Ore genesis and fluid evolution of the Sandaozhuang supergiant W-Mo skarn deposit, southern margin of the North China Craton: Insights from scheelite, garnet and clinopyroxene geochemistry

The Sandaozhuang deposit (0.55 Mt WO3 at 0.112% and 0.75 Mt Mo at 0.109%), located in the southern margin of the North China Craton, is one of the largest W-Mo deposits in the world. This study carried out in-situ elemental analyses on skarn minerals, as well as elemental and Sr-Nd isotopic analyses on scheelites from the deposit, to constrain the metal source and fluid evolution of the deposit. Three generations of scheelite (Sch A to C) were identified based on petrographic observations, including prograde skarn and diopside-plagioclase hornfels-hosted Sch A, retrograde skarn-hosted Sch B, and quartz vein-hosted Sch C. Varieties of REE patterns in the scheelite are mainly controlled by different substitution mechanisms and crystallization of REE-enriched skarn minerals. The Mo contents decrease from early generation Sch A (1.57-24.45%) to Sch B (0.19-1.52%) and finally to Sch C (0.20-0.97%), and Eu anomalies of scheelite change from negative to positive and then back to negative values. The Sandaozhuang deposit is characterized by Mg-rich clinopyroxene (Di(40-81)Hd(17-47)), and grossularite-andradite (And(22-89)Gro(6-77)), similar to well-studied oxidized W skarn deposits worldwide, but the andradite contents of one garnet grain in the late prograde stage, decrease from the core (84.09%) to rim (75.12%). Composition changes of scheelite and garnet indicate that the ore-forming fluids changed from oxidized to reduced state in the skarn stage then to oxidized state in the quartz-sulfide stage, and the variations in redox state favoured the muti-stage Mo mineralization. The relatively uniform Y/Ho ratios of the Nannihu granite porphyry and scheelite imply that the ore-forming fluids are mainly derived from the porphyry. Scheelites from Sandaozhuang are characterized by low epsilon(Nd)(t) values (-14.7 to-11.8), which are consistent with those for the Nannihu granite porphyry. Such a coincidence indicates that ore-forming fluids were mainly derived from the ore-hosting porphyry. However, the initial Sr-87/Sr-86 ratios of scheelites (0.7158-0.7295) are significantly higher than those of the Nannihu granite porphyry and other granites in the Luanchuan ore district (0.7047-0.7098), indicating that the Neoproterozoic Luanchuan Group with radiogenic Sr isotopic compositions contributed to tungsten mineralization, and the intense fluid-rock interaction between the ore-forming fluids derived from the granite porphyry and the Luanchuan Group country rocks may play an important role in tungsten mineralization.