Geochemical characteristics of apatite and amphibole within the granitic intrusions from the Suyunhe Mo deposit, Xinjiang province, NW China: Insights into petrogenesis and mineralization potential

Abstract

Textural and compositional variations in apatite and amphibole from mineralized granite (G), granite porphyry (GP), granodiorite porphyry (GDP), and barren tonalite porphyry (TDP) in the Suyunhe ore district, Xinjiang province, NW China revealed differences in their petrogenesis, magmatic properties and mineralization potentials. The higher Cl contents (0.09–0.18 wt%) and Cl/F ratios of apatite, and the higher Mg/(Mg + Fe2+) ratios (0.54–0.62) of amphibole in the barren TDP indicate that the parental magma was mainly generated via melting of the lithospheric mantle. In contrast, the parental magmas of the GDP, G, and GP were mainly derived from melting of the juvenile lower crust. The enrichment of Mn, Y, and heavy rare earth elements (HREEs), depletion of Sr, and strong negative Eu anomalies of the apatite from the GP indicate that it underwent a high degree of fractional crystallization. Moreover, the reverse core-to-rim compositional variations in REE and Sr contents and the La/Y ratios observed in some of the apatite grains, as well as the distinct differences in the major-element compositions (e.g., CaO, MnO, SiO2, and Ce2O3) of the individual zones within single apatite grains, indicate magmatic mixing. The calculated magmatic viscosities (Logη = 4.8–5.0 Pas) at Suyunhe remained below the average value for granitic melts (mostly Logη > 5 Pas) at the same temperature. These results support the hypothesis that low-viscosity magma (e.g., Logη < 5 Pas) may be essential for the formation of porphyry Mo deposits. The physicochemical conditions (including temperature, pressure, and H2O content) estimated based on the amphibole in the mineralized intrusions indicate the existence of a deep magma chamber with a large volume, which continuously released volatiles (e.g., fluorine and water) and effectively reduced the viscosity of the magma.