Thorium is a very common element in nature. It occurs in Earth’s crust, statistically, with three to four times the abundance of uranium, and approximately as abundantly as lead. Thorium is generally very well distributed, occurring at 10 parts per million roughly homogeneously throughout the crust. However, it also occurs in high concentrations in various mineral settings around the globe.
The chief mineral hosts for thorium are monazite, carbonatite, bastnaesite, thorite, thorianite, apatite and uranothorite. The three main geological systems are the following:
Thorite, uranothorite and associated material most often appear as vein mineralization or other types of intrusive complexes. These types of materials are typically associated with medium-grade thorium, are nearly always underground, and are considered difficult to process metallurgically. In these systems, thorium can often be found with uranium.
Carbonatite and bastnaesite are large underground volcanic systems associated with apatite, phosphate, and allanite material, and often contain niobium, tantalum, the lanthanide group of metals (also known as rare earth elements) and other base metals. As with the vein systems, these materials are also underground, requiring large underground mining operations, and may be difficult to process. Carbonatites are randomly distributed throughout the globe. There are significant carbonatitic systems reported in southern Africa, Brazil, Norway, Greenland, USA and elsewhere.
Monazite is a lanthanide phosphate found in granites, syenites, pegmatites, quartz-pebble conglomerate sand stones, fluviatile and beach placers. In essence, the concentrations are accumulated at the shores or current or ancient oceans. These types of deposits are nearly always at surface, and are associated with titanium oxide materials, such as ilmenite, rutile and zircon, as they are in India, Australia, southern Africa and eastern USA. They can also be associated with tin, as they are in southeast Asia.
There is considerable disagreement on what precisely are the world’s economic resources of thorium. For example, many geological information agencies have determined proprietarily that so many resources of thorium exist and can be mined for a cost of $80 per kilogram. A very similar exercise is undertaken by the same agencies with respect to the world’s uranium resources; it is clear that they have employed the same methodologies for thorium. Within this metric, however, there is no consensus. Nor is there agreement on which countries possess which resources, with the notable exception of India, who is unanimously considered to have the world’s largest resource of thorium. The primary agencies providing these estimates are: the United States Geological Survey (USGS), and the United Nations’ International Atomic Energy Agency (IAEA). The USGS claims the global “recoverable reserves” of thorium are: 1.2 million metric tonnes. However, they only extrapolate these reserves by estimating thorium grades which may be resident in measured rare earth element ores. The IAEA claims the global “Reasonably Assured Reserves” of thorium are 2.2 million metric tonnes.Table 3.2. below demonstrates the discrepancies among the two agencies.
The reason for these discrepancies and other gaps in the thorium knowledge base is due to the perhaps obvious fact that there has been no interest in the mining of thorium, save for the brief period of insignificant interest that took place at the beginning of the atomic age. The geological pursuit of thorium has largely been an academic exercise, with few exceptions. Thorium’s most significant by-product, rare earth elements, have also only had brief periods of interest, and hence also betray a lack of knowledge.
Within monazite, it is noteworthy that regional average thorium grades have been established by the IAEA, as have the grades of other monazite by-products. The IAEA published the following table (reproduced in Table 3.3. below), which demonstrates the various grades believed to exist in these ores (highlighted in blue text are the elements which would be of the greatest interest to mining companies, and hence, to Thorium One):
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