Genesis of the Tiemuli skarn W–Fe deposit in the Nanling region, South China: Constrains from multi-mineral U-Pb geochronology and scheelite geochemistry

Abstract

The Tiemuli scheelite-magnetite (W-Fe) deposit is located in the central part of the Nanling region, South China and contains five skarn-type ore bodies that are associated with the Tiemuli porphyritic granite. The mineralization process of this deposit can be divided into three stages: the prograde skarn, the retrograde skarn, and the quartz-sulfide-calcite stages. Zircon grains from the porphyritic granite yielded a U-Pb age of 138±1 Ma. The vesuvianite and garnet grains from the prograde skarn stage yielded a weighted mean 206Pb/238U age range of 139–136 Ma. The apatite and cassiterite from the retrograde stage exhibited a weighted mean 206Pb/238U age range from 138 to 136 Ma. These U-Pb ages suggest that the Tiemuli granite emplacement and W–Fe skarn mineralization occurred during the Early Cretaceous period. Based on the cathodoluminescence (CL) characteristics, two generations of scheelite were identified. Sch-I showed homogeneous luminescence, whereas Sch-II exhibited a complex core-mantle-rim texture. REE was incorporated into scheelite following the substitution mechanism of 3Ca2+ = (2REE)3+ + □Ca, allowing the scheelite to effectively reflect the REE features of the ore-forming fluids. The Mo concentration and Eu anomaly in the scheelite reflected fluctuation in the fluid conditions during the scheelite crystallization. While transiting from the prograde to retrograde skarn stage, the hydrothermal fluid shifted from a relatively reducing environment to a relatively oxidizing environment. The Tiemuli deposit is characterized by the reductive skarn mineralization that occurred during the Early Cretaceous lithospheric expansion. Variations in the physicochemical characteristics of the fluids and fluid-rock interaction played crucial roles in the W–Fe mineralization of the Tiemuli deposit. Thus, this deposit serves as a unique example of a W–Fe mineralization event in the Nanling region of South China during the Early Cretaceous.