Application of geochemical indicators to distinguish productive magma from barren in Qarah Gol area, Ardabil province using rare earth elements

Document Type : Original Article

Authors

Department of Geology, Faculty of science, University of Mohaghegh Ardabili, Ardabil, Iran

Abstract

Introduction: Qarah Gol study area is located in Ahar- Arasbaran metallogenic zone in Meshginshahr city in Moradloo section. According to field studies, the main exposed rock units in the region include volcanic, plutonic and pyroclastic igneous rocks. In the subduction zones, productive, semi-productive and fertile magma are found together, and the identification of these masses is very important from the point of view of exploration. Fertile granitoid magmas are generally formed in pre-collisional subduction setting. While productive granitoid magmas are formed in post-collision tectonic setting. It is thought that the melting of young thickened mafic lower crust or the partial crystallization of hydrous basaltic magma is the key factor in the production of various types of productive magmas, while partial melting of the young thin mafic lower crust is considered that can produce fertile magmas.
Materials and methods: In this research, 14 thin sections of surface samples and 28 thin sections of drilling cores were prepared for the petrologic study at Mineral Research and Processing Center. To determine the texture and paragenesis of ores, 30 polished thin sections and 20 polished sections of the samples were prepared. The 23 surface lithogeochemical samples were studied by ICP-MS. Also 14 core samples were studied by XRF for main oxides.
Results and discussion: Productive granitoids in the UDMB display lower values of Ni (9–31 ppm) and Cr (14–36 ppm) than barren granitoids (Ni = 11–73 ppm; Cr = 21–81 ppm). The low values of Ni (5–19.6 ppm) and Cr (14–30.2 ppm) in the igneous rocks of the Qarah Gol indicate that this magma originates from partial melting of the thick lower crust. Also, the productive magma has a higher LREE/HREE ratio than the barren magma, which is also seen in the samples of Qarah Gol area. In productive magmas, pattern of trace elements normalized to N-MORB is characterized by enrichment in Cs, Rb, Ba and Pb and depletion in Nb, Ta, Th, Zr, Hf and Ti. This process shows that garnet is present as a residual phase in the magma chamber of origin of the rocks and indicates the formation at a pressure (>15kbar) equivalent to a crustal thickness of 45-55 km (Palin et al, 2016).
Conclusion: According to geochemical results, the average concentration of copper is 138.21 ppm. Petrological observations and geochemical data show that the graniteoids of Qarah Gol area are calc alkalines type I related to subduction zone which is linked to porphyry copper mineralization. These results show that the study area is located in post-continental collision. Also, the use of La / Yb versus Sm / Yb diagram shows that the samples were made from partial melting of 10 to 20% garnet peridotite. Based on Ce against Ce / Yb diagram, the melting depth of 100 to 110 km is estimated. Productive magma has a higher LREE / HREE ratio than barren magma, which is evident in samples of the Qarah Gol area.

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References

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