Skarn Formation and Zn–Cu Mineralization in the Dachang Sn Polymetallic Ore Field, Guangxi: Insights from Skarn Rock Assemblage and Geochemistry

Abstract

The Dachang is a world-class, super-giant Sn polymetallic ore field mainly composed of Zn–Cu ore bodies proximal to the granitic pluton and Sn polymetallic ore bodies distal to the granitic pluton. In this study, we used petrographic studies and major and trace element geochemistry with calc-silicates from the Zn–Cu ore bodies to constrain the physicochemical conditions of hydrothermal fluids during skarn rock formation and the evolution of ore-forming elements. Two skarn stages were identified based on petrographic observations: Prograde skarn rocks (Stage I), containing garnet, vesuvianite, pyroxene, wollastonite, and retrograde skarn rocks (Stage II), containing axinite, actinolite, epidote, and chlorite. The retrograde skarn rocks are closely associated with mineralization. The geochemical results show that the garnets in the Dachang ore field belong to the grossular–andradite solid solution, in which the early generation of garnet is mainly composed of grossular (average Gro72And25), while the later generation of garnet is mainly composed of andradite (average Gro39And59); the vesuvianites are Al-rich vesuvianites; the pyroxenes form a diopside–hedenbergite solid solution with a composition of Di3–86Hd14–96; the axinites are mainly composed of ferroaxinite; and the actinolites are Fe-actinolite. The mineral assemblage of the skarn rocks indicates that the ore-forming fluid was in a relatively reduced state in the early prograde skarn stage. As the ore-forming fluid evolved, the oxygen fugacity of the ore-forming fluid increased. During the final skarn stage, the ore-forming fluid changed from a relatively oxidized state to a reduced state. The skarn rocks have evolved from early Al-rich to late Fe-rich characteristics, indicating that the early ore-forming fluid was mainly magmatic exsolution fluid, which may mainly reflect the characteristics of magmatic fluids, and the late Fe-rich characteristics of the skarn rocks may indicate that the late hydrothermal fluid was strongly influenced by country rocks. Trace element analyses showed that the Sn content decreased from the prograde skarn stage to the retrograde skarn stage, indicating that Sn mineralization was not achieved by activating and extracting Sn from prograde skarn rocks by hydrothermal fluids. The significant enrichment of Sn in the magmatic hydrothermal fluid is a necessary condition for Sn mineralization. There are various volatile-rich minerals such as axinite, vesuvianite, fluorite, and tourmaline in the Dachang ore field, indicating that the ore-forming fluid contained extensive volatiles B and F, which may be the fundamental reason for the large-scale mineralization of the Dachang ore field.