Characteristics of Rare Earth Elements, Zr, and Hf in Ore‐Bearing Porphyries from the Western Awulale Metallogenic Belt, Northwestern China and their Application in Determining Metal Fertility of Granitic Magma

Abstract

The western Awulale metallogenic belt in northwestern China hosts a number of small-to medium-sized porphyry Cu deposits that are associated with albite porphyry. The common presence of plagioclase (albite) as phenocrysts and the absence of hydrous minerals (amphibole and biotite) indicate that the water content of albite porphyry is low. Trace-element compositions of whole rocks and zircon grains from these orebearing porphyries were measured. Zircon grains from albite porphyries exhibit Ce 4+ /Ce 3+ ratios ranging from 7.75 to 95.1, which indicate that these porphyries have a low oxygen fugacity. Trace element compositions of ore-bearing porphyries exhibit (La/Yb) N ratios ranging from 1.09 to 11.1 and Eu/Eu * ratios ranging from 0.10 to 0.66. These ore-bearing porphyries have Zr values ranging from 171 to 707 ppm and Hf values ranging from 8.30 to 18.9 ppm. Combining these porphyries with other ore-bearing porphyries that formed in the Central Asian Orogenic Belt (CAOB) and the Pacific Rim metallogenic belt, we found that the (La/Yb) N and Eu/Eu * ratios of ore-bearing porphyries in western Awulale are low, while the Zr and Hf values are high. Specifically, REEs can be used to evaluate the degree of differentiation and degree of partial melting, and Zr and Hf can be used to evaluate the redox conditions and water content of magmatic rocks. Our findings indicate that ore-bearing porphyries in western Awulale have a lower oxygen fugacity, degree of differentiation, and water content than do others in the CAOB and the Pacific Rim metallogenic belt. Compared to those of ore-bearing porphyries with lower zircon Ce 4+ /Ce 3+ ratios, the (La/Yb) N ratios of orebearing porphyries in our study are low, and the Zr and Hf values are high. This finding indicates that, under reducing conditions, the degree of evolution and water content may have an important influence on the metal abundance in magmas. There is also a clear relationship between (La/Yb) N , Eu/Eu * , Zr, Hf, and the size of the deposits. Large-(>4 Mt Cu) and intermediate (1.5-4 Mt Cu)-sized porphyry Cu deposits are associated with granitic intrusions that have higher (La/Yb) N and Eu/Eu * ratios and lower Zr and Hf values. This finding indicates that, in addition to oxygen fugacity, the degree of evolution and water content are controlling parameters for metal abundance in magmas, especially in low oxygen fugacity porphyry Cu

Bimodal magmatism produced by delamination: geochemical evidence from late Palaeozoic volcanic rocks from the Yili Block, Western Tianshan, Northwestern China

“…The intrusive rocks in the Yili -Central Tianshan Block mainly include three periods: early Palaeozoic, late Devonianearly Carboniferous and late Carboniferousearly Permian. The early Palaeozoic intrusive rocks contain mostly calc-alkaline granite resulted from northward subduction of the Southern Tianshan Ocean in the early Palaeozoic and the metaluminous and peraluminous granite, associated with the Yili -Central Tianshan Block and the Tarim plate collision, which are present in the southern margin of the Nalati Mountains 400-490 Ma in age (Yang et al, 2006;Zhang et al, 2008;Zhu et al, 2011;Liu et al, 2016Liu & Chen, 2018. Devonianearly Carboniferous granites are calc-alkaline granites mainly found in the southern margin of the North Tianshan and Nalati Mountains.…”

“…During the Late Carboniferousearly Permian, a suite of quartz albite porphyry, monzogranite, potassium feldspar granite and diabase intruded in the Yili -Central Tianshan Block. These granitoids are characterized by adakites (Li et al, 2019) and A-type granite (Tang et al, 2008;Liu & Chen, 2018. Similar to the late Carboniferousearly Permian bimodal volcanic rocks, these rocks limit the time of the post-orogenic phase of the western Tianshan.…”

Compositions of Cu-(Fe)-Sulfides in the 109 Reduced Granite-Related Cu Deposit, Xinjiang, Northwest China: Implications to the Characteristics of Ore-Forming Fluids

“…Numerous Fe-Cu (Au) polymetallic deposits have been recently reported in the Awulale area, which formed an Awulale Fe-Cu metallogenic belt [20]. The western Awulale metallogenic belt is characterized by volcanic-subvolcanic hydrothermal Cu deposits such as Nulasai and Kezikezang and reduced porphyry Cu deposits such as Yinulasai, Qiongbulake, Qunjisayi, Qunji, 109, and Heishantou [18,19,[21][22][23].…”

“…The granitic pluton that hosts this deposit intruded early Permian bimodal volcanic rocks of the Wualng Formation, which is composed of continental intermediate-basic and felsic volcanic rocks. The petrography of the granitic pluton is albite porphyry [18,19], which contains abundant albite of magmatic origin occurring either as crystals in the groundmass or as erosion products of late-stage minerals or the matrix (Figure 3). The studied deposits contain zones affected mainly potassic alteration with weak phyllic and propylitic zones, sometimes exhibiting brownish-red coloration.…”

“…Therefore, comparison of the distribution of trace elements may reveal factors that control the trace element distribution in different Cu-(Fe)-sulfides, particularly for reduced porphyry Cu deposit. Previous studies have indicated that the Cu-(Fe)sulfides in reduced porphyry Cu deposits, which include chalcopyrite, bornite, and chalcocite, are similar to those in classic porphyry Cu deposits [5,[16][17][18][19]. However, owing to the limited number of the reduced porphyry Cu deposits, fewer studies have been conducted on sulfides in this type of deposit.…”