Download Nature and origin of Lofdal Carbonatite associated REE Deposit

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(2012, unpublished), focusing his research on xenotime.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(2012, unpublished) studied the LREE enrichment of the Emanya Intrusion, while
O’Connor (2011, unpublished) analysed the REE enrichment of the nepheline syenites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
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