THE COMPOSITION OF GARNET IN GRANITIC PEGMATITES AS A PROXY FOR MELT EVOLUTION AND RARE METAL POTENTIAL
Mitchell, Nicholas Joseph
Granitic pegmatites have been and continue to be important economic deposits of rare-metals, including Li, Ta, and rare-earth elements in addition to industrial minerals. Garnet from granitic pegmatites of the LCT, NYF, and mixed family pegmatites were analyzed for major- and trace-element contents to understand relative melt evolution, to determine compositional differences between pegmatite families, and to identify compositions indicative of pegmatites containing rare metals. Major- and trace-element contents of garnet were analyzed from the following 36 pegmatites: Alto Mirador, Altai, Avondale, Batchellerville, Bayliss, Benson, Ben Murphy, Bon Ami, Boqueirão, Capoeira 2, Carrascão, Cyprus-Foote, Cryo-Genie, Elizabeth R, Emmons, Greenfield, Greenlaw, Hoot Owl, Järkvissle, Little Three, Lord Hill, Mt. Apatite, Mt. Mica, Oceanview, Pala Chief, Palermo, Quintos, Ray, Rutherford #2, San Miguel, Sinkhole, Strzegom, Sullins-Wiseman, Szklary, Ytterby, and Yunnan. Garnet compositions reported in terms of almandine (Alm), andradite (Adr), grossular (Grs), pyrope (Prp), and spessartine (Sps) end-members (mol.%) can be described by the ranges Sps2.26-95.20Alm0.00-86.78Prp0.00-13.17Grs0.00-14.38Adr0.00-5.45, and fall within those previously reported for garnet in granitic pegmatites. Major-element composition plots of almandine vs. spessartine, FeO + MgO vs. MnO + CaO (wt.%), and Mg vs. Mn/(Mn + Fe2+) contents in garnet suggest higher relative degree of fractionation with increasing Mn contents but do not distinguish garnets from NYF and LCT pegmatites. A ternary diagram based on the molecular ratios Fe/(Fe + Mn), Fe/(Fe + Ca), and Fe/(Fe + Mg) is useful to distinguish two main compositional trends for NYF and LCT granitic pegmatites. In-situ trace-element analysis of garnet (51 elements) obtained by LA-ICP-MS shows that Li, Ga, Ge, Ti, Zn, and Zr are present in all garnets. Trace-element differences are identifiable for garnet from LCT vs. NYF granitic pegmatites. Phosphorous contents are significantly higher in garnet from LCT pegmatites compared to those in NYF pegmatites. Garnet from pegmatites which are considered to be highly evolved, as reflected by the presence of rare-metal minerals, typically have high Y/Ho (> 60) ratios, allowing these highly evolved pegmatites to be distinguished from other pegmatites. Rare-earth element contents, chondrite-normalized REE patterns, and trace-element spider diagrams display trends that are generally different for LCT and NYF pegmatites, with the former most commonly having garnet with lower REE contents than those in the latter. Moreover, garnet with low REE contents (<1,000x chondrite) is typical of highly evolved rare-metal LCT pegmatites with Li or Ta that have garnet with high Mn contents (> 30.00 wt.% MnO). These pegmatites are exemplified by the LCT pegmatites of the Borborema Pegmatite Province, Brazil, that are important hosts of economic Li and Ta mineralization and contain gem-quality minerals such as Paraiba tourmaline. Intermediate relative degree of evolution in LCT pegmatites is represented by LCT pegmatites from California that have higher REE contents (~ 1,000x chondrite), are important hosts of gem-quality minerals, and may contain minor amounts of Li. Garnet from NYF pegmatites characteristically have high REE contents (> 1,000x chondrite). A combination of REE patterns, major-element contents, and the new ternary diagram (Fe/(Fe + Mn), Fe/(Fe + Ca), and Fe/(Fe + Mg)) help classify unknown pegmatites into families and to determine the relative degree of evolution of a pegmatite within individual families. Multivariate statistical techniques were applied to trace-element compositions of garnet from 30 pegmatites of the LCT, NYF, and mixed families to determine key elements that distinguish pegmatite families. Principal component analysis (PCA) was followed by linear discriminant analysis to develop a training set. Prior to the statistical analysis, for variables that had <10% of data below the detection limit three methods of imputation (replacement, KNN, and PMM) were used to create 12 complete datasets for comparison. Following this, the centered log-ratio transformation was applied to open the data for PCA. The additive log-ratio transformation was then used for discriminant analysis with Na being chosen as the denominator based on the results of PCA. The results of PCA suggest that garnet from LCT pegmatites are associated with Li, P, Zn, Ga, Ge, Ti, Hf, Zr, and Ta, whereas garnet from NYF pegmatites have an affinity with the rare-earth elements. Discriminant analysis with 19 elements resulted in classification rates>90% for all datasets. Reduction to the best 4 elements only slightly decreased the classification rates to>88%, except for the KNN-imputed dataset in which the rate decreased to 76%. A comparison of the results of PCA in a plot of PC1 vs. PC2 with chondrite-normalized REE diagrams reveals 6 distinct garnet groups. This suggests that a pegmatite can be classified as having affinity to a family using REE diagrams and supported with PCA, as groups I, II, and II are primarily LCT pegmatites whereas group V consists of only NYF pegmatites. Rare-metal mineralized pegmatites have garnet which fall into groups I, III, and IV. Therefore, if a detrital garnet was found to be pegmatitic in origin, a REE pattern indicative of these groups might indicate the presence of a pegmatite with potential for economic rare metals.
Mitchell, Nicholas Joseph. (December 2017). THE COMPOSITION OF GARNET IN GRANITIC PEGMATITES AS A PROXY FOR MELT EVOLUTION AND RARE METAL POTENTIAL (Master's Thesis, East Carolina University). Retrieved from the Scholarship. (http://hdl.handle.net/10342/6541.)
Mitchell, Nicholas Joseph. THE COMPOSITION OF GARNET IN GRANITIC PEGMATITES AS A PROXY FOR MELT EVOLUTION AND RARE METAL POTENTIAL. Master's Thesis. East Carolina University, December 2017. The Scholarship. http://hdl.handle.net/10342/6541. September 29, 2020.
Mitchell, Nicholas Joseph, “THE COMPOSITION OF GARNET IN GRANITIC PEGMATITES AS A PROXY FOR MELT EVOLUTION AND RARE METAL POTENTIAL” (Master's Thesis., East Carolina University, December 2017).
Mitchell, Nicholas Joseph. THE COMPOSITION OF GARNET IN GRANITIC PEGMATITES AS A PROXY FOR MELT EVOLUTION AND RARE METAL POTENTIAL [Master's Thesis]. Greenville, NC: East Carolina University; December 2017.
East Carolina University