Geochemistry of REEs and trace elements in Diz alteration area, NW Iran

Diz alteration area is situated in the southern part of Ardabil province in the northwest of Iran. This alteration area is a limited part of Tarom-Hashtjin volcanic zone. The hydrothermal alteration process has been mostly taken place in pyroclastic and volcanic units such as tuff, ignimbrite, and trachyandesites. The alterations of this area are related to Eocene volcanism which has considerably developed in the northwest of Iran. The argillic alterations in Diz area are mainly seen in ignimbrite unit and the precursor rock has been intensely altered such that many parts of the parent rock has been fully leached and all of the mobile elements have been removed from the parent rock while the resistant elements such as Al, Si, and some other immobile elements have remained in the context. Considering to the special behavior of REEs in the weathering and alteration profiles, depending on the REE bearing mineral’s resistivity against weathering and alteration processes, REEs can be removed or fixed in the environment. In the studied samples different concentrations of REEs are observable.            

The drawn REE diagrams show unique patterns for the studied samples where Ce group elements (LREEs) show a slight enrichment comparing to Y group (HREEs). The comparison of LREEs with HREEs represents that LREEs have been enriched 4 times more than HREEs.

The positive correlation coefficient between ΣREE and TiO₂ (R²=0.70) represents the role of Ti bearing minerals such as ilmenite, pyroxene, rutile, and anatase in the fixation of REEs. On the other side the presence of considerable amounts of P₂O₅ in the studied samples and also the positive correlation coefficients between P₂O₅ and LREEs (R²=0.90), and P₂O₅ and ΣREE (R²=0.74) suggest that some minor minerals such as monazite (Ce,La,Nd,Th)(PO₄,SiO₄) must be considered. The positive correlation coefficient between Al₂O₃ and ΣREE shows the influence of clay minerals in the adsorption of REEs.         

The evaluation of REE patterns normalized to chondrite show a remarkable peak for Gd. Geochemically, Gd shows similarities with Ca²⁺. The Gd complexes may decompose in the presence of some elements such as Cu, Y, and REEs and Gd^{3+ ­­}can be released. Hence, CaO is a main component in the parent rock of the studied altered samples, the positive Gd anomaly is most likely related to the primary composition of the parent rock. Furthermore, the decomposition of Gd complexes in the presence of competitor elements and also the high Gd content of altering fluids can be thought as the main reasons of Gd positive anomaly in the studied samples.