REE DISTRIBUTION AND MOBILITY IN RESIDUAL DEPOSITS ASSOCIATED WITH ALTERED GRANITES: A REVIEW AND COMPARISON OF GLOBAL DATA-SETS IN THE SEARCH FOR REE-CLAY DEPOSITS OUTSIDE OF CHINA

REE ion-adsorption clay deposits are of global economic importance because some are significantly enriched in heavy REE relative to light REE. There is considerable ambiguity regarding this deposit type, which is currently found only in China: (1) source rocks for the deposits have been described as originating from unspecialized granites, and from I-type, A-type, topaz-bearing, tin-bearing, calc-alkaline, alkaline, peraluminous, ilmenite and/or magnetite-bearing granites, (2) mobilization processes may include both late-magmatic-to-deuteric alteration and deep lateritic weathering, and (3) the clays may contain ionic REE, secondary REE-carbonate and/or phosphate minerals.

Anorogenic granitic rocks of the SE US have high contents of REE and have been subjected to a history of intense chemical weathering, comparable to that of southeast China, where all the important examples of REE clay deposits are found (e.g., Zudong and Longnan). Neoproterozoic granites of the Blue Ridge (e.g., Robertson River batholith, Irish Creek, Suck Mountain, Stewartsville plutons, VA) have REE contents and patterns (n=128) that are remarkably similar to granitic rocks interpreted as source rocks for REE clay deposits of China, as well as to other global occurrences (n=450). For example, the total REE (~3000 ppm), heavy REE (100x chondrite), and patterns for granites of Irish Creek and Zudong, China, are virtually identical. A detailed comparison of weathered bedrock-soil profiles for the coeval Suck Mountain and Stewartsville (established by a new SHRIMP ion microprobe zircon age of ~680 Ma for Stewartsville) with plutons of Robertson River, suggests that primary accessory mineral composition, model content, and distribution appear to be distinguishing factors in mobility-related enrichment of REE in alluvium and soils. Our recent studies have made important advances in documenting geochemical and mineralogical pathways for REE in altered granitoid rocks in the Southeast US. Work is currently underway to establish source rock compositions, mineral pathways, and geochemical processes that can result in the formation of REE-ion adsorption clay deposits and to quantify geochemical environments and processes favorable for formation of this type of REE deposit.