Petrogenesis and mineralization of the porphyry and skarn mineralization-related Husite intrusion in the Boluokenu metallogenic belt, Western Tianshan Orogenic Belt, NW China

Abstract

Many porphyry and skarn Cu (Fe, Mo, Au, Zn) deposits have been discovered along the western Tianshan Orogenic Belt within the Central Asian Orogenic Belt. Generally, these Cu polymetallic deposits are considered to be linked to porphyries that formed in an arc setting, e.g., in a continental magmatic arc and island magmatic arc, but an agreement on their source region has not been reached. The Husite granitic complex and associated Kekesala skarn Fe-Cu deposit from the Boluokenu metallogenic belt provide significant insights into a comprehensive study of the ore-forming porphyries in Boluokenu. The ore-forming porphyries are mainly granodiorites and belong to calc-alkaline and high-K calc-alkaline series. LA-ICP-MS zircon U-Pb dating on the granodiorites yields ages of 368 Ma. The trace element compositions of the granodiorites are characterized by enrichment in large ion lithophile elements (LILE) and light rare earth elements (LREE), depletion in high field strength elements (HFSE) and heavy rare earth elements (HREE), and slightly negative Eu anomalies and are similar to rocks that form in a continental arc setting. These rocks also show large variations in their εHf (t) (+4.1 to +9.7) and Mg# (38–69) values, alongside low Ba/Th (4–230) and high Th/Yb (3.77–7.41) and Th/La (0.21–0.71) ratios, suggesting that the granodiorite magma originated from the partial melting of mafic lower crust with a mantle wedge component and involved signals of subduction-zone sediments and fluids. The Husite mafic microgranular enclaves (MMEs) exhibit high Mg# (50–62), MgO (3.25–6.93 wt%) and low SiO2 (48.8–59.5%) contents, with positive εHf(t) from +3.3 to +7.4. The zircon LA-ICP-MS U-Pb ages of the MMEs (373 Ma) indicate nearly coeval formation between the MMEs and HMPs during the Late Devonian. These features with the petrological and mineralogical characteristics (such as needle-like apatite and disequilibrium feldspar xenocrysts) suggest that the Husite MMEs formed from the mixing of felsic magma from the partial melting of mafic lower crust and mafic magma that was derived from mantle wedge, rather than restite, xenoliths or fractional crystallization of magma. Thus, the skarn Fe-Cu mineralization were probably related to hybrid intermediate-acid magma that is characterized by high water and oxygen contents and high sulfur fugacity. It can be inferred that the response to ongoing slab roll-back during the Junggar plate’s subduction beneath the Yili Block, including the underplating of mantle-derived mafic magmas from the metasomatism of subduction-zone melts, led to the partial melting of the mafic lower crust to form felsic magma. Then, the continuous reaction between the mafic and felsic magmas further increased the Fe-Cu concentrations and was closely related to the subsequent Fe-Cu mineralization.