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

Qarah Gol study area is located in Ahar- Arasbaran metallogenic zone in Ardabil province, 50 km from Meshginshahr city in Moradloo section. According to field studies, the main exposed rock units in the region include volcanic, plutonic and pyroclastic igneous rocks, microdiorite masses have injected into volcanic and igneous rocks and caused alteration and mineralization on a weak scale. Mineralized silica veins are often formed in parallel band and are sometimes seen as stockwork veins. 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 (Gammons and Williams Jones, 1997; Li et al, 2016; Shafiei et al, 2009; Shahabpour, 1992). While productive granitoid magmas are formed in post-collision tectonic setting (Li et al, 2016). Many processes have been proposed for the origin of granitoids in collisional or accretionary orogenic environments, 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 as a process that can produce fertile magmas (Hollings et al, 2005).

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 rocks collected from surface samples and drilling cores were analyzed in order to investigate rare earth elements, 23 surface lithogeochemical samples were studied by ICP-MS. method. Also 14 core samples were studied by XRF for main oxides.

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)) (Sepidbar et al, 2018). The low values of Ni (5–19.6 ppm) and Cr (14–30.2 ppm) in the igneous rocks of the Qarah Gol are comparable to their values in other productive magmas in this belt and 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. In fact, productive porphyries, unlike the barren type, usually show negative anomalies of Nb, Ta 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).

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. To identify the tectonic environment of the study area, diagrams of La/Yb versus Th/Yb and ZR3-Nb50-Ce/P2O5 can be used. 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. The values of Eu / Eu* igneous rocks in the range of Qarah Gol are between 0.7 to 1.07 and in this respect it is similar to the magmas that produce in the Urmia-Dokhtar belt.