McLeod and Shaulis reported in 2018 that the abundance of the rare earth elements (REEs) in Earth’s crust has become the intense focus of study in recent years due to the increasing societal demand for REEs, their increasing utilization in modern-day technology, and the geopolitics associated with their global distribution. Within the context of chemically evolved igneous suites, 122 REE deposits have been identified as being associated with intrusive dike, granitic pegmatites, carbonatites, and alkaline igneous rocks, including A-type granites and undersaturated rocks. These REE resource minerals are not unlimited and with a 5–10% growth in global demand for REEs per annum, consideration of other potential REE sources and their geological and chemical associations is warranted. The Earth’s moon is a planetary object that underwent silicate metal differentiation early during its history. Following ~99% solidification of a primordial lunar magma ocean, residual liquids were enriched in potassium, REE, and phosphorus (KREEP). While this reservoir has not been directly sampled, its chemical signature has been identified in several lunar lithologies and the Procellarum KREEP Terrane (PKT) on the lunar nearside has an estimated volume of KREEP-rich lithologies at depth of 2.2 × 108 km3
This reservoir therefore offers a prospective location for future lunar REE exploration. Within the context of chemically evolved lithologies, lunar granites are rare with only 22 samples currently classified as granitic. However, these extraterrestrial granites exhibit chemical affinities to terrestrial A-type granites. On Earth, these anorogenic magmatic systems are hosts to U-Th-REE-ore deposits and while to date only U-Th regions of enrichment on the lunar surface have been identified, future exploration of the lunar surface and interior may yet reveal U-Th-REE regions associated with the distribution of these chemically distinct, evolved lithologies.
The occurrence, distribution, and potential for economic extraction of the REEs have become the intense focus of world markets in recent years. This has been due to the extensive use of these elements in modern day technology, their criticality in supporting research throughout medicine, military-based technologies, and communication, and their role in the development of clean energy technologies. These elements do not occur naturally as metals like copper (Cu) or lead (Pb), instead being complexed in a wide array of different mineral types from carbonates to silicates to phosphates. According to the United States Geological Survey (USGS) [21], there are currently 799 major REE mineral-bearing deposits identified globally, of which 122 are associated with alkaline igneous activity (not including carbonatites, of which there are 149). These deposits are often specifically associated with chemically evolved, intrusive dike, granitic pegmatite, and/or alkaline igneous complexes, and are the primary focus of this review.
The mineralogy of Earth’s crust is dominated by common rock-forming silicate minerals, e.g., quartz, feldspars, ferromagnesium minerals, amphiboles, and micas. The elemental budget of Earth’s crust is therefore characterized by the cations which are integral structural components to these phases (Figure 1). Combined, the major rock-forming minerals account for 99% of Earth’s upper crustal element budget with only 2.16% of the remaining 1% being comprised of the REEs. These elements do not occur naturally as metals like copper (Cu) or lead (Pb), instead being complexed in a wide array of different mineral types from carbonates to silicates to phosphates. According to the United States Geological Survey (USGS), there are currently 799 major REE mineral-bearing deposits identified globally, of which 122 are associated with alkaline igneous activity (not including carbonatites, of which there are 149). These deposits are often specifically associated with chemically evolved, intrusive dike, granitic pegmatite, and/or alkaline igneous complexes, and are the primary focus of this review.
The mineralogy of Earth’s crust is dominated by common rock-forming silicate minerals, e.g., quartz, feldspars, ferromagnesium minerals, amphiboles, and micas. The elemental budget of Earth’s crust is therefore characterized by the cations which are integral structural components to these phases. Combined, the major rock-forming minerals account for 99% of Earth’s upper crustal element budget with only 2.16% of the remaining 1% being comprised of the REEs. They comprise <0.022% of the elemental budget of Earth’s upper crust,
The authors 1) considered and evaluated the occurrence of Earth’s REE resources associated with chemically evolved igneous alkaline suites; 2) compared and contrasted these with the association of REE-mineral bearing granitic lithologies on another planetary body which underwent metal-silicate differentiation, the Earth’s moon; and 3) evaluated the potential for economically justifiable extraction of lunar REEs.
With societies increasing reliance on natural resources, and the need for REEs to sustain the research and development of future technologies,s, the development of space-based resources is an inevitable reality of humanities future mineral exploration.
For an update, see: https://www.mdpi.com/2075-163X/8/10/455?type=check_update&version=1
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