**4. Ore deposits**

Thorianite, thorite, and uranothorite are the only true thorium minerals, but they are not recently recovered. Some resources of these minerals are coupled with quartz-pebble conglomerates in Canada (Elliot Lake region) and South Africa (Witwatersrand). World thorium resources in terms of the genetic types of ore deposits are displayed in **Table 1** [9].


**Table 1.** World thorium resources in terms of the deposit types [9].

The main recently available resources of thorium are coupled with monazite-enriched placer deposits in alluvial or marine sediments occurring mainly in Australia, India, Brazil, Venezuela, the USA, and Egypt. These deposits contain variable proportions of monazite, ilmenite, rutile, xenotime, zircon, and/or cassiterite. The main minerals, which are mined on these placer deposits, are ilmenite, rutile, zircon, and cassiterite. Associated minerals, which are rarely of economic significance, can include garnet and kyanite. Monazite, when also extracted, represents only an accidental product.

beach sands containing 10 ppm Th, and limestone averages about 2 ppm. Shale contains 10–15 ppm Th, small amounts of thorium may adsorb clay particles during weathering. Twelve isotopes of thorium are known, with atomic masses from 223 to 234. However, natural thorium is present as nearly 100% 232Th isotope. The other important natural isotope of thorium 230Th

In general, thorium occurs in relative small number of Th-enriched minerals: thorite (ThSiO<sup>4</sup>

monazite and bastnaesite. Monazite is a primary source of light REE. Monazite concentrates, which are mined from beach sands in India, Brazil, the USA, Malaysia, Korea, and Sri Lanka,

Other, especially, potentially based resources of thorium are coupled with carbonate-

Kola peninsula (Russia) contain rare REE-enriched mineral loparite (Na,REE,Ca) (Ti, Nb) O2

apatite ore deposit on the Kola Peninsula in the Russia. Apatite containing higher concentrations of thorium occurs also in the alkalic magmatic rocks on the Vishnevyye Mountains of the Urals range in the Russia [1]. A large variety of other minerals contain minor amounts of

Thorianite, thorite, and uranothorite are the only true thorium minerals, but they are not recently recovered. Some resources of these minerals are coupled with quartz-pebble conglomerates in Canada (Elliot Lake region) and South Africa (Witwatersrand). World thorium

resources in terms of the genetic types of ore deposits are displayed in **Table 1** [9].

**Deposit type Metric tons of thorium**

Placers 2,182,000 Carbonatites 1,783,000 Vein-type 1,528,000 Alkaline igneous rocks 584,000 Others 135,000 Total 6,212,000

**Table 1.** World thorium resources in terms of the deposit types [9].

enriched magmatic rocks (carbonatites), containing bastnaesite (up to 2.8 wt.% ThO<sup>2</sup>

], bastnaesite [(Ce, La)CO3

and 40.7–65.0 wt.% REO (rare earths oxides). However, monazites

), and synchysite [CaREE(CO3

[8]. Some higher concentrations of thorium have also important

.

1−x(OH)4−x]. However, the main world resources of thorium are coupled with

). Highly rare alkali-rich nephelinite syenites from the Lovozero pluton on the

),

), parisite

(F, OH)] (up to

F], and thorogum-

)2

is generally presented in uranium minerals.

204 Descriptive Inorganic Chemistry Researches of Metal Compounds

), monazite [(Ce, La, Nd,Th,U)PO<sup>4</sup>

from some granitic rocks could contain up to 27 wt.% ThO2

thorium (e.g., allanite, xenotime, zircon, and uraninite).

] (up to 4.0 wt.% ThO<sup>2</sup>

thorianite (ThO2

)

contain 3.1–14.32 wt.% ThO<sup>2</sup>

(CO3 )3 (F,OH)2

**4. Ore deposits**

with up to 1.6 wt.% ThO2

mite [Th(SiO<sup>4</sup>

[CaREE2

5.0 wt.% ThO2

Placer deposits are found where water waves have concentrated heavy mineral grains on a sea beaches. These deposits may occur in both modern and ancient shorelines. Many of the heavy mineral sand deposits are concentrated by wave action in both parallel and transgressive dunes. Monazite placers are reported from Australia, Egypt, India, Liberia, Brazil, Burma, Malaysia, Sri Lanka, and the USA.

The placer deposits in Australia are mined for their ilmenite, rutile, and zircon content. The monazite content in heavy mineral concentrates varied from 0.2 to 1.5 wt.%. Present-day shoreline deposits are evolved on the east coast in the SE Queensland. The most important placer sand deposits are coupled with the Tertiary fossil shoreline deposits in the Murray Basin, in the southwest Australia. Monazite grades in this deposits are around 1–1.5 wt.%.

Although monazite occurs associated with ilmenite and other heavy minerals in beach sands, skirting the entire Peninsular India, its economic concentration is confined to only some areas with suitable physiographic conditions. The west coast placers are essentially beach or barrier deposits with development of dunes evolved on dry months. On the other hand, the east coasts deposits consist of extensive dunes fringing the coasts. The beach sands of Chavara bar (Kerala) on the West coast contain 73 vol.% heavy minerals, 60–70 vol.% ilmenite, 4–7 vol.% garnet, 5–8 vol.% zircon, and 0.5–1.0 vol.% monazite. The east coast beach placers and dunes are low grade with 8–20 vol.% of heavy minerals. The most important placer deposit on the East coast is the Chatrapur deposit (Orissa) with about 20 vol.% heavy minerals and 0.5 vol.% monazite. In the Malaysian deposits, monazite is associated with columbite, xenotime, and cassiterite. The cassiterite placers at Trengganu contain as much as 58 vol.% monazite. In Sri Lanka, the largest placer deposit near Pulmoddai contains 3 million tons of sand with 0.4 vol.% monazite, 18 vol.% rutile, and 62 vol.% ilmenite [10].

In Brazil, monazite occurs associated with ilmenite and zircon in placer deposits evolved along the eastern and south-eastern Atlantic coast. In Burma, placer deposits occur in the southern Shan states. Weathering of quartz veins and pegmatite dykes injected into the granites derives considerable quantities of cassiterite and wolframite occurring in the placers.

In the USA, alluvial deposits of monazite are known to occur in the intermountain valleys of Idaho, the Carolina Piedmont of North and South Carolina, and the beach deposits of north-eastern Florida to south-eastern Georgia. The three monazite placer districts, namely the North and South Carolina stream deposits, Idaho stream deposits, and Florida-Georgia beaches, are the largest volume known alluvial thorium deposits in the USA. The modern and raised Pleistocene and Pliocene beach deposits of north-eastern Florida and south-eastern Georgia host low-grade but persistent concentrations of thorium. Heavy minerals constitute a small part of the beach sands. The most abundant heavy mineral in this beach deposits is ilmenite, in many places forming more than 50 vol.% of the heavy-mineral fraction. Monazite forms a minor part of the heavy-mineral fraction, usually less than 1 vol.%. The beach placer deposits of this region contain total reserves of about 14,700 tons of ThO<sup>2</sup> , which occur in 330,000 tons of monazite. These placer deposits were mined primarily for ilmenite and rutile. Mining ceased in this area in late 1978 since increasing environmental regulations made mining operations more costly [11].

The ore bodies constituting quartz-pebble conglomerates are represented in particular by the Blind River-Elliot Lake deposits in Ontario, Canada and the Witwatersrand deposits in South Africa. These ore bodies occur mainly in pyrite-bearing oligomictic conglomerates. In the Blind River-Elliot Lake deposits, thorium together with uranium occurs mainly in a brannerite-uraninite-monazite mineral assemblage. Principal ore minerals are uraninite, brannerite, and monazite, with minor coffinite, uranothorite, xenotime, and gummite. Uraninite is partly enriched in thorium with average content of 6.5 wt.% ThO2 . The Witwatersrand reefs are not only rich in gold but also represent significant uranium deposit. The principal uranium minerals are uraninite and lesser uranothorite, brannerite, and coffinite. Uraninite from this deposit is enriched in thorium (average 3.9 wt.% Th) [12].

Carbonatites, which recently represented the most important source of REE, are also considered as potential source of thorium. Carbonatites are igneous rocks containing >50% of primary carbonate minerals. The most carbonatites are actually polygenetic and show evidence of hydrothermal and metasomatic reworking. These rocks generally contain <50 ppm Th; however, some contain higher concentrations. The majority of carbonatites occur in association with broadly coeval ultramafic and alkaline silicate rocks.

From the mid-1960s to 1985, the carbonatite-hosted Mountain Pass deposit in the USA was the world's main source of REE, producing over 20 kt REO at its zenith. Recently, almost all (~97%, or 120–130 kt REO in 2006–2010) of the world's REE supply comes from China, with 40–50% of this production contributed by the giant Fe–REE–Nb deposit at Bayan Obo [8].

Carbonatite-related deposits can be subdivided into deposits where magmatic and/or hydrothermal processes are important and those where secondary processes such as supergene enrichment and laterization predominate. The most important primary carbonatite-related deposit is the Bayan Obo deposit in Inner Mongolia, China, which represents 70% of the world's REE resources. The most abundant REE minerals at Bayan Obo are monazite and bastnaesite. Carbonatites with appreciable REE and Th mineralization have been reported also at Khibiny (Kola, Russia), Ozerny and Arshan (Siberia, Russia), Fen (Norway), Sokli (Finland), Mount Weld, Cummins Range, Mud Tank (Australia), Palabora (South Africa), Khanneshin (Afghanistan), Amba Dogar (India), Barra do Itapirapuã (Brazil), Tundulu and Kangankunde (Malawi), and Wigu Hill (Tanzania) among many others [13].

Highly potential sources of REE and Th represent intrusions of alkaline and peralkaline igneous rocks. Some well-known examples of these rock series being the Ilímaussaq intrusion in southern Greenland, Lovozero and Khibiny alkaline plutons on the Kola Peninsula (Russia), the Red Wine-Letitia alkaline province in Canada, alkaline laccolith at Poços de Caldas (Brazil), and alkaline syenite body at Pilanesberg (South Africa) [14, 15]. All these magmatic complexes are zoned or layered, enriched with Na and K, and contain a variety of relatively rare minerals including thorite, monazite, loparite, zircon, and apatite. Thorium is present in thorite, monazite, zircon, loparite, and some other accessory minerals of REE. Local thorium contents in these rock complexes may range up to 1500 ppm Th, but overall they rarely contain in excess of 50 ppm Th [16].

a small part of the beach sands. The most abundant heavy mineral in this beach deposits is ilmenite, in many places forming more than 50 vol.% of the heavy-mineral fraction. Monazite forms a minor part of the heavy-mineral fraction, usually less than 1 vol.%. The beach placer

330,000 tons of monazite. These placer deposits were mined primarily for ilmenite and rutile. Mining ceased in this area in late 1978 since increasing environmental regulations made min-

The ore bodies constituting quartz-pebble conglomerates are represented in particular by the Blind River-Elliot Lake deposits in Ontario, Canada and the Witwatersrand deposits in South Africa. These ore bodies occur mainly in pyrite-bearing oligomictic conglomerates. In the Blind River-Elliot Lake deposits, thorium together with uranium occurs mainly in a brannerite-uraninite-monazite mineral assemblage. Principal ore minerals are uraninite, brannerite, and monazite, with minor coffinite, uranothorite, xenotime, and gummite. Uraninite is partly

not only rich in gold but also represent significant uranium deposit. The principal uranium minerals are uraninite and lesser uranothorite, brannerite, and coffinite. Uraninite from this

Carbonatites, which recently represented the most important source of REE, are also considered as potential source of thorium. Carbonatites are igneous rocks containing >50% of primary carbonate minerals. The most carbonatites are actually polygenetic and show evidence of hydrothermal and metasomatic reworking. These rocks generally contain <50 ppm Th; however, some contain higher concentrations. The majority of carbonatites occur in associa-

From the mid-1960s to 1985, the carbonatite-hosted Mountain Pass deposit in the USA was the world's main source of REE, producing over 20 kt REO at its zenith. Recently, almost all (~97%, or 120–130 kt REO in 2006–2010) of the world's REE supply comes from China, with 40–50% of this production contributed by the giant Fe–REE–Nb deposit at Bayan Obo [8].

Carbonatite-related deposits can be subdivided into deposits where magmatic and/or hydrothermal processes are important and those where secondary processes such as supergene enrichment and laterization predominate. The most important primary carbonatite-related deposit is the Bayan Obo deposit in Inner Mongolia, China, which represents 70% of the world's REE resources. The most abundant REE minerals at Bayan Obo are monazite and bastnaesite. Carbonatites with appreciable REE and Th mineralization have been reported also at Khibiny (Kola, Russia), Ozerny and Arshan (Siberia, Russia), Fen (Norway), Sokli (Finland), Mount Weld, Cummins Range, Mud Tank (Australia), Palabora (South Africa), Khanneshin (Afghanistan), Amba Dogar (India), Barra do Itapirapuã (Brazil), Tundulu and Kangankunde

Highly potential sources of REE and Th represent intrusions of alkaline and peralkaline igneous rocks. Some well-known examples of these rock series being the Ilímaussaq intrusion in southern Greenland, Lovozero and Khibiny alkaline plutons on the Kola Peninsula (Russia), the Red Wine-Letitia alkaline province in Canada, alkaline laccolith at Poços de Caldas

, which occur in

. The Witwatersrand reefs are

deposits of this region contain total reserves of about 14,700 tons of ThO<sup>2</sup>

enriched in thorium with average content of 6.5 wt.% ThO2

tion with broadly coeval ultramafic and alkaline silicate rocks.

(Malawi), and Wigu Hill (Tanzania) among many others [13].

deposit is enriched in thorium (average 3.9 wt.% Th) [12].

ing operations more costly [11].

206 Descriptive Inorganic Chemistry Researches of Metal Compounds

On the Ilímaussaq complex is bounded highly interested REE-Y-U-Th mneralization at Kvanefjeld (Kuannersuit) coupled on agpaitic nepheline syenites. Recently performed exploration is concentrated on potential recovery of REO, Y, together with production of U and Zn as valuable by-products. The Lovozero complex in the Kola Peninsula is represented by layered intrusion of varied varieties of nepheline syenite with loparite as main economic interested mineral. Several loparite-rich units were mined since 1951 as the major source of LREE, Nb, and Ta for Soviet industry [13].

Some other potential sources of thorium represent Th-bearing granites and pegmatites, which are known in many parts of world. The thorium-enriched pegmatites most commonly occur in near granitic or syenitic bodies or in high-grade metamorphic rock series near their contacts with granitic stocks or batholiths. The principal thorium-bearing minerals in these deposits are uraninite, thorite, brannerite, uranothorite, monazite, and some other REE-, Th-, and U-bearing accessory minerals.

Most of the Th-enriched granites and pegmatites are not of commercial importance at present but provide a large reserve of thorium and uranium for the future. Thorium-bearing granitic rocks and pegmatite-bearing rock complexes occur in the Bancroft area of Ontario (Canada), Rössing (Namibia), Crockers Well, Greenbushes, Radium Hill (Australia). The Th-bearing minerals are represented by allanite, betafite, brannerite, davidite, monazite, thorite, and some others rare REE-, Th-, and U-bearing minerals. The ore zones at the Greenbushes contain low levels of thorium with average grades in range 3–25 ppm Th [17]. The uranium deposits in the Bancroft area coupled with anatectic pegmatites produced a total of 5700 tons U between 1956 and 1982. The principal ore minerals were uraninite, with up to 10 wt.% ThO<sup>2</sup> , and uranothorite.

Uranium mineralization associated with leucogranite dykes on the Rössing deposit in Namibia was derived by the partial melting of U-rich sedimentary rocks. Uraniferous leucogranite bodies are located in high-grade metasediments of the Damara Pan-African belt. Within mineralized leucogranites, the distribution of uranium can be extremely variable. Main uranium mineral is Th-bearing uraninite with 3.3–8.0 wt.% ThO<sup>2</sup> . The Rössing uranium deposit is mined from 1976 and total production by the end of 2007 was slightly over 90,000 tones U [18].

Relatively rare thorium-bearing vein deposits are distributed throughout the world. They are localized in shear zones, faults, breccias zones in metasedimentary and metavolcanic rocks and are often associated with alkalic rocks complexes and carbonatites. The principal Th-bearing minerals in vein deposits are thorite, thorogummite, and monazite, which are associated with some REE-minerals (allanite, bastnaesite, and xenotime). Examples of these ore deposits include the Lemhi Pass in the USA, Steenkampskraal and Vanrhynsdorp in South Africa, Eskisehir deposit in Turkey, and Nolans Bore in Australia [16, 19].

The Lemhi Pass district on the Montana-Idaho border in the USA contains numerous thoriumrich veins in the central Beaverhead Mountains. This district is thought to represent the largest concentration of thorium resources in the USA. The district contains total reserves of 64,000 tons of ThO2 [20]. Most of mineralized veins are quartz-hematite-thorite veins, which fill fractures, shears, and brecciated zones in quartzitic rocks. Rare-earth- and Th-bearing allanite and monazite are locally abundant. The thorite veins of the Lemhi Pass district are approximately equally enriched in thorium and REE. The total REE-oxide contents range from 0.07 to 2.20 wt.%, with an average value of 0.43 wt.%. The average concentrations of Th are 0.43 wt.%. [11].

At Steenkampskraal (South Africa) from the 1950s to 1963 about 50,000 tons of monazite concentrates were extracted, which contained between 3.3 and 7.6 wt.% Th before operation of the mine was halted. New economic assessment of this deposit was completed in 2012 and currently were established resources of 86,900 wt.% of REO [9].

The Nolans Bore deposit in the Northern Territory (Australia) is coupled with mineralized shear zones evolved in variably deformed and altered granitic gneiss, pegmatite, and minor calcsilicate rocks. Massive fluorapatite dykes enriched in REE and Th form the mineralization. The thorium content of Nolans Bore fluorapatite generally ranges from 0.07 to 0.59 wt.% Th (average 0.23 wt.% Th) [21].
