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Nature and origin of Lofdal Carbonatite associated REE Deposit, Khorixas, Namibia MSc. Proposal By Sarah Bodeving Supervisor: Prof. A.E. Williams-Jones Committee: Prof. Jeanne Paquette, Prof. Don Francis 28th February 2013 MSc. Proposal – Bodeving Sarah 28th February 2013 1. Introduction Since China, the greatest rare earth elements (REE) producer, has decided to stockpile its resources therefore creating artificial shortage in the world, intensive exploration for REE, comprising La-Lu+Y, Sc (Lottermoser, 1992), has increased. Lofdal intrusive complex, located in northwestern Namibia, currently explored by Namibia Rare Earths Inc. (NRE) shows striking high concentrations in valuable heavy REE (HREE). The average of total rare earth elements is 2.34 % (Swinden, 2012) of which 74,9% are HREE+Y (Swinden, 2012). In comparison, the two largest carbonatite deposits associated with REE are Bayan Obo in Inner Mongolia, with an average value of 5.41 wt. % REO (Castor, 2001) and Mountain Pass in the US with an average value of 8.9% REO (Castor, 2008). Both are highly enriched in LREE vs. HREE. The US Department of Energy predicts that Dy, Tb, Eu, Nd and Y, needed for new green technology development, are facing a critical risk of supply shortage on a short and medium term basis. In this respect, Lofdal is a unique deposit of special interest due to its unusual REE fractionation, not showing the characteristic LREE dominance of carbonatite hosted REE mineralisation. In addition, HREE enrichment in Lofdal carbonatites seems to be associated with late stage hydrothermal processes (Swinden, 2012), in contrast to other carbonatite deposits e.g. Mountain Pass, which is of magmatic origin. (Walters, 2011) At Lofdal, two specific areas, area 2 and area 4 have been declared first exploration targets by NRE, on account of their extreme enrichment in HREE. Previous work on area 4 investigates the geological controls of its HREE enriched hydrothermal system by Loye (2012, unpublished), focusing his research on xenotime. Wall (2008) conducted the first U-Pb dating of xenotime overgrowing on zircon in carbonatite dikes at Lofdal. Kruger (2012, unpublished) studied the LREE enrichment of the Emanya Intrusion, while O’Connor (2011, unpublished) analysed the REE enrichment of the nepheline syenites and phonolite dikes. Preliminary analyses conducted by myself on the petrography of the Main Intrusion revealed the occurrence of pyrochlore, fluorapatite, zircon, k-feldspar, calcite and Nb-rutile, of which zircon has been further investigated on its elemental composition on the electron microprobe. In addition, xenotime, a HREE-bearing mineral phase, occurring in 4 samples from area 4, dolomite hill and area 5A has been quantitatively analysed on the electron microprobe. The focus of this MSc. Thesis is to investigate the origin of LREE/HREE enrichment by establishing a relation between basement structures, alteration and REE mineralisation in different areas e.g. carbonatite dikes and carbonatite plugs to determine their association with early magmatic or late stage hydrothermal fluids. By comparing Lofdal to other carbonatite deposits around the world, we aim to figure out which processes led to this extreme enrichment in HREE in contrast to LREE and provide new knowledge to the on-going exploration at Lofdal. 2 MSc. Proposal – Bodeving Sarah 28th February 2013 2. Objectives 1. Determination of magmatic or hydrothermal origin of REE hosting minerals and fluids in HREE enriched carbonatite dikes and mainly LREE enriched carbonatite plugs e.g. Main Intrusion and Emanya Intrusion • Mineralogical and petrographical characterization of carbonatite dikes and plugs at Lofdal intrusive complex. • Determination of elemental composition in REE bearing mineral phases, e.g. zircon, xenotime, garnet, fluorapatite. • Establish relation between structural features, alteration (metasomatism) and REE mineralisation in alkali syenites and carbonatite dikes and plugs. 2. Comparison of mineralogy, alteration, structural controls and source of fluids between distinct HREE enriched Lofdal areas and other known carbonatite associated HREE deposits around the world. 3. Literature Review Rare earth element mineralisation can occur in sedimentary, igneous or metamorphic rocks, facilitated by three different processes or a combination of them: 1. Supergene enrichment of REE, resulting from chemical weathering or sedimentary processes (Walters, 2011), e.g. Mt. Weld laterite REE deposits in Australia (Castor, 2011) 2. Hydrothermal REE remobilisation (Thompson, 2011) seen at Gallinas Mountains in New Mexico (Williams-Jones, 2000) 3. Primary crystallization, as exhibited at Mountain Pass Carbonatite, US or Lovozero in Russia. (Walters, 2011). Carbonatites, as Lofdal Carbonatite Complex, constitute one of the most common hosts of REE and contain some of the highest REE concentrations in the Earth’s crust. Concentrations have typical values ranging from several hundreds ppm up to over 10wt% (Wall, 1997). Carbonatites are mostly intrusive igneous rocks, containing more than 50 modal% carbonate minerals. They are commonly associated with alkaline silicate rocks and occur as late stage intrusions, inside or as satellite to these intrusive complexes. They are frequently developed as small intrusions, dikes, sills or plugs in stable cratonic regions or often related to rift settings. (Bell, 1989) Three main processes responsible for carbonatite genesis have been proposed by Winter (2009): 1. Direct partial melting of a hydrous-carbonated lherzolite 2. Liquid 3 MSc. Proposal – Bodeving Sarah 28th February 2013 immiscibility between silica saturated and silica unsaturated magmas 3. Fractional crystallization. Trace element and isotopic studies favour a mantle derived, igneous origin as opposed to remobilisation or assimilation of limestone, despite uncertainty of the exact mantle source. A distinct feature of intrusive carbonatites is the presence of metasomatic aureoles enveloping the carbonatites and sometimes also the alkaline silicate rocks. They are characterized by the conversion of wall rock quartz and feldspar to aegerine and alkali rich amphiboles. This process of fenitization, resulting from alkali rich fluids moving along a network of fractures, leads to enrichment in alkalis, progressive desilicification and visible replacement textures. (Winter, 2009) Carbonatite geochemistry typically contains low amounts of SiO2 and unusually high amounts of Ba, Sr, LREE and high-field strength elements (Ti, Zr, Hf, Nb, Ta) (Chakhmouradian, 2009). LREE are prevalently hosted in the most frequent REE minerals, e.g. bästnasite REE(CO3)F, monazite REEPO4, allanite (REE, Ca, Y)2(Al, Fe3+)3(SiO4)3(OH) and apatite Ca5(PO4)3(F, Cl, OH) (Thompson, 2011). HREE are often incorporated in xenotime REEPO4, which is a highly uncommon accessory phase in carbonatites, however occurring frequently at Lofdal, contributing to its unique nature. (Wall, 2008) 4. Geology of Lofdal District Lofdal Intrusive Complex is located in northwestern Namibia in the Welwitschia inlier on the southern edge of the Archean Congo Craton. Volcanic and sedimentary rocks of the neoproterozoic Damaran Orogeny, representing a Pan African Belt between Congo and Kalahari Craton, border the Welwitschia inlier. The basement rocks of the Welwitschia Inlier consist of the paeloproterozoic polydeformed and locally migmatised Huab Metamorphic Complex (HMC) (Swinden & Siegfried, 2011). Post tectonically intruded by the Fransfontein Granite, which was imprecisely dated at 1871±30 and 1730±30, HMC is believed to have an age of around 2Ga, as no direct dating has been conducted so far (Swinden & Siegfried, 2011). 4 MSc. Proposal – Bodeving Sarah 28th February 2013 Figure 1: Geology of Lofdal District, NW Namibia -‐ NRE 2012 A series of alkali silicate rocks and carbonatites intruded this metamorphic unit around 750 Ma ago (O’Connor, 2011). The emplacement of the Oas Quartz Syenite, Lofdal Carbonatite Complex and Lofdal Nepheline Syenite is more or less coeval with the Naawpoort Formation volcanism in the Damaran Orogen. Coarse diatreme breccias are widespread and are apparently related to the intrusion of the syenite. Carbonatite dikes are closely linked with phonolite dikes and follow an E-W trend of folds. They cross cut all other dikes in the Lofdal area and represent the youngest intrusive event. These carbonatite dikes show a high concentration in HREE, most likely resulting from late stage hydrothermal alteration (Swinden, 2012). The two largest carbonatite plugs are referred to as “Main Intrusion” and “Emanya Intrusion” (Swinden & Siegfried, 2011). Xenotime dated by U-Pb method carried out by Wall et. al (2008) revealed an age of 765 ±16, which is more or less equivalent with the OAS Syenite (Swinden & Siegfried, 2011). Figure 2: Location of Lofdal a reas, colored grid showing thorium channel from 2010 airborne survey – NRE 2012 5 MSc. Proposal – Bodeving Sarah 28th February 2013 5. Methodology a. Fieldwork Fieldwork at Lofdal in Namibia is planned for May or June 2013. It will involve extensive sampling of outcrops in the different units, surface mapping, core sampling and macroscopic descriptions of the occurring rock types. b. Analytical Methods In order to meet the objectives above, the rocks will be described macroscopically and analysed petrographically using polarized light microscopy to determine the modal proportions and textural relationships of the different mineral phases. Cathodoluminescence microscopy will help to identify REE bearing mineral phases, reveal different types of carbonate phases, show the intensity of metasomatic processes in albite, reveal zonation and help to identify different generations of minerals. The scanning electron microprobe will be used to semi-quantitatively analyse mineral phases with electron dispersive spectroscopy (EDS) and provide backscattered images to describe textural relationships. The concentrations of major elements in various minerals will be determined by conducting quantitative analyses on the electron microprobe, providing element compositional data that will be plotted in e.g. REE fractionation trend and element ratio diagrams. In addition, mapping of element concentrations in minerals will help to identify pristine from hydrothermally altered mineral phases. Comparing whole-rock lithogeochemical analyses may assist in identifying hydrothermal processes and different types of alteration. 6. Anticipated Significance to Research This project will provide information on the origin of the LREE and HREE enrichment at Lofdal, by determining if the source of the fluid is magmatic and if the interaction of the fluid and carbonatite occurred at near magmatic or much lower hydrothermal temperatures. This new knowledge may help to provide better understanding of the formation model of carbonatite associated REE deposits and contribute to the on-going exploration of Lofdal Complex. 6 MSc. Proposal – Bodeving Sarah 28th February 2013 7. Timeline Course Work, Literature Review Analyses: EMPA, SEM, CL, XRD Petrography New Sample Analyses: EMPA, SEM, CL MSc. Thesis Field Work Conferences 2012 2013 December January-April December January-April 2014 April-November June-November December January – April Mai-June SEG GAC-MAC SEG Goldschmidt 8. References Bell, K., 1989, CARBONATITES Genesis and Evolution – Unwin Hyman, London Castor, S. B., 2008, The Mountain Pass rare-earth carbonatite and associated ultrapotassic rocks, California: The Canadian Mineralogist, vol. 46, p.779-806. Castor, S. B. & Hedrick, J.B., 2011, Rare Earth Elements – Industrial Minerals and Rocks, p.769-792 Chakhmouradian, A., 2009, The geochemistry of carbonatites revisited: Two major types of continental carbonatites and their trace-element signatures – Geophysical Research Abstracts, vol.11, EGU2009-10806 Gray, D.R., Foster, D. A., Meert, J. G., Goscombe, B. D., Armstrong, R., Trouw, R. A. J. and Passchier, C.W., 2008, A Damara Orogen perspective on the assembly of southwestern Gondwana – Geological Society, London, Special Publications, v. 294, p.257-278 7 MSc. Proposal – Bodeving Sarah 28th February 2013 Kruger, T., 2012, A petrological study of the LREE-enriched Emanya carbonatite plug, Lofdal, Namibia – Honours Research Project, Stellenbosch University (SUN) Lottermoser, B. G., 1992, Rare earth elements and hydrothermal ore formation processes: Ore Geology Reviews, vol.7, p.25-41 Loye, E., 2012, The geological controls on the heavy rare earth element enriched hydrothermal system of area 4, Lofdal, Khorixas, Namibia– MSc. Thesis, University of Exeter, UK O’Connor, D.E., 2011, Petrogenesis of nepheline syenites and phonolites fom the Lofdal Intrusive Complex, Kunene Region, Namibia– Bachelor Thesis, Dalhousie University, Halifax, Nova Scotia Swinden, H.S. & Siegfried, P., 2001, Amended 43-101 Technical Report on the RareEarth Element Occurrences in the Lofdal Carbonatite Complex, Kunene Region, Khorixas District, Namibia – Prepared for Namibia Rare Earths Inc. Swinden, H.S. & Burton, D., 2012, Lithogeochemistry of the Lofdal carbonatite complex, north-central Namibia: unusual late stage hydrothermal HREE enrichment – GAC2012_1 Power Point Presentation, Namibia Rare Inc. Thompson, W., Lombard, A., Santiago, E., Singh, A., 2011, Mineralogical studies in assisting beneficiation of rare earth element minerals from carbonatite deposits – 10th International Congress for Applied Mineralogy, Trondheim 2011 US Department of Energy Critical Materials Strategy, 2010, Technology Metals Research LLC 2011 Verplanck, P. & Van Gosen, B., 2011, Carbonatite and Alkaline Intrusion-Related Rare Earth Element Deposits-A Deposit Model – Open File Report 2011-125, USGS Wall, F., Niku-Paavola, V.N., Storey, C., Müller, A., Jeffries, T., 2008, Xenotime-(Y) from carbonatite dykes at Lofdal, Nambia: Unusually low LREE:HREE ratio in carbonatite, and the first dating of xenotime overgrowths on zircon- The Canadian Mineralogist, Vol. 46, p. 861-877 Wall, F., Zaitsev, A., Jones, A.P., Mariano, A.N., 1997, Rare-earth rich carbonatites: A review and latest results – MAEGS-10, Section 3 8 MSc. Proposal – Bodeving Sarah 28th February 2013 Walters, A. & Lusty, P., 2011, Rare Earth Elements: British Geological Survey, Nov. 2011 www.MineralsUK.com Williams-Jones, A.E., 2000, The Genesis of Hydrothermal Fluorite-REE Deposits in the Gallinas Mountains, New Mexico – Economic Geology, vol.95, p.327-342 Winter, D.W., 2009, Principles of Igneous and Metamorphic Petrology – Prentice Hall, 2nd Edition 9