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Trace elements in calcite from carbonatite of the Bear Lodge deposit, northeast Wyoming: evidence of hydrothermal alteration? Danielle Olinger, Texas Tech University, [email protected] The Bear Lodge Complex Introduction Calcite Textures The mobilization and concentration of rare earth elements (REE) have received a great deal of attention in the geochemical literature as a result of increased attempts to understand the formation of hydrothermally derived rare earth element deposits. Hydrothermal fluids with hard ligands (F - , CO 3 2- , Cl - , SO 4 2- ) are capable of mobilizing REE and recent studies have shown that F - , CO 3 2- , and Cl - are important ligands for mobilization and preferential partitioning of light rare earth elements at elevated temperatures (Wood 1990, Migdisov and Williams-Jones 2010, Williams-Jones et al. 2012). This study presents data on the changes in REE concentration and abundance patterns as magmatic calcite undergoes recrystallization in the presence of hydrothermal fluid(s). The results indicate mechanisms in which ore-grade REE carbonates can form as a result of hydrothermal alteration of carbonatite calcite. Carbonatite and REE Mineralization Conclusions Results Figure 1: Locality of the Bear Lodge complex (after Rare Element Resources). BL10-04 BL10-11 Figure 2: Geology of the Bear Lodge complex (after Richardson 2010). Abstract ID V43C-2850 Acknowledgements 1 10 100 1000 10000 100000 1 10 100 1000 10000 100000 1 10 100 1000 10000 100000 La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu coarse-grained calcite calcite w/ well defined cleavage Figure 4: Photomicrographs of (a) coarse grained, euhedral calcite with well defined cleavage, (b) coarse grained, consertal calcite, and (c) fine grained, anhedral calcite. A B C fine-grained calcite coarse-grained, consertal calcite fine-grained calcite bulk rock bulk rock bulk rock Funding for this research was provided by Rare Element Resources. This research was completed under the guidance of Cal Barnes, Callum Hetherington, Jim Clark, and Moira Ridley. Additional help was provided by Melanie Barnes and Mark Grimson. A B Figure 3: (a) A Bear Lodge carbonatite sample with calcite (cal), sphalerite (sp), K-feldspar (kspar), and hexagonal pseudomorphs (hex) composed of ancylite, bastnaesite group minerals, strontianite, barite, pyrite, and calcite, (b) photomicrograph of hexagonal pseudomorphs and phlogopite aggregates, (c) BSE image of hexagonal pseudomorph composed of calcite( cal), strontianite (str), ancylite (anc), syncysite (syn), and minor rutile (rt). The Bear Lodge complex is one of a group of igneous complexes that extend along a N75W trend from the northern Black Hills, through Wyoming and into Montana (Duke 2005, Richardson 2010) (Figure 1). Intrusion age and degree of silica saturation decrease to the northwest, with the youngest (Tertiary) and most alkaline rocks existing in the Bear Lodge complex (Duke 2005). The complex consists Bear Lodge carbonatite is primarily composed of calcite with <5 to 20 volume percent hexagonal pseudomorphs thought to be after primary burbankite or ancylite. Pseudomorphs are composed of ancylite, strontianite, calcite, barite/celestite, minor phlogopite, and fibrous aggregates of parisite with syntaxial intergrowths of synchysite or bastnaesite and interstitial hematite. Accessory phases include fluorite, barite, phlogopite, rutile, and orthoclase. Secondary phases are pyrite, sphalerite, galena, ilmenite, and hematite. hex kspar cal sp C cal str anc syn rt of three main rock units: (1) Roof pendants of Precambrian gneiss and granite, (2) Paleozoic and Mesozoic sedimentary units, and (3) Tertiary carbonatite and alkaline silicate intrusions (Staatz 1983) (Figure 2). References The Bear Lodge REE deposit is hosted in a carbonatite dike swarm located in the north-central area of the complex within and proximal to the Bull Hill and Carbon Hill diatreme structures (Richardson 2010) (Figure 2). Target REE mineralization is secondary LREE- carbonate minerals found in pseudomorphs after primary burbankite or ancylite (Table 1). The secondary REE mineral assemblage is predominantly ancylite in unoxidized carbonatite and predominantly bastnaesite group minerals (Table 1) in near surface, oxidized carbonatite. Mineral Formula REE oxide wt% Ancylite Sr(La,Ce)(CO 3 ) 2 OH*H 2 O 45 Burbankite (Na,Ca) 3 (Sr,Ba,Ce) 3 (CO 3 ) 5 12 Bastnaesite (La,Ce)(CO 3 )F 75 Parisite Ca(Ce,La) 2 (CO 3 ) 3 F 2 60 Synchysite Ca(Ce,La)(CO 3 ) 2 F 50 Table 1: Rare earth element mineralization present in the Bear Lodge complex. Burbankite is presumed to be the primary REE-carbonate mineral replaced by ancylite +minor bastnaesite group minerals in unoxidized zones of carbonatite while bastnaesite group minerals are the predominant secondary REE-mineralization near surface. Bastnaesite group minerals are indicated with an asterisk. (Anthony et al., Handbook of Mineralogy) The majority of Bear Lodge carbonatite samples contain calcite of two or more of the following textures: 1) Euhedral to subhedral, coarse-grained calcite with well defined cleavage 2) Coarse-grained, consertal to serrated, equant to elongate calcite with abundant inclusions of fluorite, calcite, and, to a lesser extent, ancylite and burbankite 3) Fine-grained, anhedral, inclusions free calcite frequently displaying 120 o contacts. This texture is predominantly found proximal to hexagonal pseudomorphs Figure 5: Chondrite normalized (Sun and McDonough 1989) rare earth element diagrams with calcite patterns compared to whole rock carbonatite patterns for three carbonatite samples from northwest Bull Hill. A B C Coarse-grained calcite with well defined cleavage displays steep, log-linear, negatively sloped REE patterns which parallel whole rock REE patterns (Figure 5a) This pattern is considered to be characteristic of magmatic calcite crystallizing as the main mineral phase from a LREE-rich carbonatite melt (Hornig-Kjarsgaard 1998, Halama et al. 2005). Fine-grained, anhedral, inclusion free calcite and coarse-grained, consertal calcite with abundant inclusions have convex-up, “humped” REE patterns (Figure 5b). Such “humped” REE patterns could be the product of calcite recrystallization in the presence of a hydrothermal fluid. Specifically, the light rare earth elements would be preferentially partitioned into hydrothermal fluid(s) with hard ligands (F - and CO 3 2- ) during calcite recrystallization resulting in the observed “humped” patterns (Halama et al. 2005, Wood 1990, Migdisov and Williams-Jones 2009). This explanation is consistent with the presence of fluorite and calcite inclusions in consertal calcite, late stage fluorite-calcite veins throughout the complex, and secondary REE- fluorocarbonate mineralization. Fine-grained, anhedral to consertal calcite proximal to hexagonal pseudomorphs displays relatively flat REE patterns (Figure 5c). The process producing these patterns is not clear but could be the result of late stage calcite crystallization after a LREE-rich phase or recrystallization in the presence of a hydrothermal fluid (Xu et al. 2010). Anthony, J.W., Bideaux, R.A., Bladh, K.W., Nichols, M.C., Eds., Handbook of Mineralogy, Mineralogical Society of America: Chantilly, VA, http://www.handbookofmineralogy.org. Duke, G.I. (2005). Doctorate of Philosophy Thesis: South Dakota School of Mines and Technology. Halama, R., Vennemann, T., Siebel, W., Markl, G (2005). Journal of Petrology 46, pp191-217. Hornig-Kjarsgaard, I. (1998). Journal of Petrology 39 (11+12), pp 2105-2121. Migdisov, A.A. and Williams-Jones, A.E. (2009). Geochimica et Cosmochimica Acta 73, pp 7087-7109. Rare Element Resources (2010). Corporate overview: targeting rare earth elements and gold, Bear Lodge project. Management Presentation June 2010, http://www.rareelementresources.com. Richardson, M.P. (2010). Technical report: preliminary economic assessment (scoping study) of the Bear Lodge Rare- Earths Project—a national instrument 43-101 report. Staatz, M.H. (1983). Geological Survey Professional Paper 1049D. Wood, S.A. (1990). Chemical Geology 88, pp 99-125. Xu, C., Kynicky, J., Chakhmouradian, A.R., Campbell, I.H., and Allen, C.M. (2010). Lithos 118, pp 145-155. Williams-Jones, A.E., Migdisov, A.A., and Samson, I.M. (2012). Elements 8, pp 355-360. •Calcite was the main crystallizing phase as indicated by REE patterns that parallel whole rock REE patterns. •The variable calcite textures are the product of recrystallization in the presence of a hydrothermal fluid with hard ligands such as F - and CO 3 2- . •Flat REE patterns are either the product of calcite recrystallization in the presence of a hydrothermal fluid or late stage calcite crystallization after a LREE-rich phase. The author is currently awaiting samples from the Mountain Pass mine to test the latter hypothesis. •The LREE were likely sequestered into secondary REE-carbonate minerals present in pseudomorphs throughout the system given the increase in REO content from primary burbankite at depth to secondary bastnaesite group minerals present near surface.
