Anenburg and 3 co-authors reported in late 2020 that REEs are incompatible during all igneous differentiation stages in carbonatites. In alkali-free carbonatites, REEs concentrate via short-distance fluid transport in potentially economic domains within the carbonatites, where they are hosted by the relatively insoluble REE carbonates and phosphates (Fig. 4, A and B). Alkali-bearing carbonatites are capable of exsolving REE-rich fluids that can migrate long distances while retaining high REE solubilities (Fig. 4C). Both Na and K produce higher HREE/LREE ratios in the transported fluids, but potassic fluids preferentially transport HREE relative to sodic fluids. Therefore, while both LREE and HREE are expected in sodic fenites, we expect potassic fenites to contain the highest HREE/LREE ratios, consistent with observations from nature (22, 48, 56).
Carbonatites intruding siliceous rocks can assimilate SiO2, which negates this ore-forming potential by partitioning some (but not all) REEs into early-crystallizing apatite [e.g., at Nolans Bore, Australia (78)] and by loss of alkalis into early-crystallizing silicate minerals (73). Therefore, REE mineralization is confined to the carbonatite body itself (54), and valuable REEs such as Nd and Dy are diluted with the LREE La and Ce. Fenites around these Si-bearing carbonatites are expected to be small and barren (Fig. 4D).
Fig. 4 is schematic representation of four potential scenarios based on our experimental results. “L/HREE” annotation denotes higher or lower LREE/HREE ratios, with letter thickness proportional to concentration. Dotted line separating melt and fluid fields represents a continuous, ill-defined transition:
A) Alkali-free carbonatite intruding silica-undersaturated rocks (i.e., nepheline or other feldspathoids) produces minor REE-enrichment in late apatite. Most REE hosted in carbonates and phosphates (e.g., bastnäsite, ancylite, and monazite) distributed homogenously throughout the low-temperature zone of the carbonatite. If volumetrically limited, then REE contents may be of economic grade.
B) Alkali-free carbonatite intruding silica-saturated rocks (i.e., feldspars ± quartz) loses most REE into early crystallizing apatite. Late-stage apatite may be HREE enriched, and minor LREE mineralization is expected to occur in late-stage REE carbonates and phosphates. The economic potential of the deposit will be upgraded if late-stage alteration redistributes REE into carbonates. This scenario is analogous to Nolans-type deposits.
C) Alkali-rich carbonatite intruding silica-undersaturated rocks with most of its LREE concentrated into alkali REE carbonates such as burbankites and some HREE migrating into an extensive peripheral fenite alteration zone. Burbankite is unlikely to survive alteration (by fluids varying from deuteric to weathering) although its former presence is established by REE-rich pseudomorphs. This scenario is analogous to deposits hosted in ferrous dolomite carbonatites and associated fenites (e.g., Malawi type).
D) Alkali-rich carbonatite intruding silica-saturated rocks, with most of its alkalis found within silicates (e.g., aegirine and phlogopite). The resultant fenite is spatially limited with no extensive REE, and most REE are found within the carbonatite body, similar to (A). This scenario is analogous to deposits found in the Mianning-Dechang belt, China.
The authors concluded that the formation of LREE deposits are dominant in carbonatites. In addition, some potential HREE carbonatite–associated deposits are expected to be hosted in potassic fenites formed around silica poor carbonatites. Alternatively, HREE deposits within carbonatites are expected to be hosted in either silica-rich and/or alkali-poor systems. Even when fenite-hosted HREE deposits are not economic, they can indicate that their host carbonatite had experienced LREE concentration by alkali-fluid migration, with potential deposition in burbankite-rich zones. Thus, the higher HREE/LREE signature of fenites is a potential exploration indicator that can help vector toward a mineralized carbonatite (10, 13, 22).
The authors indicated that their results are applicable to not only carbonatites sensu stricto, but also other REE-rich and HREE-enriched occurrences hosted in carbonatite-absent alkaline systems, in which post-magmatic fluids are commonly rich in alkalis, carbonate, and halogens (52, 53, 71).
Original working URL: https://advances.sciencemag.org/content/6/41/eabb6570
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