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| Titel |
Genesis of ion-adsorption type REE ores in Thailand |
| VerfasserIn |
K. Sanematsu, K. Yoshiaki, Y. Watanabe |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250065231
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| Zusammenfassung |
| Ion-adsorption type REE deposits, which have been economically mined only in southern
China, are predominant supply sources for HREE in the world. The ore bodies consist of
weathered granites called ion-adsorption ores. The majority of REE (>50 %) are
electrostatically adsorbed onto weathering products in the ores and they can be extracted by
ion exchange using an electrolyte solution (e.g., ammonium sulfate solution). Recently the
occurrences of ion-adsorption ores have been reported in Indochina, SE Asia. In this study,
we discuss geochemical and mineralogical characteristics of parent granites and
weathered granites in Thailand in order to reveal the genesis of ion-adsorption
ores.
Permo-Triassic and Cretaceous-Paleogene granite plutons are distributed from northern
Thailand to western Indonesia through eastern Myanmar and Peninsular Malaysia. They are
mostly ilmenite-series calcalkaline biotite or hornblende-biotite granites. REE contents of the
granites range from 60 to 600 ppm and they are relatively high in Peninsula Thailand.
REE-bearing minerals consist mainly of apatite, zircon, allanite, titanite, monazite and
xenotime. Some I-type granites contain REE fluorocarbonate (probably synchysite-(Ce)) in
cavities and cracks in feldspars and it is the dominant source of REE for ion-adsorption ores
because the fluorocarbonate is easily soluble during weathering. In contrast, insoluble
monazite and xenotime are not preferable for ion-adsorption ores although they are common
ore minerals of placer REE deposits.
Weathered granites show REE contents ranging from 60 to 1100 ppm in Thailand because
REE are relatively immobile compared with mobile elements (e.g., Na, K, Ca). In the
weathered granites, REE are contained in residual minerals and secondary minerals
and are adsorbed onto the surface of weathering products. A weathering profile
of granite with ion-adsorption type mineralization can be divided into upper and
lower parts based on REE enrichment and Ce anomalies reflecting a redox condition
during weathering. The upper part of the profile is a leached zone characterized by
positive Ce anomaly and lower REE contents compared with the parent granite. This
indicates that acidic soil water in an oxidizing condition immobilized Ce4+ as CeO2
and transported REE3+ downward. In contrast, the lower part of the profile is an
accumulation zone characterized by negative Ce anomaly and higher REE contents. This
indicates that the transported REE3+ (depleted in Ce) were adsorbed onto weathering
products and/or distributed into secondary minerals such as rhabdophane. This
immobilization of REE results from the increase of pH and subsequent decrease
of REE solubility due to the contact with higher pH groundwater. Percentages of
adsorbed REE to whole-rock REE are also higher in the accumulation zone than in
the leached zone. As the majority of REE in the weathered granites are present in
the ion-adsorption phase with negative Ce anomaly, the percentages of adsorbed
REE are positively correlated with the whole-rock negative Ce anomaly. Although
fractionation of LREE/HREE is controlled by the occurrences of REE-bearing
minerals and adsorption by weathering products, the ion-adsorption phase tends to
be enriched in LREE relative to whole-rock compositions of weathered granites. |
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