Synchysite (Ca:Ln = 1.0) was rarely encountered during our study of oxide- and transition-zone samples, however, its presence is reported by other workers (e.g., Moore et al., 2015; personal communication, Anthony N. Mariano), particularly in deeper transition-zone or unoxidized carbonatite. The variation in relative REE abundance of Ca-REE fluorocarbonates is shown in the chondrite-normalized profiles of Fig. 10a–d. Profiles decrease from La through Dy/Y, except for those which show a negative Ce anomaly. Fluorocarbonates which exhibit prominent negative Ce anomalies have the greatest 2r values for Ce and other LREEs, principally La and Nd (Table 2). For instance, Bull Hill fluorocarbonate sample B58-752 has a La2O3 range of 30.84–45.18 wt% and Ce2O3 range of 3.63–15.73 wt% giving rise to 2r values of 9.92 and 8.63, respectively (Table 2; Appendix Table 2). Fluorocarbonates which exhibit La- or Nd-dominance are those with a pronounced negative Ce anomaly. Bull Hill parisite-(La) sample B55C-943 contains 25.53 wt% La2O3 and 8.13 wt% Nd2O3, but only 21.67 wt% Ce2O3. La-dominant parisite is now an IMA-recognized variant (Menezes Filho et al., 2016), however our parisite-(La) may alternatively be described as Ce-deficient parisite. In some samples, fluorocarbonates show almost complete Ce depletion. Carbon fluorocarbonate sample

11P79A-14 contains 24.81 wt% La2O3 and 19.39 wt% Nd2O3, but only 2.50 wt% Ce2O3 (Table 2; Fig. 10c). Fluorocarbonates with prominent negative Ce anomalies are accompanied by finegrained cerianite (Fig. 4b). In the La-Ce-Nd ternary diagram (Fig. 11a), two major trends emerge among the major REE cations. The Bull Hill trend shows extensive variation in the La/Ce ratio but low Nd/La ratios and Nd concentrations rarely exceeding 10 wt% Nd2O3. The Whitetail trend represents fluorocarbonates with higher Nd/La ratios along a silimar range of La/Ce ratios and 11.87 to 19.39 wt% Nd2O3. The two trends also emerge when comparing the percentage of REE which P P are HREE ( HREE + Y/ REE + Y) to Ca and P concentrations and magnitude of the Ce anomaly (Fig. 11b–d). Fluorocarbonates at Bull Hill contain less HREE than those from all other peripheral areas, irrespective of their Ca content (Fig. 11b). Less than 0.52% of the REE in Bull Hill bastnäsite, parisite, and intermediate fluorocarbonates are HREE, and Tb and Dy are below detection by EPMA (Table 2, Fig. 10a). HREE constitute 0.87–13.31% of total REE in fluorocarbonates from Whitetail, Carbon, and Taylor. This is consistent with whole rock analyses, which show that HREE concentrations are lower in Bull Hill carbonatites. Fluorocarbonates from Whitetail and Carbon exhibit a prominent positive correlation between the P P HREE + Y/ REE + Y ratio and Ca content (Fig. 11b), which may be the result of REE incorporation in the octahedral Ca site. This is consistent with other studies (e.g., Wall, 2000) which show that in many carbonatites, Ca-dominant varieties (e.g., synchysite) typically contain a greater proportion of HREE, while bastnäsite is always the most highly LREE-enriched. A similar correlation exists

A.K. Andersen et al. / Ore Geology Reviews 89 (2017) 780–807

795

P P Fig. 11. Variability in Bear Lodge Ca-REE fluorocarbonate chemistry. HREE + Y/ REE + Y is the percentage of total REE which are Tb and heavier based on atoms per formula unit. (a) Ternary with major REE cation vertices showing two predominant trends among Bear Lodge fluorocarbonates. (b) Correlation of HREE enrichment with Ca. Ca-rich varieties typically contain the greatest proportion of HREE at localities peripheral to Bull Hill. (c) Correlation between HREE enrichment and magnitude of the negative Ce anomaly. (d) HREE enrichment vs P content. Note the positive correlation for fluorocarbonates at Taylor. (e) Percentage of total REE which are HREE in fluorocarbonates by locality. Carbon fluorocarbonates share characteristics of both Bull Hill and Whitetail samples.

796

A.K. Andersen et al. / Ore Geology Reviews 89 (2017) 780–807

P

P between the HREE + Y/ REE + Y ratio and magnitude of the negative Ce anomaly for the same fluorocarbonates (Fig. 11c). However, a pronounced negative Ce anomaly does not always correlate with HREE-enrichment as evidenced by the Carbon and Bull Hill fluoroP P carbonate samples with HREE + Y/ REE + Y percentages less than 3 (Fig. 11c, Table 2). P P A positive correlation between HREE + Y/ REE + Y and P for Taylor fluorocarbonates (Fig. 11d) shows that HREE are probably incorporated as REE phosphates, either as structurally bound intergrowths or adjacent micron- or submicron-sized crystals unavoidable during EPMA even while using a focused beam. Phosphate intergrowths or inclusions may account for HREE-enrichment at Taylor, but the HREE-P correlation is absent in fluorocarbonates at Bull Hill, Whitetail, and Carbon. Whitetail fluorocarbonates display the greatest range in HREE content (2.27–13.31% of total REE), while Bull Hill fluorocarbonates contain a low restricted range in HREE content (0.0–0.52% of total REE) (Fig. 11e). In terms of fluorocarbonate chemistry, the Carbon area is an intersection of Bull Hill and Whitetail styles of mineralization. Bear Lodge fluorocarbonates also show considerable variation in F concentration. Many samples contain only about one-half to two-thirds of the expected F and no Cl, a feature confirmed independently for Bear Lodge fluorocarbonates by Wall (2000).

P

Nd

Ce

Ca

4.3.2. REE phosphates at Taylor Fluorapatite of residual primary or secondary hydrothermal origin may be the most abundant phosphate in oxidized and transition zone carbonatite, although it rarely contains >10 wt% TREO. At Taylor, hydrothermal or supergene monazite and rhabdophane represent a significant portion of the total REE budget as they typically contain 50–60 wt% TREO. Rhabdophane-group REE phosphates occur as sheaf-like clusters, 20–100 mm in diameter, with oscillatory zoning that appears to show differences in REE solubility and fluctuations in HREE/LREE ratios from core to rim (Fig. 4d, e). X-ray element maps show cores of zoned phosphate clusters are enriched in LREE (Ce and Nd), while acicular rims are enriched in Y and Ca (Fig. 12). Microprobe oxide totals for LREE domains (typically cores) are 85.0–94.5%, which is consistent with the theoretical 7% H2O in rhabdophane, while allowing for natural variation and devolatilization during analysis. LREE cores are chemically similar to published rhabdophane analyses (cf., Nagy and Draganits, 1999; Nagy et al., 2002; Krenn and Finger, 2007; Berger et al., 2008); however, the presence of 1.0 wt% F suggests this phase may contain OH
in place of, or in addition to, H2O (Table 3). Additional F
and/or OH
may help offset the charge imbalance in LREE domains resulting from P2O5 concentrations (22.23–27.13 wt%) which are

Y

BSE

100 μm

Fig. 12. X-ray element maps of fine-grained rhabdophane-group REE phosphates. Images show the relative abundance of the analyte, with reds representing higher concentrations and blues lower. Cores are LREE-enriched (Ce and Nd) while the more acicular rims are Y,Ca,HREE-enriched.

797

A.K. Andersen et al. / Ore Geology Reviews 89 (2017) 780–807 Table 3 Average rhabdophane, monazite, and fluorapatite analyses from Bear Lodge. rhabdophane-(Ce) L11SZ07

rhabdophane-(Y) H11SZ07

n=19

2r

n=8

2r

Na2O SiO2 P2O5 SO3 K2O CaO V2O5 MnO FeO SrO BaO Y2O3 La2O3 Ce2O3 Pr2O3 Nd2O3 Sm2O3 Eu2O3 Gd2O3 Tb2O3 Dy2O3 Ho2O3 Er2O3 Tm2O3 Yb2O3 Lu2O3 ThO2 UO2 F –O=F TOTAL H2O

0.09 0.42 23.94 0.29 0.07 4.37 bdl 0.07 – 2.82 0.56 2.88 8.46 18.67 2.14 9.01 3.94 1.46 4.16 0.39 1.31 bdl 0.13 bdl bdl bdl 3.19 0.06 1.05 0.44 88.95 11.05

0.14 0.37 2.53 0.18 0.09 0.68 – 0.07 – 1.40 0.56 2.65 2.47 5.56 0.55 1.59 1.78 0.79 2.29 0.25 0.89 – 0.14 – – – 2.58 0.09 0.37 0.16 6.20

0.21 0.40 31.81 0.06 0.86 8.83 bdl 0.18 – 0.57 0.15 18.98 1.14 1.54 0.47 2.76 3.46 2.16 8.93 1.49 6.09 0.69 0.84 bdl 0.12 bdl 1.43 bdl 0.17 0.07 93.27 6.73

0.36 0.57 2.48 0.08 0.68 2.05 – 0.72 – 0.34 0.18 4.09 1.37 1.14 0.43 2.47 1.96 0.83 2.67 0.22 0.51 0.08 0.18 – 0.02 – 1.00 – 0.08 0.04 2.49

Na Si P S K Ca V Mn Fe Sr Ba Y La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Th U F P A P T P Z P REE+Y P P ( HREE+Y/ REE+Y) x100

0.031 0.076 3.698 0.040 0.017 0.855 – 0.011 – 0.299 0.040 0.279 0.569 1.252 0.142 0.587 0.248 0.091 0.252 0.023 0.077 – 0.007 – – – 0.132 0.002 0.608 4.91 3.81 0.61 3.53 10.95

0.048 0.066 0.201 0.025 0.019 0.107 – 0.010 – 0.157 0.038 0.239 0.151 0.413 0.035 0.090 0.110 0.049 0.135 0.014 0.050 – 0.008 – – – 0.108 0.004 0.240

0.060 0.059 3.932 0.006 0.159 1.380 – 0.022 – 0.048 0.008 1.473 0.062 0.082 0.025 0.145 0.175 0.108 0.434 0.072 0.286 0.032 0.039 – 0.005 – 0.048 – 0.080 4.66 4.00 0.08 2.94 65.14

monazite-(Ce) M12SZ10C n=12

0.10 0.17 27.21 1.94 0.02 3.75 0.02 0.06 0.05 0.84 0.29 0.31 14.15 28.39 3.25 11.61 1.24 0.28 0.49 bdl 0.08 bdl bdl bdl bdl bdl 0.49 0.07 0.72 0.30 95.24 4.76 Formula calculated to 16 (O) 0.102 0.031 0.083 0.028 0.114 3.723 0.009 0.235 0.122 0.005 0.260 0.648 – 0.003 0.087 0.009 – 0.007 0.029 0.079 0.010 0.018 0.262 0.027 0.079 0.843 0.060 1.679 0.025 0.192 0.141 0.670 0.109 0.069 0.047 0.015 0.152 0.026 0.013 – 0.022 0.004 0.004 – 0.007 – – – 0.001 – – – 0.036 0.018 – 0.003 0.037 0.369 4.34 3.99 0.37 3.53 0.87

monazite-(Ce) M12SZ10C-2

fluorapatite R60-591/595

2r

n=16

2r

n=23

0.02 0.17 0.84 0.28 0.01 0.45 0.05 0.03 0.03 0.21 0.20 0.08 2.31 1.37 0.38 1.83 0.26 0.08 0.13 – 0.05 – – – – – 0.53 0.03 0.15 0.06 0.85

0.11 0.19 27.12 1.91 0.03 3.58 0.02 0.07 0.07 0.81 0.26 0.28 18.83 28.20 2.60 8.57 0.85 0.22 0.37 0.02 0.06 bdl bdl bdl bdl bdl 0.49 0.07 0.72 0.30 95.13 4.87

0.03 0.20 1.36 0.26 0.01 1.11 0.03 0.02 0.05 0.11 0.15 0.18 2.74 1.38 0.38 1.46 0.23 0.07 0.14 0.06 0.06 – – – – – 0.52 0.05 0.18 0.08 1.08

0.006 0.026 0.071 0.036 0.002 0.071 0.005 0.004 0.004 0.020 0.013 0.007 0.143 0.093 0.021 0.104 0.014 0.004 0.007 – 0.003 – – – – – 0.019 0.001 0.077

0.033 0.031 3.718 0.232 0.006 0.620 0.002 0.009 0.010 0.076 0.016 0.024 1.126 1.673 0.154 0.496 0.047 0.012 0.020 0.001 0.003 – – – – – 0.018 0.003 0.371 4.35 3.98 0.37 3.56 0.79

0.010 0.032 0.070 0.033 0.003 0.174 0.004 0.002 0.006 0.009 0.009 0.015 0.180 0.134 0.025 0.089 0.012 0.003 0.007 0.003 0.003 – – – – – 0.019 0.002 0.102

0.93 0.29 bdl – 39.86 1.31 0.03 0.04 na – 51.05 1.92 bdl – 0.14 0.37 0.07 0.20 0.82 0.17 bdl – 0.27 0.14 0.29 0.14 1.12 0.44 0.20 0.10 1.08 0.37 0.33 0.08 0.10 0.04 0.29 0.11 bdl – 0.09 0.05 bdl – bdl – bdl – bdl – bdl – 0.05 0.04 bdl – 3.70 0.80 1.56 0.34 98.87 2.41 – – calculated to 26 (O) 0.316 0.105 – – 5.902 0.059 0.004 0.005 – – 9.567 0.166 – – 0.021 0.058 0.010 0.031 0.084 0.017 – – 0.025 0.012 0.018 0.009 0.072 0.029 0.013 0.006 0.067 0.024 0.020 0.005 0.006 0.002 0.017 0.006 – – 0.005 0.003 – – – – – – – – – – 0.002 0.002 – – 2.045 0.400 10.24 5.91 2.04 0.24 12.63

na = not analyzed; bdl = below detection limit. Individual phosphate mineral analyses used to calculate averages are compiled in Appendix Table 3.

2r

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Fig. 13. Chondrite normalized REE profiles of secondary REE phosphates and primary fluorapatite in Bear Lodge carbonatite.

lower than the 28.0–30.0 wt% P2O5 typically reported in natural rhabdophane and monazite. LREE domains are classified as rhabdophane-(Ce) with the following cation distribution: Ce > Ca > Nd  La > Sr  Y > other Ln. HREE domains (acicular rims) have oxide totals between 91.7 and 95.5 wt% and contain Y > Ca > Gd > Dy > other Ln, a lanthanide distribution similar to xenotime-(Y) [Y(PO4)] or churchite-(Y) [Y(PO4)2H2O] (Table 3). The acicular HREE-rich crystals are hexagonal in cross-section normal to the c-axis, in contrast to monoclinic needles flattened along [0 1 0] as with churchite-(Y). Zoned phosphate clusters show no evidence of a morphological core-to-rim inversion from a hexagonal to monoclinic structure. Lower totals, near 85%, would be expected if the hydrated phase had incorporated additional H2O forming churchite-(Y) [Y(PO4)2H2O] and, together with the morphological observations, we infer these rims to be rhabdophane(Y). Published analyses of churchite-(Y) (Lottermoser, 1990; Lapin, 1994; Plášil et al., 2009; Göb et al., 2011) report between 26.06 and 47.52 wt% Y2O3, while Bear Lodge rhabdophane-(Y) contains only 14.44–21.25 wt% Y2O3, owing to the substitution of Gd, Dy, and other HREE for Y (Table 3). Rhabdophane-(Y) has a negaP tive Ce anomaly, <6.0 wt% La2O3–Nd2O3, 18.98 wt% Y2O3, and chondite-normalized profiles which peak at Gd or Tb (Fig. 13, Table 3). Like the Ce-deficient fluorocarbonates, Ce-deficient rhabdophane-(Y) is accompanied by neighboring cerianite. Negative Ce anomalies are more pronounced in rhabdophane-(Y) rims relative to rhabdophane-(Ce) cores (Fig. 13), possibly recording a transition to more oxidizing fluids. The most notable deviation from typical rhabdophane or monazite chemistry is the high Ca content (avg. of 4.37 wt% CaO in cores and 8.83 wt% in rims). High-Ca rhabdophane of similar composition is reported by Dorfman et al. (1993) and Nagy et al. (2002). Krenn and Finger (2007) report a similar charge imbalance in their rhabdophane analyses, and note that Y and Ca contents vary greatly with small, patchy zoning. In REE phosphate minerals, charge balance is maintained by incorporation of Ca and Th accord-

ing to the following coupled substitutions (Gramaccioli and Segalstad, 1978; Watt, 1995; Van Emden et al., 1997):

2REE3þ $ ðTh; UÞ

4þ

þ ðCa; SrÞ2þ ðcheralite=brabantite
typeÞ 4þ

REE3þ þ P5þ $ ðTh; UÞ

þ Si

4þ

ðhuttonite=thorite
typeÞ

ð1Þ ð2Þ

In apatite group minerals, REE are incorporated into the phosphate structure and charge-balanced via two principal coupled substitutions (Roeder et al., 1987; Rønsbo, 1989; Fleet et al., 2000; Pan and Fleet, 2002): 4


2þ REE3þ þ SiO4 $ PO3
4 þ Ca

REE3þ þ ðNaþ ; Kþ Þ $ 2Ca2þ

ðbritholite
typeÞ

ðbelov ite
typeÞ

ð3Þ ð4Þ

Substitution 1, is the primary charge-compensating mechanism in both rhabdophane-(Ce) and rhabdophane-(Y) at Bear Lodge (Fig. 14a). Coupled substitution 2, involving end-member ThSiO4 polymorphs huttonite or thorite, and coupled substitution 3 are insignificant due to low SiO4 concentration. Excess alkalis are present only in rhabdophane-(Y) and correlate with increasing Ca2+, suggesting that both 1+ and 2+ cations are incorporated to help compensate for excess Th4+ and REE3+. The entrance of Ca2+ is commonly accompanied by minor F
and OH
to compensate for valency differences in natural monazite/huttonite (Chang et al., 1996), and may explain the negative correlation between P and F for rhabdophane-(Ce) (Fig. 14b). Excess Ca2+ and F
may be a remnant of precursory fluorapatite during in situ pseudomorphic replacement. Breakdown of precursor fluorapatite would also contribute to the Ca2+ and F
necessary for crystallization of co-existing Ca-REE fluorocarbonates (Fig. 4e). Fluorapatite accounts for a fraction of the total REE budget at Taylor, and in some veins fluorapatite and fluorite are the principal REE-bearing minerals. In BLC, multiple generations of apatite have been identified by cathodoluminescence. Primary fluorapatites

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2

a

rhabdophane-(Y) 1.6 Ca+Sr+Th+U apfu

rhabdophane-(Ce) 1.2

monazite-(Ce)

0.8

0.4

ap-F

1.60 mm

0 2

2.4

2.8 3.2 ∑REE+Y apfu

3.6

Fig. 15. Cathodoluminescence photomicrograph of primary REE-poor fluorapatite (lavender) with pink-yellow, REE-rich rims (image courtesy of Adrian Van Rythoven). The pink-yellow luminescence arises from the presence of mid-atomic number lanthanides (Sm3+ and Dy3+). Collected at a depth of 227.4 m, this sample appears to have been altered by REE-rich, non-oxidizing fluids (early carbohydrothermal stage?). Orange luminescence is the result of Mn2+ activation in the surrounding calcite matrix.

4

4.1

b

rhabdophane-(Y) 4

P apfu

3.9 3.8

rhabdophane-(Ce)

3.7 monazite-(Ce)

3.6 3.5 3.4 0

0.2

0.4

0.6

1

0.8

F apfu 0.5

c 0.45

from unoxidized carbonatite at Bull Hill contain 2.9–4.9 wt% TREO (Table 3) and have a lavender luminescence owing to LREE activation (Sm3+>Dy3+) (Mariano, 1978a, 1989c). The importance of coupled substitution 4 is supported by the positive correlation between Na and REE + Y (Fig. 14c) and negative correlation of each with Ca. Chondrite-normalized REE patterns of rhabdophane-(Ce) mimic REE patterns of primary fluorapatite (Fig. 13). Apatite in oxidized carbonatite at Taylor is characterized by yellow-orange luminescence (560–600 nm), the dominant activators for which are Sm3+, Dy3+, and Mn2+, and to a lesser extent Tb3+ and Eu3+ (Mariano, 1978a, 1989c; Roeder et al., 1987), suggesting this secondary generation of apatite contains a greater percentage of mid- to high-atomic number lanthanides. In some unoxidized subsurface samples, yellow-orange luminescent rims have developed on lavender fluorapatite, indicating incipient alteration by a REErich fluid (Fig. 15). The yellow-orange luminescence and pitted or corroded appearance of secondary apatite (Figs. 4c, 15) is likely an indicator of recrystallization during hydrothermal alteration, or may reflect the original crystal growth in the lateritic weathering environment (Roeder et al., 1987).

Na apfu

0.4

0.35 fluorapatite 0.3

0.25

0.2 0.15

0.2

0.25 ∑REE+Y apfu

0.3

0.35

Fig. 14. Plots of atomic proportions in rhabdophane and monazite recalculated on basis on 16 oxygens and fluorapatite on basis of 26 oxygens. (a) Ca + Sr + Th + U vs. P REE + Y; Trends among Bear Lodge phosphates are compared with the ideal cheralite– or ‘‘brabantite” –type substitution. (b) P vs. F; Fluorine substitution in monazite can help compensate for valency differences arising from Ca2+ substituP tion for Ln3+. (c) Na vs. REE + Y; In fluorapatite, the charge imbalance that arises from substitution of Ln3+ for Ca2+ is resolved by simultaneous incorporation of monovalent alkalis.

4.3.3. Monazite-(Ce) at Taylor South The REE mineralogy changes considerably outward and upward from the central carbonatite dikes to the laterite-like veins at Taylor South, where the principal REE-bearing phases are monazite-(Ce), and to a much lesser extent, pyrochlore. The fine-grained, acicular morphology of monazite-(Ce) (Fig. 5a) is similar to rhabdophane-group REE phosphates at Taylor, but individual crystals are smaller, lack the clear hexagonal form, and do not experience degradation due to dehydration during EPMA. Oxide totals (93.95–96.06 wt%) are more appropriate for natural monazite, with perhaps some degree of hydration, and XRD patterns of bulk material match that of typical LREE-rich monazite. Although veins contain abundant late silica, contamination was avoided during EPMA as monazite-(Ce) analyses average only 0.18 wt% SiO2. Chondrite-normalized REE patterns decrease from La through Y with a slope greater than that of rhabdophane-(Ce) (Fig. 13). Monazite-(Ce) contains 60 wt% TREO compared with rhabdophane-(Ce) (52.54 wt% TREO) and rhabdophane-(Y) (48.67 wt% TREO), which is a result of lower Th and Ca contents (substitution 1).

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bet

a

bt

40 μm

SiO2

rt

b

4.3.4. Pyrochlore Two minerals of the pyrochlore group have been identified by EDS. In biotite-rich selvages of unoxidized carbonatite, pyrochlore group crystals are an U-Ca-Ti variety, likely uranpyrochlore [(U,Ca, Ce)2(Nb,Ti,Ta)2O6(OH,F)] or betafite [(Ca,U)2(Ti,Nb,Ta)2O6(OH)] (Fig. 16a). In some cases, aggregates of fersmite [(Ca,Th,REE)(Nb, Ti)2(O,OH)6] or a similar secondary, acicular Nb oxide form pseudomorphs after betafite when nearly all U has been removed. In the monazite veins at Taylor South, complex zoned, partially metamict pyrochlore is associated with rutile/ilmenorutile where it appears to have exsolved and crystallized along cleavage planes (Fig. 5b). Radial cracks extending from pyrochlore into surrounding rutile are a result of volume changes during a-decay-induced metamictization (Fig. 16b) (Lumpkin and Ewing, 1988; Melgarejo and Martin, 2011). This generation of pyrochlore consists of alternating bands and irregular domains of uranpyrochlore or betafite and plumbopyrochlore [(Pb,Y,U,Ca)2-xNb2O6(OH)] (Fig. 16b,c). X-ray element maps show distinct U,Ca- and Pb-rich domains with little or no variation of Ti (Fig. 17). Although selective leaching during alteration and ion exchange processes do not typically affect the ratio of B-site cations (Nb, Ta, Ti) (Lumpkin and Ewing, 1995; Chakhmouradian and Mitchell, 2002), Nb concentration is noticeably higher in Pb-dominant domains. More prismatic, crystalline domains are plumbopyrochlore, while porous and recrystallized domains have a U-rich, and likely defect-rich, pyrochlore composition (Fig. 17). Other U,Ca,Ti-rich domains may be relict U-rich pyrochlore/betafite, like that found in biotite selvages of unaltered carbonatite. Oscillatory zoning is common, particularly in more prismatic plumpopyrochlore crystals (Fig. 16b). Secondary alteration has resulted in patchy zoning superimposed on primary oscillatory zoning, similar to pyrochlore alteration described by Chakhmouradian and Mitchell (2002) (Fig. 16c).

10 μm 5. Discussion

c

25 μm Fig. 16. BSE images of pyrochlore group minerals. (a) Interstitial U-rich pyrochlore/betafite (bet) in biotite-rich (bt) carbonatite vein selvage. (b) Oscillatory zoned plumbopyrochlore with radial cracks extending into host rutile (rt) as a result of volume changes during metamictization. (c) Complexly zoned, partially metamict pyrochlore cluster with evidence of both primary (oscillatory) zoning and secondary (wavy, turbid patches) alteration. Bright white zones are plumbopyrochlore. Darker gray domains are a U- and Ti-rich, possibly defect, variety similar to uranpyrochlore or betafite.

5.1. Vertical zonation and REE enrichment Oxide and transition zone carbonatite samples show an enrichment in REE relative to unoxidized carbonatite (Tables 1a and 1b; Figs. 7, 8). REE-enrichment is reflected by vertical changes in the principle REE minerals. Burbankite is thought to be an important primary REE mineral, which has been replaced by a secondary pseudomorphic assemblage of ancylite + strontianite ± barite ± carbocernaite within unoxidized carbonatite, generally observed 200m from the surface and deeper (Fig. 3a-e). Similar replacement of burbankite by an ancylite + strontianite + barite assemblage, and subsequent replacement by Ca-REE fluorocarbonates, is documented at several carbonatite complexes of the Kola Peninsula (Afrikanda, Khibina, Vuoriyarvi, Sallanlatvi, Seblyavr) and at Kangankunde, Malawi (Wall and Mariano, 1996; Bulakh et al., 1998; Zaitsev et al., 1998, 2002; Wall, 2000; Zaitsev and Chakhmouradian, 2002; Chakhmouradian and Zaitsev, 2012). Wall and Mariano (1996) conclude that replacement of early burbankite or carbocernaite is an open-system hydrothermal process, further supported by the reactions calculated by Zaitsev et al. (1998) for Khibina carbonatites. We have calculated two similar reactions for Bear Lodge carbonatites using REE mineral formulas from Moore et al. (2015) and this study where possible. Replacement of burbankite by the ancylite + strontianite + barite assemblage may be represented by reaction 5, while the subsequent replacement by an assemblage of bastnäsite + barite + cerianite may be represented by reaction 6.

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Nb

Pb

Ca

U

Ti

BSE

50 μm

Fig. 17. X-ray element maps of zoned pyrochlore cluster in carbonatite laterite at Taylor South. Images show the relative abundance of the analyte, with reds representing higher concentrations and blues lower. Distinct U,Ca- and Pb-rich domains are discernable with Nb concentrations noticeably higher in Pb-dominant domains and lower in Uand Ca-rich domains.


3þ 3:0ðNa2:20 Ca0:70 ÞP 2:90 ðSr1:50 Ca0:60 Ln0:60 Ba0:20 ÞP 2:90 ðCO3 Þ5 þ 0:6SO2
þ 3:6H2 O 4 þ 0:4F þ 0:6Ln burbankite

¼ 2:0ðSr0:65 Ln1:20 Ca0:05 ÞP 1:90 ðCO3 Þ2 ðF0:20 OH0:80 Þ  H2 O þ 3:20SrCO3 þ 0:6BaSO4 þ 6:6Naþ þ 3:8Ca2þ þ 1:6Hþ þ 7:80CO2
3 strontianite

ancylite

barite

ð5Þ

2:0ðSr0:65 Ln1:20 Ca0:05 ÞP 1:90 ðCO3 Þ2 ðF0:20 OH0:80 Þ  H2 O þ 3:20SrCO3 þ 0:6BaSO4 þ2:08F
þ 0:30Ca2þ þ 1:58Ln3þ ancylite

strontianite

barite

þ ¼ 4:0ðLn0:92 Ca0:10 Sr0:02 ÞP 3:00 ðCO3 ÞðF0:62 OH0:38 Þ þ 0:6BaSO4 þ4:42Sr2þ þ 3:2CO2
3 þ 1:48H2 O þ 0:3LnO2 þ1:12H ::

bastnasite

barite

cerianite

ð6Þ

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As pointed out by Moore et al. (2015), the secondary REE mineralization in BLC is likely the result of interaction with (carbo-) 3
hydrothermal fluids enriched to varying degree in F
, CO2
3 , PO4 , 2
and SO4 . Late-stage fluids responsible for modifications to the primary carbonatite mineralogy (i.e., replacement of burbankite by ancylite + strontianite + barite) were likely carbo-hydrothermal in nature with high CO2 activity, as supported by excess CO2
in 3 reaction 5 above. Within the upper oxidized portions of the carbonatite stockwork, Ca-REE fluorocarbonates have formed at the expense of ancylite + strontianite ± barite ± carbocernaite pseudomorphs (Fig. 3a, f–h) suggesting that fluorine was increasingly important in the precipitation REE minerals at shallower levels. Reaction 5 shows that S, F, and REE are added, while Na, Ca, and CO3 are removed during replacement of burbankite by the anyclite assemblage. The modal abundance of secondary REE phases is highly variable; therefore, trace amounts of Sr and Ba may also be introduced at this stage, comparable to the fluids at Khibina (Zaitsev et al., 1998). Reaction 6 shows that F, REE, and trace Ca are added, while Sr, CO3, and acidic water are lost during replacement of ancylite and strontianite by bastnäsite and cerianite. Whole rock analyses show higher F/CO2 ratios in oxide and transition zone samples, although this ratio is largely controlled by the degree of matrix carbonate dissolution. Decreasing CO2 activity (increasing F/CO2 ratios) as fluorocarbonates replace ancylite-dominant pseudomorphs is consistent with experimental data, which show ancylite-type REE carbonates crystallize at higher CO2 activities than bastnäsite-type hydroxycarbonates (Kutty et al., 1985; Moore et al., 2015). Replacement of the ancylite-dominant assemblage by Ca-REE fluorocarbonates is also recorded by a marked decrease of Sr in oxidized carbonatite from all areas relative to unoxidized carbonatite (Fig. 8, reaction 6) as Sr is largely incompatible in the fluorocarbonate structure, but greatly expands the ancylite stability field as a major component in its structure. These replacement reactions are based on the assumption that there was no major heterogeneity of primary REE minerals, and that this is a multi-stage open system process. We suggest, based on textural evidence, that burbankite and/or carbocernaite were the primary REE minerals through the entire vertical extent of carbonatite studied, and that a series of reactions like those above have resulted in the secondary assemblages. Pseudomorphs with fluorocarbonates partially replacing the ancylite + strontianite ± barite ± carbocernaite assemblage (Fig. 3g) are most abundant within the transition zone, supporting the general burbankite ? ancylite ? fluorocarbonate paragenesis. Synchysite appears to be most abundant in these pseudomorphs replacing ancylite, within the transition zone. We suggest synchysite crystallization was favored deeper in the carbonatite stockwork near the transitionunoxidized zone boundary where greater Ca activity increased synchysite stability relative to bastnäsite or parisite. In addition to the replacement of Sr-rich ancylite + strontianite + barite ± carbocernaite ± celestine pseudomorphs, major Sr depletion in oxidized samples (494–8489 ppm Sr) is the result of matrix carbonate (Sr-rich calcite and strontianite) dissolution. Oxidation of sulfides in the wearthering environment probably resulted in acidic fluids which enhanced matrix calcite dissolution, leaving behind a friable residuum of Fe, Mn, and Ti oxides, silicates (K-feldspar, micas, and clays), REE minerals, and native gold, equivalent to a gossanous cap through the network of dikes and veins. This volume loss is largely responsible for the near-surface REE-enrichment. Isocon mass balance calculations by Hutchinson (2016), which account for changes in porosity and volume during weathering of the BLC, show that REE are conserved and enrichment may be solely due to matrix removal. The results of Hutchinson (2016) confirm that Sr is lost from the system during weathering, but show little or no preferential mobilization

of LREE or HREE outside of the expected concentration due to weathering. The presence of cerianite, negative Ce-anomalies in nearly all secondary fluorocarbonates and phosphates analyzed (Figs. 10, 11c, 13), and cubic Fe oxide pseudomorphs after pyrite suggest higher f O2 in late-stage fluids, which may be the result of an increased meteoric water fraction. The pronounced negative Ce anomalies of Ca-REE fluorocarbonates suggest direct precipitation from late-stage oxidized solutions. At Taylor, zoned rhabdophane crystals appear to have recorded a transition from moderately oxidizing to more strongly oxidizing fluids as Ce is partitioned into cernaite, coupled with a transition from LREE to HREE precipitation (Figs. 12 and 13). While fluorocarbonates and phosphates display prominent negative Ce anomalies on chondrite normalized patterns, whole rock samples including those from the oxide zone, display only weak negative Ce anomalies (Fig. 7). Compared to unoxidized carbonatite, many oxide zone samples show Ce-enrichment (Fig. 8). This suggests Ce was fractionated, yet not transported great distances, as Ce balance is maintained by the co-crystallization of cerianite (Fig. 4b, f). Cerianite is stable in acidic to alkaline aqueous environments (pH = 3–12), but its stability is highly dependant on redox conditions (Loges et al., 2012; Pan and Stauffer, 2000). In alkaline environments cerianite is stable under reducing conditions; however, acidic environments must also become more oxidizing for cerianite to remain stable (Hutchinson, 2016). We suggest late-stage solutions at Bear Lodge became oxidizing as they mixed with meteoric water. Stable C and O isotope compositions reported by Moore et al. (2015) combined with unpublished analyses from our study further support the involvement of meteoric water during the latest modifications to carbonatite mineralogy. Moore et al. (2015) reports high d18O values for oxidized, transition zone carbonatite (12.5–13.6‰) and oxide zone carbonatite (18‰). The isotopic trend toward higher d18O values coincide with increasing sulfide oxidation, carbonate dissolution, and the replacement of ancylite-type pseudomorphs by fluorocarbonate minerals. These modifications were probably the result of low temperature (<250 °C) alteration by fluids with lower CO2/H2O ratios, and an increasing proportion of meteoric water in the supergene environment. Fluorcarbonates and phosphates which crystallized in the supergene environment with the involvement of meteoric water likely crystallized at lower temperatures. The few studies of fluorocarbonate-hosted fluid inclusions (cf., Yang et al., 1998; Williams-Jones et al., 2000; Hou et al., 2009; Xie et al., 2009) suggest moderate to low temperatures (210–410 °C) of precipitation, although bastnäsite mineralization at the Maoniuping REE deposit, China, is also thought to have occurred during a late, low-T (100– 200°) stage dominated by meteoric water (Yuan et al., 1995; Hou et al., 2009). Rhabdophane is stable only at low temperatures, and upon heating, it loses water as it is converted to monazite (Akers et al., 1993; Nagy and Draganits, 1999, and references herein). Experiments by Akers et al. (1993) show an upper stability limit of 200°C for rhabdophane in the system LaPO4-H2O, at H2O pressures of 500–2000 bar. Temperatures as low as 100–200 °C may not be unreasonable for secondary REE mineralization in oxidized portions of the BLC stockwork, particularly at Taylor where rhabdophane is abundant.

5.2. Carbonatite laterite and supergene REE mineralization at Taylor South Perhaps the best example of supergene REE mineralization at Bear Lodge is the network of silicified veins at Taylor South, which consist of yellow-green polycrystalline masses of acicular mon-

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a

b

2.0 cm

c

d

Fig. 18. (a) Outcrop of silicified yellow-green supergene monazite veins at Taylor South. (b) Laterite-like veins consist almost exclusively of monazite, jarosite and Fe oxides, rutile/pseudorutile, barite, plumbopyrochlore/U-rich pyrochlore, trace K-feldspar, and late concentrically banded silica. Hand sample shown consists of monazite, jarosite, and silica. (c) Supergene monazite in ferric iron-rich laterite at Araxá, Brazil (photo courtesy of Anthony N. Mariano). (d) Supergene monazite replacing apatite at Araxá, Brazil (photo courtesy of Anthony N. Mariano).

azite, with subordinate jarosite, barite, rutile, and pyrochlore (Fig. 5a, b). This style of acicular supergene monazite has been recognized in several other carbonatite laterites, including Araxá, Catalão I, and Morro do Ferro, Brazil; Mt. Weld, Australia; and Tomtor, Russia (Mariano, 1989a,b; Lottermoser, 1990; Waber, 1992; Kravchenko and Pokrovsky, 1995). Monazite in Fe3+-rich laterite at Araxá, Brazil, bears particular resemblance both texturally and chemically to monazite mineralization at Taylor South (Fig. 18a–d). The fine-grained acicular monazite morphology has been attributed to crystallization from colloidal solutions at near-surface temperatures and pressures, and often occurs as pseudomorphic replacement of apatite after the selective leaching of Ca and Mg (Mariano, 1979, 1989a). For reasons not yet completely understood, monazite formation was favored at Taylor South, peripheral to rhabdophane mineralization at Taylor. Mariano (1989a) suggests monazite may form by dehydration of rhabdophane, and although water loss may occur at relatively low-T (200 °C), much higher temperatures (500–900 °C) are required for a rhabdophane?monazite structure transformation (Jonasson and Vance, 1986). It is unclear whether dehydration of rhabdophane to form monazite is a common natural phenomenon, and there is no evidence (e.g., hexagonal pseudomorphs) of precursory rhabdophane or apatite at Taylor South. Similarities between carbonatite laterites (e.g., Araxá, Catalão I, Mt. Weld, Tomtor) (Mariano, 1979; Mariano, 1989a,b; Kravchenko and Pokrovsky, 1995) and the monazite + jarosite + rutile + barite + plumbopyrochlore assemblage at Taylor South suggest similar processes like eluvial enrichment and decalcification were responsible for the REE + HSFE mineralization. Accumulation of cationdeficient hydrated pyrochlore, enriched in Sr, Ba, Pb, K, Mn, Fe2+, Th, U, and REE, is a characteristic feature of laterite horizons over-

lying carbonatites, as these elements are liberated during dissolution of feldspars and carbonate minerals (Lumpkin and Ewing, 1995, 1996; Hogarth et al., 2000). X-ray element maps of complex zoned pyrochlore at Taylor South show evidence of A-site cation exchange with U- and Pb-rich domains (Fig. 17). Selective leaching of A- and Y-site components resulted in secondary irregular and turbid patches superimposed on primary oscillatory zoning, which suggests the pyrochlore-bearing veins were subjected to lowtemperature alteration in the lateritic weathering environment (e.g., Lottermoser and England, 1988). If supergene monazite formed at the expense of apatite, as noted in other carbonatite laterites, then apatite at Taylor South was probably much more abundant than in any primary unaltered carbonatite yet examined from the BLAC. Alternatively, weathering of carbonatite may have produced a horizon of residual apatite and ilmenite, which were subsequently altered to monazite and rutile by late-stage fluids that introduced REE. Other alkaline igneous rocks and local Paleozoic sediments appear to be insufficient sources of phosphorus, as P2O5 concentrations rarely exceed 1.0 wt% in each. An addition of phosphorus, by either magmatic, hydrothermal, or supergene processes is required to explain the elevated P2O5 concentrations at both Taylor (avg. 5.71 wt% P2O5) and Taylor South (avg. 6.88 wt% P2O5). The predominance of monazite and complete lack of carbonate minerals at Taylor South sug
gests a change in fluid chemistry, possibly higher PO3
and 4 /F 3
2
PO4 /CO3 ratios, where a greater amount of available phosphorous facilitated precipitation of monazite rather than REE fluorocarbonates. Like the REE-rich carbonatite veins, sensu stricto, the yellowgreen monazite veins cut across argillically altered Eocene-age alkaline silicate rocks, suggesting they are a lateritically weathered

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carbonatite equivalent. As the stratigraphically highest major REE occurrence in the BLAC, these particular carbonatite veins may have been exposed to more intense chemical weathering. While the BLAC no longer experiences the humid conditions necessary for laterite formation, flora and fauna of the nearby Green River Formation suggest a moist temperate to sub-tropical climate was prevalent at these latitudes during the Eocene, creating conditions more favorable to lateritization for at least some time after carbonatite emplacement. An unconformity between the Late EoceneOligocene White River Formation and underlying Paleogene intrusive rocks and pre-Cenezoic sedimentary rocks indicates that these rocks were exposed at the surface prior to deposition of the White River Formation (Lillegraven, 1993; Staatz, 1983; Hutchinson, 2016). Thus, the Late Eocene probably represents the first period of significant weathering that could have affected carbonatite intrusions. Lillegraven (1993) suggests that the region experienced widespread and intense erosion between 42 Ma and 37 Ma, which may be contemporaneous with the latest pulse of magmatic activity in the BLAC (Duke, 2005; Hutchinson, 2016). 5.3. LREE-HREE lateral zonation In addition to the near surface REE-enrichment, HREE/LREE ratios increase laterally away from the main carbonatite dike swarm adjacent to the Bull Hill diatreme (Fig. 9a). This enrichment results from a general conservation or slight increase in LREE concentrations and more pronounced increase in HREE concentrations (Figs. 7, 8). The total REE budget, concentration of individual REE, and slope of chondrite-normalized patterns are controlled mainly by the proportions of secondary REE-bearing phases throughout the carbonatite stockwork. Samples dominated by LREE-selective minerals, ancylite and carbocernaite (Bull Hill, unoxidized carbonatite), have the steepest LREE-enriched patterns, while samples containing appreciable amounts of REE phosphates in addition to Ca-REE fluorocarbonates and cerianite (Taylor, oxidized carbonatite) have flatter chondrite-normalized patterns and a greater proportion of HREE (Fig. 7). At Taylor and Taylor South, HREEenrichment corresponds with an increase in P and greater abundance of secondary REE phosphates, but a HREE-P correlation is absent at the larger Whitetail/Carbon area (Figs. 9b, 11d). Fluorocarbonate analyses show two major trends in REE distribution, the Bull Hill trend and Whitetail trend (Fig.11a). The higher Nd/ La ratios and HREE concentrations in Whitetail fluorocarbonates suggest fluids at peripheral localities were enriched in HREE relative to those that crystallized fluorocarbonates at Bull Hill. We consider three possibilities for the peripheral HREE-enrichment: 1) fractionation during crystallization of LREE selective minerals, such as ancylite, carbocernaite, and Ca-REE fluorocarbonates near Bull Hill, 2) REE liberated from matrix calcite, with higher HREE/LREE ratios, available during the crystallization of secondary fluorocarbonates and phosphates, and 3) differential transport of REE in flu2
ids with higher PO3
and F
/CO2
ratios, leading to 4 /CO3 3 phosphate and fluorocarbonate mineralization. Higher HREE/LREE ratios in fluids at peripheral localities are reasonably expected considering the amount of LREE removed from solution during crystallization of LREE selective minerals in the center of the complex. Oxide zone carbonatite at Bull Hill conP tains up to 19.92 elemental wt.% La-Nd, but less than 0.22 wt% P Tb-Lu (Tables 1a and 1b). However, this requires that a single fluid was responsible for fluorocarbonate mineralization as it emanated outward from Bull Hill toward Whitetail and Carbon. HREEenriched fluorcarbonate mineralization at Whitetail could as easily be explained by a separate magmatic or hydrothermal pulse slightly more enriched in HREE. The overlapping styles of fluorocarbonate mineralization at Carbon (Fig. 11e) suggest temporal changes in the HREE/LREE ratio.

Alteration and carbonate dissolution in the supergene environment have clearly contributed to the HREE-enrichment at peripheral (and shallower) areas, as HREE-enrichment is observed in nearly all oxidized samples regardless of there location (Figs. 7, 8). The HREE-enriched (although still LREE-dominant) character of late-stage fluids is further supported by the REE patterns of early-crystallized calcite (Olinger, 2012; Moore, 2014; Moore et al., 2015) dissolved during late hydrothermal or supergene stages. The results of Olinger (2012) and Moore et al. (2015), show relatively flat, middle lanthanide-enriched profiles for multiple generations of matrix calcite in unoxidized carbonatite (Fig. 6 of Moore et al., 2015). Late-stage fluids enriched in the REE liberated during partial dissolution are likely to reflect the middle to heavy lanthanide-enriched character of primary calcite. Even with the volume loss resulting from partial dissolution, LREE concentrations are conserved and even increase due to recrystallization. Ca-REE fluorocarbonates (70 wt% TREO) form at the expense of ancylite (40 wt% TREO) and also account for some LREE liberated from primary calcite and apatite. This is reflected by the whole rock geochemistry, which shows an increase of both LREE and HREE in oxidized samples relative to unoxidized carbonatite samples (Figs. 7, 8; Tables 1a and 1b). This model requires that major amounts of Ca and CO3 are removed in the supergene environment, while the REE remain available to form the secondary mineral assemblage. Our third scenario involves the preferential transport of HREE during hydrothermal or supergene stages. In some hydrothermal systems, LREE are expected to be more mobile than HREE, particularly those where Cl
is the predominant transport ligand (Migdisov et al., 2009, 2016; Williams-Jones et al., 2012). However, thermodynamic data do not yet exist to effectively predict REE 3
behavior in carbonatitic fluids where F
, CO2
3 , and PO4 may exist in a competitive environment at concentrations that greatly exceed that of Cl
and SO2
4 . Carbonate was likely an important ligand during an early carbo-hydrothermal stage and crystallization of ancylite and carbocernaite, while late-stage fluids emanating from and overprinting the carbonatites were characterized by higher PO3
4 /
2
CO2
3 and F /CO3 ratios, leading to phosphate and pseudomorphic fluorocarbonate mineralization. Although phosphates like monazite and xenotime have very low solubility, phosphates (not fluorocarbonates) are the exclusive host of REE in the most distal REErich veins in the BLAC. Taylor sample R25169 which consists mainly of fluorite and fluorapatite is enriched in HREE and depleted in LREE (Tables 1a and 1b, Figs. 4c, 7, 8). Small peripheral veins of this nature appear to represent some of the most highly fractionated fluids with high F, P, and HREE/LREE ratios. Given the HREE-P association at Taylor and HREE-enrichment of fluorocarbonates at Whitetail and Carbon, transport of REE in hydrothermal fluids with higher F
and PO3
4 contents may have contributed to the observed fractionation and preferential outward mobilization of HREE. It is relevant to note that veins on the far east side of the BLAC are also characterized by an assemblage of REE phosphates (xenotime and brockite), Ti-Nb oxides (Nb-rich anatase), and fluorite, further supporting our hypothesis that highly fractionated late-stage fluids were enriched in HREE, HFSE, P, and F (Andersen et al., 2016).

6. Conclusions The network of carbonatite dikes and veins at the center of the BLAC hosts a major resource of REE, at concentrations comparable to other large carbonatite-hosted REE deposits, such as Mountain Pass, California and Maoniuping, China. Mineral speciation and distribution of individual REE are important factors in assessing the economic potential of a REE occurrence, carbonatite-hosted or

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otherwise. Thus, any geological processes which may fractionate or concentrate the REE are of particular interest. The complex REE mineralogy and replacement textures in Bear Lodge carbonatites are the result of subsolidus processes, including auto-fenitization, late hydrothermal alteration and recrystallization, and supergene oxidation, which were important for the concentration and distribution of REE in a dynamic carbonatite-fluid system. Mineralogical, chemical, and isotopic evidence support a multistage, yet continuous, temporal evolution through: (1) magmatic; (2) carbo-hydrothermal; (3) late hydrothermal; and (4) supergene weathering stages. A vertical, toward-surface increase in REE concentrations correlates with replacement of REE(±Sr,Ca,Na,Ba) carbonate minerals by Ca-REE fluorocarbonate minerals, dissolution of matrix calcite, development of Fe- and Mn-rich gossan during oxidation, crystallization of cerianite and accompanying negative Ce anomalies in secondary fluorocarbonates and phosphates, and increasing d18O values. Collectively, these vertical changes demonstrate the importance of low-T oxidizing fluids during the most recent and noticeable chemical and mineralogical modifications to the carbonatite stockwork, which concentrated the REE to levels of a potential economically extractable resource. Ca-REE fluorocarbonates are the principal host of REE in oxide and transition zone carbonatite and major lateral REE zonation throughout the stockwork is reflected by fluorocarbonate chemistry. Bastnäsite and parisite are the most abundant, although synchysite and intermediate varieties are present and indicate variable Ca activity during fluorocarbonate crystallization. A growing body of evidence from several fluorocarbonate occurrences (e.g., Khibina, Kola Peninsula, Russia; Tamazert, Morocco; Bayan Obo, China; Bear Lodge, U.S.A.) shows that Ca-rich varieties (parisite and synchysite) contain a greater proportion of HREE than their Ca-poor counterpart, bastnäsite. With greater Ca activity, formation of parisite and mixed-layer bastnäsite-synchysite varieties are favored, perhaps increasing the capacity for HREE. These results show that in spite of the fluorocarbonate group’s affinity for LREE, minerals of this group can also be an important source of HREE and those REE deemed economically critical. Mineral speciation and lanthanide partitioning are important factors in metallurgical processing, as different techniques may be required to liberate REE from fluorocarbonates, phosphates, and cerianite. While HREE/LREE ratios are similar at peripheral localities, Whitetail, Carbon, Taylor, and Taylor South, the distribution of REE among different mineral species is quite different:  Whitetail and Carbon: Ca-REE fluorocarbonates and subordinate cerianite  Taylor: Ca-REE fluorocarbonates, rhabdophane-group phosphates, cerianite, fluorapatite  Taylor South: REE phosphates (supergene monazite) Mid to high atomic number REE (Eu-Lu and Y) are distributed between fluorocarbonates and rhabdophane-group phosphates at Taylor, while fluorocarbonates are the predominant host at Bull Hill, Whitetail, and Carbon. Optimum value will likely be achieved by recovering REE from all minerals in which REE are a major component. Acknowledgments The authors would like to thank Washington State University’s GeoAnalytical Laboratory and technical staff. We especially thank Owen Neill for improvement of microprobe routines. This research was financially supported by Rare Element Resources, Ltd., and we gratefully acknowledge former RER staff involved with this project, with special thanks to Adrian Van Rythoven, John Ray, and Jason Felsman for their contributions. We thank the University of Ore-

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gon’s CAMCOR facility and the assistance of Julie Chouinard. This manuscript and our overall understanding of Bear Lodge geology and REE mineralogy has benefited greatly from the advice and guidance of Dr. Anthony N. Mariano and Anthony Mariano Jr. We also thank an anonymous reviewer and Iain Samson for constructive reviews.

Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.oregeorev.2017. 06.025.

References Akers, W.T., Grove, M., Harrison, T.M., Ryerson, F.J., 1993. The instability of rhabdophane and its unimportance in monazite paragenesis. Chem. Geol. 110, 169–176. Andersen, A.K., Clark, J.G., Larson, P.B., Neill, O.K., 2016. Mineral chemistry and petrogenesis of a HFSE(+HREE) occurrence, peripheral to carbonatites of the Bear Lodge alkaline complex, Wyoming. Am. Mineral. 101, 1604–1623. Andersen, A.K., Cosca, M.A., Larson, P.B., 2013. Timing of carbonatite magmatism in the Bear Lodge alkaline complex, WY. Geol. Soc. Am. Abstracts Prog. 45 (7). Andrade, F.R.D., Möller, P., Lüders, V., Dulski, P., Gilg, H.A., 1999. Hydrothermal rare earth elements mineralization in the Barra do Itapirapuã carbonatite, southern Brazil: behaviour of selected trace elements and stable isotopes (C, O). Chem. Geol. 155, 91–113. Berger, A., Gnos, E., Janots, E., Fernandez, A., Giese, J., 2008. Formation and composition of rhabdophane, bastnäsite and hydrated thorium minerals during alteration: implications for geochronology and low-temperature processes. Chem. Geol. 254, 238–248. Boatner, L.A., 2002. Synthesis, structure, and properties of monazite, pretulite, and xenotime. In: Kohn, M.J., Rakovan, J., Hughes, J.M. (Eds.), Phosphates: Geochemical, Geobiological, and Materials Importance. Reviews in Mineralogy and Geochemistry, 48. Mineralogical Society of America, Washington D.C., 337– 362. Bühn, B., 2008. The role of the volatile phase for REE and Y fractionation in lowsilica carbonate magmas: implications from natural carbonatites, Namibia. Mineral. Petrol. 92, 453–470. Bulakh, A.G., Le Bas, M.J., Wall, F., Zaitsev, A.N., 1998. Ancylite-bearing carbonatites of the Seblyavr massif, Kola Peninsula, Russia. Neues Jahrb. Mineral. Monatshefte 4, 171–192. Chakhmouradian, A.R., Mitchell, R.H., 2002. New data on pyrochlore- and perovskite-group minerals from the Lovozero alkaline complex, Russia. Eur. J. Mineral. 14, 821–836. Chakhmouradian, A.R., Zaitsev, A.N., 2012. Rare-earth mineralization in igneous rocks: sources and processes. Elements 8, 347–353. Chang, L.L.Y., Howie, R.A., Zussman, J., 1996. Rock-Forming Minerals, 2nd ed. Nonsilicates. Sulphates, Carbonates, Phosphates and Halides, vol. 5B. Longman, Harlow, UK, 383p. Dahlberg, P.S., Noble, A.C., Pickarts, J.T., Rose, W.L., Jaacks, J.A., 2014. Technical Report on the Reserves and Development of the Bul Hill Mine, Wyoming. Rare Element Resources, Ltd., Bear Lodge Project Canadian NI 43–101 Pre-Feasibility Technical Report, Oct. 9, 2014. Donovan, J.J., Kremser, D., Fournelle, J.H., 2007. Probe for EPMA User’s Guide and Reference. Probe Software Inc, Eugene, OR, p. 355p. Donovan, J.J., Lowers, H.A., Rusk, B.G., 2011. Improved electron probe microanalysis of trace elements in quartz. Am. Mineral. 96, 274–282. Donovan, J.J., Synder, D.A., Rivers, M.L., 1993. An improved intereference correction for trace element analysis. Microbeam Anal. 2, 23–28. Donnay, G., 1953. Roentgenite, 3CeFCO32CaCO3, a new mineral from Greenland. Am. Mineral. 38, 868–870. Donnay, G., Donnay, J.D.H., 1953. The crystallography of bastnaesite-(Ce), parisite(Ce), roentgenite-(Ce) and synchysite-(Ce). Am. Mineral. 38, 932–963. Dorfman, M.D., Gorshkov, A.I., Nechelyustov, G.N., 1993. Calcium rhabdophane, a new variety. Transactions (Doklady) of the Russian Academy of Sciences 331, 583–586. Duke, G.I., 2005. Geochemistry and Geochronology of Paleocene-Eocene Alkalic Igneous Rocks, Northern Black Hills, South Dakota and Wyoming, Ph.D. dissertation, South Dakota School of Mines and Technology, 291p. Duke, G.I., 2009. Black Hills-Alberta carbonatite-kimberlite linear trend: slab edge at depth? Tectonophysics 464, 186–194. Duke, G.I., Singer, B.S., DeWitt, E., 2002. 40Ar/39Ar laser incremental-heating ages of Devil’s Tower and Paleocene-Eocene intrusions of the northern Black Hills, South Dakota and Wyoming. Geol. Soc. Am. Abstr. Prog. 34 (6). 207-7. Felsman, J.M., 2009. Geology, Hydrothermal Alteration, and Gold-Telluride Mineralization in the vicinity of Carbon Hill and Taylor Ridge, Bear Lodge Mountains, Wyoming, M.S. thesis, University of Idaho, 305p.

806

A.K. Andersen et al. / Ore Geology Reviews 89 (2017) 780–807

Fleet, M.E., Liu, X., Pan, Y., 2000. Rare-earth elements in chlorapatite [Ca10(PO4)6Cl2]: uptake, site preference and degradation of monoclinic structure. Am. Mineral. 85, 1437–1446. Göb, S., Wenzel, T., Bau, M., Jacob, D.E., Loges, A., Markl, G., 2011. The redistribution of rare-earth elements in secondary minerals of hydrothermal veins, Schwarzwald, SW Germany. Can. Mineral. 49, 1305–1333. Goldstein, J., Newbury, D., Echlin, P., Joy, D., Fiori, D., Lifshin, E., 1984. Scanning Electron Microscopy and X-Ray Microanalysis. Plenum, New York, p. 690p. Gramaccioli, C.M., Segalstad, T.V., 1978. A uranium- and thorium-rich monazite from a south-alpine pegmatite at Piona, Italy. AmericanMineralogist 63, 757– 761. Guastoni, A., Nestola, F., Giaretta, A., 2009. Mineral chemistry and alteration of rare earth element (REE) carbonates from alkaline pegmatites of Mount, Malawi. Am. Mineral. 94, 1216–1222. Hogarth, D.D., Williams, C.T., Jones, P., 2000. Primary zoning in pyrochlore group minerals from carbonatites. Mineral. Mag. 64, 683–697. Hou, Z., Tian, S., Xie, Y., Yang, Z., Yin, S., Yi, L., Fei, H., Zou, T., Bai, G., Li, X., 2009. The Himalayna Mianning-Dechang REE belt associated with carbonatite-alkaline complexes, eastern Indo-Asian collision zone, SW China. Ore Geol. Rev. 36, 65– 89. Hutchinson, M. B., 2016. REE Enrichment in Weathered Carbonatite, Bull Hill: Bear Lodge Mountains, Wyoming. M.Sc. Thesis. Colorado School of Mines, Golden, Colorado. 72p. Jenner, G.A., 1984. Tertiary alkalic igneous activity, potassic fenitization, carbonatitic magmatism, and hydrothermal activity in the central and southeastern Bear Lodge Mountains, Crook County, Wyoming. M.Sc. thesis. University of North Dakota, Grand Forks, North Dakota, 232p. Jonasson, R.G., Vance, E.R., 1986. DTA study of the rhabdophane to monazite transformations in rare earth (La-Dy) phosphates. Thermochim. Acta 108, 65– 72. Kravchenko, S.M., Pokrovsky, B.G., 1995. The Tomtor Alklaine Ultrabasic Massif and related REE-Nb Deposits, Northern Siberia. Econ. Geol. 90, 676–689. Krenn, E., Finger, F., 2007. Formation of monazite and rhabdophane at the expense of allanite during Alpine low temperature retrogression of metapelitic basement rocks from Crete, Greece: microprobe data and geochronological implications. Lithos 95, 130–147. Kutty, T.R.N., Tareen, J.A.K., Mohammed, I., 1985. Correlation between the stability of carbonates in ternary Ln2O3-H2O-CO2 hydrothermal systems and lanthanide systematics. J. Less Common Metals 105, 197–209. Lapin, A.V., 1994. Churchite from lateritic weathered mantles on carbonatites and the behavior of rare earths in the supergene alteration zone. Transactions (Doklady) of the Russian Academy of Sciences. Earth Sci. Sect. 327, 135–139. Laputina, I.P., Batyrev, V.A., Yakushev, A.I., 1999. A new EPMA technique for determination of rare earth elements with the use of automated peak-overlap and modelled background corrections. J. Anal. At. Spectrom. 14, 465–469. Lillegraven, J.A., 1993. Correlation of Paleogene Strata Across Wyoming – A User’s Guide, In: Snoke, A.W., Steidtmann, J.R. and Roberts, S.M. (Eds.), Geology of Wyoming: Geological Survey of Wyoming Memoir No. 5, pp. 414–477. Loges, A., Wagner, T., Barth, M., Bau, M., Göb, S., Markl, G., 2012. Negative Ce Anomalies in Mn oxides: the role of Ce4+ mobility during water-mineral interaction. Geochicimica et Cosmochimica Acta 86, 296–317. Lottermoser, B.G., 1990. Rare-earth element mineralization within the Mt. Weld carbonatite laterite, Western Australia. Lithos 24, 151–167. Lottermoser, B.G., England, B.M., 1988. Compositional variation in pyrochlores from the Mt. Weld carbonatite laterite, Western Australia. Mineral. Petrol. 38, 37–51. Lumpkin, G.R., Ewing, R.C., 1988. Alpha-decay damage in minerals of the pyrochlore group. Phys. Chem. Miner. 16, 2–20. Lumpkin, G.R., Ewing, R.C., 1995. Geochemical alteration of pyrochlore group minerals: pyrochlore subgroup. Am. Mineral. 80, 732–743. Lumpkin, G.R., Ewing, R.C., 1996. Geochemical alteration of pyrochlore group minerals: betafite subgroup. Am. Mineral. 81, 1237–1248. Mariano, A.N., 1978a. The Application of Cathodoluminescence for Carbonatite Exploration and Characterization, In: Proceedings, 1st International Symposium on Carbonatites, Pocos de Caldas, Minas Gerais, Brazil, June 1976, ed. Braga, C.J., 39–57, Brasilia: Brasil, Departamento Nacional da Producão Mineral, 49p. Mariano, A.N., 1978b. A Petrographic Examination of Selected Drill Core From the Bear Lodge Project, Crook County. Unpublished report to Molycorp, Wyoming, p. 34p. Mariano, A.N., 1979. Rare Earth Mineralization at Araxá Minas Gerais. Unpublished report to Molycorp, Brazil, p. 88p. Mariano, A.N., 1981. Rare Earth Mineralization in the Bear Lodge Carbonatite Complex Crook County. Unpublished report to Molycorp, Wyoming, p. 38p. Mariano, A.N., 1989a. Nature of economic mineralization in carbonatites and related rocks, In: Bell, K. (Ed.), Carbonatites; genesis and evolution, pp. 149–176. Mariano, A.N., 1989b. Economic Geology of Rare Earth Elements. In: Lipin, B.R. and McKay, G.A. (Eds.), Geochemistry and Mineralogy of Rare Earth Elements, Reviews in Mineralogy 21, pp. 309–337. Mariano, A.N., 1989c. Cathodoluminescence Emission Spectra of Rare Earth Element Activators in Minerals. In: Lipin, B.R. and McKay, G.A. (Eds.), Geochemistry and Mineralogy of Rare Earth Elements, Reviews in Mineralogy 21, pp. 339–348. McDonough, W.F., Sun, S.S., 1995. The composition of the earth. Chem. Geol. 120, 223–253. McDowell, F.W., 1971. K-Ar ages of igneous rocks from the western United States: Isochron/West. 2, 1–16.

Melgarejo, J.C., Martin, R.F., 2011. Atlas of Non-silicate Minerals in Thin Section. Canadian Mineralogist Special Publication 7, 522p. Menezes Filho, L.A.D., Chaves, M.L.S.C., Chukanov, N.V., Atencio, D., Scholz, R., Pekov, I., Magela da Costa, G., Morrison, S.M., Andrade, M., Freitas, E., Downs, R.T., Belakovskiy, D.I., 2016. Parisite-(La), IMA 2016–031. CNMNC Newsletter No. 32, August 2016, page 920. Mineral. Mag. 80, 915–922. Meng, D.W., Wu, X.L., Han, Y., Meng, X., 2002. Polytypism and microstructures of the mixed-layer member B2S, CaCe3(CO3)4F3 in the bastnaesite-(Ce)–synchysite(Ce) series. Earth Planet. Sci. Lett. 203, 817–828. Migdisov, A.A., Williams-Jones, A.E., Brugger, J., Caporuscio, F.A., 2016. Hydrothermal transport, deposition, and fractionation of the REE: experimental data and thermodynamic calculations. Chem. Geol. 439, 13–42. Migdisov, A.A., Williams-Jones, A.E., Wagner, T., 2009. An experimental study of the solubility and speciation of the Rare Earth Elements (III) in fluoride- and chloride-bearing aqueous solutions at temperatures up to 300°C. Geochim. Cosmochim. Acta 73, 7087–7109. Moore, M., 2014. Carbonatite-related rare-earth mineralization in the Bear Lodge alkaline complex, Wyoming: Paragenesis, geochemical and isotopic characteristics. M.Sc. Thesis. University of Manitoba, Winnipeg, Manitoba. 240p. Moore, M., Chakhmouradian, A.R., Mariano, A.N., Sidhu, R., 2015. Evolution of rareearth mineralization in the Bear Lodge carbonatite, Wyoming: mineralogical and isotopic evidence. Ore Geol. Rev. 64, 499–521. Nagy, G., Draganits, E., 1999. Occurrence and mineral-chemistry of monazite and rhabdophane in the Lower and ?Middle Austroalpine tectonic units of the ¨ sterr. 42, 21– southern Sopron Hills, (Austria). Mitt. Ges. Geol. Bergbaustud. O 36. Nagy, G., Draganits, E., Demény, A., Pantòa, G., Arkai, P., 2002. Genesis and transformations of monazite, florencite and rhabdophane during medium grade metamorphism: examples from the Sopron Hills, Eastern Alps. Chem. Geol. 191, 25–46. Nielsen, C.H., Sigurdsson, H., 1981. Quantitative methods for electron microprobe analysis of sodium in natural and synthetic glasses. Am. Mineral. 66, 547–552. Ni, Y.X., Hughes, J.M., Mariano, A.N., 1993. The atomic arrangement of bastnästite(Ce), Ce(CO3)F, and structural elements of synchysite-(Ce), röntgenite-(Ce), and parisite-(Ce). Am. Mineral. 78, 415–418. Ni, Y.X., Hughes, J.M., Mariano, A.N., 1995. Crystal chemistry of the monazite and xenotime structures. Am. Mineral. 80, 21–26. Olinger, D., 2012. Characterization and Genetic Relation of Carbonatite and Associated Alkaline Silicate Rocks of Northwest Bull Hill, Bear Lodge Mountains, Northeast Wyoming. M.Sc. Thesis. Texas Tech University, Lubbock, Texas. 299p. Pan, Y., Fleet, M.E., 2002. Compositions of the apatite-group minerals: Substitution mechanisms and controlling factors. In: Kohn, M.J., Rakovan, J., Hughes, J.M. (Eds.), Phosphates: Geochemical, Geobiological, and Materials Importance. Reviews in Mineralogy and Geochemistry, 48. Mineralogical Society of America, Washington D.C., pp. 13–49. Pan, Y., Stauffer, M.R., 2000. Cerium anomaly and Th/U fractionation in the 1.85Ga Flin Flon Paleosol: Clues from REE- and U-rich accessory mineral and implications for paleoatmospheric reconstruction. Am. Mineral. 85, 898–911. ˇ ejka, J., Škoda, R., Goliáš, V., 2009. Supergene mineralization of Plášil, J., Sejkora, J., C the Medveˇdín uranium deposit, Krkonoše Mountains, Czech Republic. J. Geosci. 54, 15–56. Pyle, J., Spear, F., Wark, D.A., 2002. Electron Microprobe analysis of REE in apatite, monazite and xenotime: protocols and pitfalls. In: Kohn, M.J., Rakovan, J., Hughes, J.M. (Eds.), Phosphates: Geochemical, Geobiological, and Materials Importance. Reviews in Mineralogy and Geochemistry, 48. Mineralogical Society of America, Washington D.C., pp. 337–362. Roeder, P.L., 1985. Electron-microprobe analysis of minerals for rare-earth elements: use of calculated peak-overlap corrections. Can. Mineral. 23, 263– 271. Roeder, P.L., MacArthur, D., Ma, X.-P., Palmer, G.R., Mariano, A.N., 1987. Cathodoluminescence and microprobe study of rare-earth elements in apatite. Am. Mineral. 72, 801–811. Rønsbo, J.G., 1989. Coupled substitutions involving REE and Na and Si in apatites in alkaline rocks from Ilimaussaq, South Greenland, and the petrological implications. Am. Mineral. 74, 896–901. Ruberti, E., Enrich, G.E.R., Gomes, C.B., Comin-Chiaramonti, P., 2008. Hydrothermal REE fluorocarbonate mineralization at Barra do Itapirapuã, a multiple stockwork carbonatite, southern Brazil. Can. Mineral. 46, 901–914. Salvi, S., Williams-Jones, A.E., 1990. The role of hydrothermal processes in the granite-hosted Zr, Y, REE deposit at Strange Lake, Québec-Labrador: evidence from fluid inclusions. Geochim. Cosmochim. Acta 54, 2403–2418. Sheard, E.R., Williams-Jones, A.E., Heiligmann, M., Pederson, C., Trueman, D.L., 2012. Controls on the concentration of zirconium, niobium, and the rare earth elements in the Thor Lake rare metal deposit, Northwest Territories, Canada. Econ. Geol. 107, 81–104. Smith, M.P., Henderson, P., Campbell, L.S., 2000. Fractionation of the REE during hydrothermal processes: constraints from the Bayan Obo Fe-REE-Nb deposit, Inner Mongolia, China. Geochim. Cosmochim. Acta 64, 3141–3160. Staatz, M.H., 1983. Geology and Description of Thorium and Rare-Earth Deposits in the Southern Bear Lodge Mountains, Northeastern Wyoming. U.S. Geological Survey Professional Paper 1049-D, 52p. Taylor, S.R., McLennan, S.M., 1985. The Continental Crust: Its Composition and Evolution. Blackwell Scientific Publications, Oxford, p. 312p.

A.K. Andersen et al. / Ore Geology Reviews 89 (2017) 780–807 Van Emden, B., Thornber, M.R., Graham, J., Lincoln, F.J., 1997. The incorporation of actinides in monazite and xenotime from placer deposits in Western Australia. Can. Mineral. 35, 95–104. Van Landuyt, J., Amelinckx, S., 1975. Multiple beam direct lattice imaging of a new mixed-layer compound of the bastnaesite-synchysite series. Am. Mineral. 60, 351–358. Van Wambeke, L., 1977. The Karonge rare earth deposits, Republic of Burundi: new mineralogical–geochemical data and origin of the mineralization. Miner. Deposita 12, 373–380. Waber, N., 1992. The supergene thorium and rare-earth element deposit at Morro do Ferro, Poços de Caldas, Minas Gerais, Brazil. J. Geochem. Explor. 45, 113–157. Wall, F., 2000. Mineral chemistry and petrogenesis of rare earth-rich carbonatites with particular reference to the Kangankunde carbonatite, Malawi. Ph.D. dissertation. University of London. 284p. Wall, F., Mariano, A.N., 1996. Rare earth minerals in carbonatites: a discussion centred on the Kangankunde Carbonatite, Malawi. In A.P. Jones, F. Wall, C.T. Williams, Eds., Rare Earth Minerals: Chemistry, Origin and Ore Deposits. Mineralogical Society Series, 7, 193–225. Wall, F., Niku-Paavola, V.N., Storey, C., Muller, A., Jefferies, T., 2008. Xenotime-(Y) from carbonatite dykes at Lofdal, Namibia: unusually low LREE/HREE ratio in carbonatite, and the first dating of xenotime overgrowths on zircon. Can. Mineral. 46, 861–877. Walter, A.-V., Flicoteaux, R., Parron, C., Loubet, M., Nahon, D., 1995. Rare-earth elements and isotopes (Sr, Nd, O, C) in minerals from the Juquiá carbonatite (Brazil): tracers of a multistage evolution. Chem. Geol. 120, 27–44. Watt, G.R., 1995. High-thorium monazite-(Ce) formed during disequilibrium melting of metapelites under granulite-facies conditions. Mineral. Mag. 59, 735–743. Williams, C.T., 1996. Analysis of rare earth minerals. In A.P. Jones, F. Wall, C.T. Williams, Eds., Rare Earth Minerals: Chemistry, Origin and Ore Deposits. Mineralogical Society Series, 7, 327–348.

807

Williams-Jones, A.E., Migdisov, A.A., Samson, I.M., 2012. Hydrothermal mobilisation of the rare earth elements–a tale of ‘‘ceria” and ‘‘yttria”. Elements 8, 355–360. Williams-Jones, A.E., Samson, I.M., Olivo, G.R., 2000. The genesis of hydrothermal fluorite-REE deposits in the Gallinas Mountains, New Mexico. Econ. Geol. 95, 327–341. Wu, X.L., Meng, D.W., Pan, Z.L., Yang, G.M., Li, D.X., 1998. Transmission electron microscopic study of new, regular, mixed-layer structures in calcium–rareearth fluorocarbonate minerals. Mineral. Mag. 62, 55–64. Xie, Y.L., Hou, Z.Q., Yin, S.P., Simon, C.D., Xu, J.H., Tian, S.H., Xu, W.Y., 2009. Continuous carbonatitic melt-fluid evolution for a REE mineralization system: evidence from inclusions in the Maoniuping REE deposit in the western Sichuan, China. Ore Geol. Rev. 36, 89–104. Yang, G.M., Chang, C., Zuo, D.H., Liu, X.L., 1998. Geology and Mineralization of the Dalucao REE Deposit in Dechang County, Sichuan Province. Open file of China University of Geosciences, Wuhan, pp. 1–89 (in Chinese). Yuan, Z.X., Shi, Z.M., Bai, G., Wu, C.Y., Chi, R.A., Li, X.Y., 1995. The Maoniuping Rare Earth Ore Deposit, Mianning County, Sichuan Province. Seismological Press, Beijing. 150 p., (in Chinese). Zaitsev, A.N., Chakhmouradian, A.R., 2002. Calcite–amphibole–clinopyroxene rock fromthe Afrikanda complex, Kola Peninsula, Russia: mineralogy and a possible link to carbonatites. II. Oxysalt minerals. Can. Miner. 40, 103–120. Zaitsev, A.N., Wall, F., LeBas, M.J., 1998. REE–Sr–Ba minerals from the Khibina carbonatites, Kola Peninsula, Russia: their mineralogy, paragenesis, and evolution. Mineral. Mag. 62, 225–250. Zaitsev, A.N., Demény, A., Sindern, S., Wall, F., 2002. Burbankite-group minerals and their alteration in rare earth carbonatites—source of elements and fluids (evidence from C-O and Sr–Nd isotopic data). Lithos 62, 15–33.