Occurrences of thorium, yttrium and rare earth elements in Namibia are known to be associated with carbonatites and granites and pegmatites of Namibian and Mesozoic age. Placer deposits of these minerals have formed mainly in Tertiary to Recent times in the marine environment.
Thorium,
yttrium and rare earth element mineralization associated with
carbonatites
The
Lofdal-Bergville carbonatite dykes and plugs.
The
Lofdal-Bergville carbonatites are
located 30 km west of Khorixas in Damaraland on the farms Lofdal
491 and Bergville 490, where syenite, porphyry, tinguaite,
lamprophyre, fenite and carbonatite dykes and plugs have intruded
Huab Complex gneiss.
The
carbonatites are highly radiogenic and contain xenotime, bastnaesite
and thorite. Other alkaline intrusives occur south of Lofdal
491 on the farm Oas 486. The Oas
syenite intruded rocks of the Nosib Group.
Carbonatites
on Lofdal-Bergville were investigated in 1982, and thorium and
yttrium values from 0.17% to 14.4%
ThO2 and
0.05% to 0.63% yttriumwere obtained. In addition to the radiogenic
minerals and calcite, the carbonatites contain limonite, hematite,
magnetite, zircon, fluorite and apatite. A sample of a carbonatite
dyke assayed as follows: lanthanum 1.5%, cerium 0.87% and neodymium
0.74%.
Namibia
Rare Earths(NRE)
filed a maiden resource for its Lofdal rare earths elements project,
confirming the presence of high levels of heavy rare-earth
enrichment (HREE) in certain areas of the project. The National
Instrument 43-101-compliant resource estimate, covering Area 4
of the project located in the north-west of Namibia, pointed to
“exceptional” levels of HREE of between 75% and 93%
HREE, depending on the cut-off grade, with corresponding total
rare earth oxide grades (TREO) ranging from
0.27% to 1.26%.
Mining consultancy The MSA Group of South Africa prepared the estimate had identified the presence of an indicated resource, at a 0.3% TREO cut-off, of 900 000 t at 0.62% TREO, with 86% HREE, and an inferred resource of 750 000 t grading 0.56% TREO, with 85% HREE. The resource was drilled to a depth of 150 m and remained open at depth and along strike. At a low-grade cut-off of 0.1% TREO, the resource estimate provided for 2.88-million tons grading 0.32% TREO, with 76% HREE in the indicated category, and 3.28-million tons grading 0.27% TREO, with 75% HREE in the inferred category.
The company said it was still studying the most appropriate cut-off grade. NRE said a metallurgical study programme was also currently underway with Commodas Ultrasort, in Germany, and South African metallurgical specialist Mintek, to demonstrate the viability of extracting the rare earths from Area 4.
0.27% to 1.26%.
Mining consultancy The MSA Group of South Africa prepared the estimate had identified the presence of an indicated resource, at a 0.3% TREO cut-off, of 900 000 t at 0.62% TREO, with 86% HREE, and an inferred resource of 750 000 t grading 0.56% TREO, with 85% HREE. The resource was drilled to a depth of 150 m and remained open at depth and along strike. At a low-grade cut-off of 0.1% TREO, the resource estimate provided for 2.88-million tons grading 0.32% TREO, with 76% HREE in the indicated category, and 3.28-million tons grading 0.27% TREO, with 75% HREE in the inferred category.
The company said it was still studying the most appropriate cut-off grade. NRE said a metallurgical study programme was also currently underway with Commodas Ultrasort, in Germany, and South African metallurgical specialist Mintek, to demonstrate the viability of extracting the rare earths from Area 4.
The
Eureka carbonatite dykes
Monazite-bearing carbonatite
dykes on the farm Eureka 99, located
approximately 38 km west of Usakos, and about 2 km north of the
Usakos-Swakopmund road, contain rare earth mineralization.
With
the exception of a single occurrence of a sovite dyke, all the
carbonatite dykes at Eureka are beforsitic in
composition, but have varying concentrations of monazite.
The
monazite-rich beforsites are
characterised by rare earth oxide concentrations that range
between 33% and 40%.
Drilling
established proven reserves of 30 000 t of ore to a depth of 20 m,
containing 1 900 t of rare earth elements. E-Tech Metals is exploring
the carbonatiete dykes.
The
Kalkfeld Alkaline Complex
Thorium,
yttrium and rare earth element mineralization is known to be
associated with four carbonatite complexes in the Otjiwarongo and
Grootfontein Districts. These complexes belong to a
northeast-trending line of over 20 intra-plate-type, subvolcanic,
ring complexes of the Damaraland Alkaline Province.
Within
these complexes, thorium, yttrium and rare earth element
mineralization is associated with late-stage plugs and dykes of
mainly beforsitic composition, or with iron-rich, late-stage
metasomatites. Three of the carbonatite complexes also carry
disseminated pyrochlore.
The
Kalkfeld complex is situated on the farm Eisenberg 78, about 11 km
northwest of Kalkfeld, and measures about 5 km in diameter. The
complex consists of confocal rings of granite, syenite, foyaite and
carbonatite.
A
plug of massive iron ore occupies the central area of the complex.
The carbonatites and the iron ore show an enrichment in lanthanum(500
to 5 000 g/t), cerium (2 000 to 8
000 g/t) and neodymium (1 000 to
2 500 g/t).
The
Ondumakorume Complex
The
Ondumakorume Complex forms a prominent hill on the farmEtaneno
44, about 10 km northeast of Kalkfeld.
In addition to syenite, nepheline syenite, volcanic breccia and iron
ore, micaceous sovite, grey sovite and beforsite are the main types
of carbonatite. Rare earth minerals such as monazite, ancylite,
cerianite and carbocerianite occur within beforsite with whole rock
concentrations of up to 9 000 g/tcerium.
The
Osongombe Complex
Osongombe
is the smallest of the Damaraland carbonatite complexes and is
situated about 12 km southwest of Kalkfeld on the farms Osongombe 80,
Sud Osongombe 83 and Okarume 82.
The
diatreme consists mainly of volcanic breccia and beforsite together
with iron ore. The beforsite occupies the central area of the complex
and is composed of manganiferous ankerite, apatite-rich aggregates
magnetite-siderite, quartz and accessory, finely disseminated,
yellowish octahedra of pyrochlore.
The
Okorusu Alkaline Complex
The
Okorusu Alkaline Complex is situated 45 km north-northeast of
Otjiwarongo. The complex is composed of a series of alkaline rocks
including hortonolite monzonite, various syenites, foyaite, urtite,
tinguaite, nephelinite and carbonatite.
The
southern portion of the complex is characterised by the presence of
various metasomatites including aegirine fenite, limonitic iron ore
and numerous ore bodies of fluorite that were formed by replacement
of dolomitic marbles.
The
carbonatites at Okorusu occur as pluglike bodies and dykes that have
variously intruded aegirine fenite, rocks of the metasomatic aureole
and the central area of the complex.
On
the farm Brandenberg 87 a number
of beforsitic carbonatite dykes and carbonate fluorite- bearing
metasomatites carry appreciable amounts of rare earth element
mineralization of mainly pale yellowish-brown synchisite, green
monazite and yttrio-fluorite. Some of
the dykes are up to 20 m wide and can be followed for up to 300 m
along strike.
Synchisite
occurs as fibrous needle-shaped or as plate-like crystals and is
mainly associated with
carbonate-quartz-fluorite-barite-thorite-monazite and
xenotime-bearing assemblages.
The
total rare earth oxide content in the siliceous rocks varies from
1.5% to 7%. The thorium and yttrium values of these rocks range from
0.4% to 3.5% thorium and from 0.2% to 1.01% yttrium.
The
Agate Mountain Carbonatite Complex
About
8 km northeast of False Cape Fria, on the north-western coast of
Namibia, a carbonatite complex has intruded Karoo Sequence volcanics.
Bastnaesite is associated with late-stage beforsitic (Mg-rich)
carbonatite and occurs in irregularly distributed patches.
Thorium,
yttrium and rare earth element occurrences associated with granites
and pegmatites
Some
of the post-tectonic biotite granites of late Pan African age and, in
particular, Pan African pegmatites, are known to contain accessory
monazite, gadolinite, allanite, thorianite and yttrio-fluorite.
Wlotzkasbaken
allanite occurrence
Phenocrysts
of allanite, up to 60 mm in length, occur in a coarse-grained Pan
African granite northeast of Wlotzkasbaken, 30 km north of
Swakopmund. The allanite crystals are extremely well disseminated and
the granite has not been investigated for other rare eartk minerals
or sampled for total rare earth content.
The
Brandberg Alkaline Complex
Zoned
allanite occurs in potassium metasomatised biotite
granite of the Brandberg Alkaline Complex, whereas chevkinite,
monazite and fluorite are commonly associated with potash-altered,
biotite granite.
Peralkaline
granites and sodium-rich fenites that are associated with the Amis
complex, located on the southwestern periphery of the Brandberg
complex, contain anomalous whole rock yttrium (up to 2 000 g/t) and
thorium concentrations (up to 700 g/t).
Fenitised
peralkaline granites and agpaitic pegmatites of the Amis complex
contain Y-fluorite, monazite, xenotime, bastnaesite and
fergusonite. The Amis Complex represents a late intrusive
phase associated with the Brandberg anorogenic granite intrusion.
It
consists of peralkaline, arfvedsonite-bearing granitic and pegmatitic
dikes and sills and is characterized by locally extreme enrichments
in REE and rare metals with high charge-ionic radius ratios, such as
Zr and Nb.
The
highest concentrations (e.g., 1.7 wt % Zr, 0.3 wt % Nb, 0.5 wt %
total REE) are found in aegirine-albite aplites that formed around
arfvedsonite pegmatite cores.
.jpg)
Xenotime
(high interference colors) crystals in a fine-grained matrix made of
quartz and feldspar (I order gray). Brandberg Complex (Namibia). XPL
image, 10x (Field of view = 2mm)
Thorium and rare earth element occurrences associated with placer deposits
Thorium and rare earth element occurrences associated with placer deposits
Toscanini
monazite occurrence
A
monazite-bearing marine placer deposit was found
in the Skeleton Coast Park near Toscanini. The monazite-bearing sands
occur along a coastal strip some 22 km long, stretching from about 31
km south of Torra Bay to 20o 40’ south.
The
extent of the deposit is outlined by a prominent radiogenic anomaly.
The bedrock consists of quartz latites of the Etendeka Formation and
sediments of the Toscanini Formation. The source of the monazite is
unknown, but it can be speculated that it is possibly derived from
Pan African granites that occur south of the deposit.
The
monazite was presumably transported in a northerly direction by the
Benguela current and subsequently concentrated and deposited by
marine processes.
