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

Abstract

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 (Di40-81Hd17-47), and grossularite–andradite (And22–89Gro6-77), 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 εNd(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 87Sr/86Sr 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.