SO-CALLED “ION-ADSORPTION TYPE” REE DEPOSITS FOUND IN WEATHERED CRUST OF ILMENITE-SERIES GRANITE IN NORTHERN VIETNAM

The demand for Rare Earth Elements (REE), especially that for heavy-REE (HREE) is sky-rocketing due to the rapidly growing needs of REE magnet for EV. On the other hand, the production of HREE depends almost entirely on specific REE deposits named "ion-adsorption type" which exist within weathered crust of granitic rocks in Longnan area, southern China. Wu et al. (1992) explained that REE ions are adsorbed on the surface of clay minerals in these REE deposits, but the detailed mechanism of “adsorption” is not clarified.

A significant HREE enrichment was found in the limited area in weathered crust of Nui Phao granite (NPG), which is classified into ilmenite-series granite, near Dai Tu, Thai Nguyen district, northern Vietnam. The authigenic weathered crust, about 10 m in thickness, develops in an area of 20 km² on NPG body.

In the northern part of its distribution, total REE concentration is generally high (Ave. 549 ppm) and in the southern part, total REE concentration is around 300 ppm which is only two times higher than that of fresh NPG (155 ppm). Stepwise leaching test indicates that concentration of ion-exchangeable REE in the northern part is also generally high (Ave. 67% of all REE) and that of ion-exchangeable REE in the southern part is also low (Ave. 18% of all REE). The low concentration found in southern part could be explained by residual enrichment if the REE-bearing mineral(s) in granite are refractory. However, the high concentration of REE in the northern part is not explained by the mechanism and additional enrichment process is needed.

By observing under FE-SEM, we found that most of the REE in weathered crust of the northern part occur as secondary phosphate which chaotically mixes with kaolin-group mineral. The amount of the REE-bearing secondary phosphate is equivalent to the excess amount of REE in the specimen. Therefore, it is indicated that the REE occur not as adsorped on the surface of kaolin-group mineral. Careful measurement using Gandolfi camera suggests that the secondary phosphate is likely to be amorphous.

We assume that the “ion-adsorption type” REE deposits in China, at least a part of them, might have similar characteristics with the weathered crust of NPG.