Microlite

Although magmatic carbonatites and some A-type granites are currently the primary sources of rare earth elements (REEs), certain S-type granitic rocks, particularly those derived from sedimentary protoliths such as pegmatites, can also host significant REE concentrations. Because REEs are generally incompatible in major felsic minerals, which dominate highly evolved magmatic systems, they tend to become increasingly enriched during late-stage magmatic differentiation. In such settings, REE-bearing minerals of the microlite group are common in pegmatites.

Is REE mineralization primarily source-controlled or process-controlled? While a high-REE source is undoubtedly advantageous, however, many regions with similar geological settings lack significant REE mineralization. This observation suggests that magmatic processes also play a critical role. Understanding the key processes behind REE enrichment may help us predict potential “missing” deposits in otherwise favorable terranes.

Two main processes may enrich REEs in magmas: mineral fractionation and volatile fluxes. Fractionation depends on the partition coefficients of REEs between melt and crystallizing minerals. Crystallization of hydrous mafic minerals such as amphibole, biotite, or F-rich apatite tends to deplete REEs from the melt. In contrast, fractionation of other phases can progressively enrich REEs. Therefore, forming REE-rich pegmatites,regardless of the source, may require the suppression of mafic cumulates or their remelting during decompression. Additionally, the introduction of volatiles such as fluorine that promotes REE microlite-group mineral precipitation.

The specimen shown here is a striking example of fluornatromicrolite, exhibiting unusually large, well-formed crystals with the finest honey color i observed. It contrasts beautifully with the bright white albite matrix.

Fluornatromicrolite on albite, 3.8 cm

Shigar Valley, Shigar District, Gilgit-Baltistan, Pakistan

Jingnan (Tom) Zhang Coll & Photo