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

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 ore-bearing porphyries were measured. Zircon grains from albite porphyries exhibit Ce4+/Ce3+ 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 Ce4+/Ce3+ ratios, the (La/Yb)(N) ratios of ore-bearing 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 deposits. Such a conclusion may be useful in the exploration for other concealed porphyry Cu deposits