Authors
1
professor, School of Geology, College of Science, University of Tehran, Tehran, Iran
2
M.Sc. student, School of Geology, College of Science, University of Tehran, Tehran, Iran
3
Associate professor, School of Geology, College of Science, University of Tehran, Tehran, Iran
Abstract
Introduction
Rare earth elements are important strategic elements and have a wide range of technological applications. Sangan iron ore deposit is located in 308 km southeast of Mashhad and 18 km north-east of Sangan city and is part of Khaf-Kashmar-Bardsan volcanic-belt (Karimpour, 2003). The study site lies on the southern C anomaly, which is an anomaly of western part of Sangan iron mine associated with magnetite skarn mineralization. The purpose of this study is to determine nature and origin of REE mineralization fluids, physicochemical conditions of REE concentration and the pattern of rare earth element distributions in this zone. Samples of skarn-ore and host rocks were collected from this zone for petrographic and geochemical studies. The samples show LREE enrichment relative to HREE in the iron ore and skarn zones. The obtained results suggesting that REE mineralization was associated with emplacement of intrusive rocks with simultaneous iron ore deposition.
Methodology
Mineralogy of Skarns
The skarn in this zone is in terms of mineralogical, including amphibole skarns (actinolites) and garnet skarns. The broadest skarn zone in the southern C anomaly is the amphibole skarn zone. The main mineral that forms this skarne unit is amphibole with the combination of ferroactinolite. The expansion of the garnet skarn zone in the low area is garnet of the andradit-grosolar type.
Geochemistry
To determine the pH of the formation setting, the La/Y parameter can be used such that values less than 1 of this ratio represent the acidic setting and more than one of them represents the alkaline setting (Crinci and Jurkowi, 1990). Rare earth elements are typically deposited in alkaline conditions (Patino et al., 2003). The separation degree of LREE relative to HREE can be calculated from the ratio (Gd/Yb)N and (La/Yb)N. These ratios are used to determine the degree of separation of LREE from HREE in geochemical processes (Aubert et al., 2001; Yosoff et al., 2013). To calculate the Ce/Ce* ratio, the following equation is used:
Ce/Ce*={(2Ce)sp/Ce(chon)}/{(La)sp/La(chon)}+{(Pr)sp/Pr(chon)}
This equation is a measure of Ce anomalies, so that values greater than 1 represent positive anomalies and values less than 1 represent a negative anomie of Ce.
Determine the origin of the mineralized fluid
In the Magmatic origin of skarn deposits, the values of Eu/Eu*, Ce/Ce* and (Pr/Yb)N indices increase with increasing amounts of rare earth elements, reflecting the characteristics of rare earth elements in magmatic fluid, while in atmospheric waters the amount of rare earth elements is very low (Kato, 1999). Hence, in skarn deposits with atmospheric waters origin, (Pr/Yb)N ratios decrease with decreasing amounts of rare earth elements, but significant changes in Ce/Ce* values are not observed with decreasing rare earth elements (Kato, 1993).
Results and discussion
In the skarn and iron ore zone, the frequency of LREE varies between 59.56 to 305.91 and 54.95 to 147.1ppm and HREE ranges from 5.79 to 45.86 and 40 to 22.89 ppm, indicating the LREE is enriched to HREE in this zone. The ratio of La/Y in skarn and iron ore studied is about 0.44 to 5.95 and 0.5 to 2.4, respectively, which indicates the alkaline environment for most of the samples, leading to the deposited of rare earth elements from fluids in this is the environment. The calculated values for skarn and iron ore for Ce/Ce* are between 0.13 to 0.84 and 0.02 to 0.15, respectively, which indicates a negative anomaly. In the diagrams (Pr/Yb)N and Ce/Ce* (Fig. 1), the skarn and iron ore zones are located near the magmatic waters and tend to be very low in the atmospheric waters.
Figure 1. Chondrite normalized diagrams, (a) (Pr/Yb) N and (b) Ce/Ce*, versus the total REE, are used to distinguish magmatic fluids from the atmosphere that can play a role in mineralization (Kato, 1999).
Conclusions
Due to the nature of the low pH of the primary fluids, REEs are likely to form the complex with ligands and are removed from the environment by washing. Given that the La/Y mean can be used to determine the pH of the formation setting, this ratio can be indicative of the alkaline environment governing the formation of the skarn zone. Therefore, rare earth elements appear to be deposited and enriched in the Skarn zone due to system temperature degradation and the rise of the pH of the fluids.
From the Ce/Ce* and (Pr/Yb)N parameters it can be concluded that the effective fluids in mineralization in the southern C anomaly deposit were a mixture of magmatic and atmospheric waters, which made atmospheric fluids a very small part of this mixed fluid. In parallel with injection, deposition and crystallization of deep intrusive masses, a significant amount of REE fluid through the shear zone of the region into the carbonate rocks of the injection area and eventually leads to the formation of iron ore. It can be said that the originator of the ore has a reproductive relationship with the intrusive mass in the region. Mineralogy and geochemical studies of rare earth elements indicate that factors such as changing the physico-chemical conditions of the environment (pH, Eh and temperature), alterations, ligand activity and the presence of sub-mineral phases play an important role in the concentration of rare earth elements in skarn samples.
References
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Crinci. J., Jurkowi. C., 1990, Rare earth elements in Triassic bauxites of Croatia Yugoslavia,Travaux, Vol: 19, p: 239-248.
Karimpour M. H., 2003, Mineralogy, Alteration, Rock of Origin and Tectonic Environment of Fe-oxide Cu-Au Reserves and Examples of Iran, Yazd University, pp. 184-187.
Kato. Y., 1993, REE geochemistry of aluminous skarn in the representative Japanese skarn deposits. Resource Geology, Vol: 15, p: 393–400.
Kato. Y., 1999, Rare Earth Elements as an Indicator to Origins of skarn deposits: Examples of the kamioka Zn-Pb and Yoshiwara-Sannotake Cu (-Fe) deposits in Japan, Resource Geology,Vol: 49, p: 183-198.
Patino. L. C., Velbel. M. A., Price. J. R., Wade. J. A., 2003, Trace element mobility during spheroidal weathering of basalts and andesites in Hawaii and Guatemala, Chemical Geology, Vol: 202, p: 343-364.
Yosoff. Z. M., Ngwenya. B. T., Parsons. I., 2013, Mobility and fractionation or REE during deep weathering of geochemically contrasting granites in a tropical setting, Malaysia, Chemical Geology, Vol: 349-350, p: 71-76.
Keywords
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