Chlorite chemistry of Tongshankou porphyry-related Cu–Mo–W skarn deposit, Eastern China: Implications for hydrothermal fluid evolution and exploration vectoring to concealed orebodies

Abstract

Recent years, the depletion of shallow mineral resources has drawn increasing attention to the exploration of hidden ore bodies in deep of skarn type deposits. Chemical composition of chlorite is an effective tool for tracing the hydrothermal fluid evolution, and has been increasingly used for deep-level exploration of porphyry Cu (–Au) deposits (PCDs). However, the technique was rarely applied onto (porphyry-related)-skarn deposits. The Tongshankou Cu–Mo–W deposit (0.55 Mt Cu @ 0.86%, 0.01 Mt Mo @ 0.10% and 0.12 Mt WO3 @ 0.19%) is one of the largest skarn–porphyry deposits in the Middle–Lower Yangtze River Metallogenic Belt (MLYRB) in Eastern China. In this study, new petrographic and trace element data of the hydrothermal chlorites from Tongshankou were presented to constrain the ore-fluid evolution, and to explore the vectoring potential of the concealed orebodies at Tongshankou. Four types of chlorite have been identified, i.e., G-type chlorite that replaced the original magmatic biotite or hornblende, V-type chlorite in chlorite–quartz–sulfides veins in the granodiorite porphyry (alteration zones I and II), S-type chlorite that replaced garnet in the intensive skarn alteration zone (III) and M-type chlorite–quartz–calcite–sulfides veins that cut the (dolomitic)-marble in the distal alteration zones IV and V.All the four types of chlorites are typically trioctahedral chlorite. The Fe-Mg, Tschermark and di-trioctahedral substitution mechanisms likely control the geochemical variations of the Tongshankou chlorites. Empirical thermometer indicates that the Tongshankou chlorites were crystallized at 169–322 ℃. Compared with typical metamorphic chlorites, the higher Zn and Sr contents but lower Al content in the Tongshankou chlorites can potentially distinguish the skarn-type hydrothermal chlorites from typical metamorphic ones. Compared with the chlorites in the typical PCDs, the Tongshankou G-type chlorite has higher Fe, Zn, As and Sn contents, but lower Al, Mg, Co and Cu contents, which likely reflect its protolithic and/or fluid geochemical inheritance. The Tongshankou V-, M- and S-type chlorites have high contents of mobile elements (such as Sr, Ca, Sn and B) and metals (such as Fe, Cu, As, Zn and Pb), indicating that they were directly precipitated from the hydrothermal fluids. From the granodiorite porphyry to the marble wallrocks, the spatial chlorite geochemical variations at Tongshankou indicate that the increasing Zn, Mn, Ga, Ge and Pb contents, and decreasing V, Sc and Ti contents are potential vectors toward concealed mineralized porphyries at Tongshankou. Our study highlights that the chlorite geochemistry is a potential tool for ore-fluid tracer and vectoring tool toward concealed skarn–porphyry orebodies in the MLYRB.