1. I measured REY concentrations for weathered whole rocks (anorthosite complex, syenitic rocks, metamorphic complex) and each clay fraction extracted therefrom. The results showed good correlation between Eu/Eu* anomaly and total REY concentrations.
2. In addition, diagram of (Th+U)n vs. REY concentrations also showed good correlation.
I want to understand the REY fractionation based on physico-chemical factors such as electrovalence and ionic radii.
There are two major types of supergene REE accumulations which you should pay attention to as you seek for an answer:
1. Lateritic REE deposits on mainly igneous rocks such as Mount Weld, Araxa, Kangankunde
2. Ion-adsorption clays which are widespread on felsic plutonic rocks such as Longnan, Xunwu
The question is whether the transition from the crystalline bedrock to the residual clays concerns a bastnäsite (carbonate-) or monazite (LREE)- and xenotime (HREE)-type. They show a different response to the meteoric fluids and create different physico-chemical regimes the closer they are on the parent material (saprock). Higher up in the profile these differences may become less pronounced and take on the regime typical of the existing morphoclimatic zone.
• bauxite deposits contain elevated REE contents
• in Fe-rich bauxite horizons (scavenging by goethite) not observed in the boehmite-rich horizons
• REE accessory minerals (bastnaesite, hydroxyl-bastnaesite-(Nd), goyazite-(Nd), monazite-(Nd), synchysite, florencite, rhabdophane-(Nd, Sm, Ce)
The above phosphates occur under different pH regimes and along with that the release and differentiation of REE, U, Th and Y in the saprolite goes different ways. This is also of impact on the REE which are redox- sensitive like Ce and Eu, or hexavalent and tetravalent U . Also have a look at the radioactive equilibrium where the different Th and U isotopes/decay products show different mobilities and as a conseqeunce of this a separation and shift of the U/Th ratio may take place.
It is a very complex process where without any meticulous mineralogy, radio-chemical investigation your job will only be half-way done. If you deal with natural products straight from the pit or field it will be mandatory to consider the morphoclimatic situation, hydrology and age of the mineralization.
Any simple correlation with each and everything yields nice clusters in x-y plots but with no meaning or getting you closer to your goal of determining the origin of these REE deposits.
My frequently stated credo
1. Geology-geomorphology (field)
2. Mineralogy (XRD, EMPA, SEM-EDX) in this case clay minerals, oxide-hydroxides, heavy minerals
3. Major-minor element chemistry (XRF, NAA)
4. Advanced level: radiometric age dating and radio chemistry (instable isotopes with passage into the stable isotopes)
H.G.Dill
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