Triassic gold-silver metallogenesis in Qingchengzi orefield, North China Craton: Perspective from fluid inclusions, REE and H–O–S–Pb isotope systematics

Abstract

The Qingchengzi Au–Ag orefield is a key Au province in the North China Craton. The fault-controlled orebodies are hosted in the Paleoproterozoic meta-sedimentary rocks of the Dashiqiao and Gaixian Formations. We present new bulk-ore REE, fluid inclusion (FI) microthermometric and mineral H–O–S–Pb isotopic data of these Au–Ag deposits, and discuss their ore fluid/material source and metallogenesis. Our FI microthermometric data indicate that the ore fluids are of medium temperatures (260–310 °C), low salinity (≤10 wt% NaCl eq.), and belonged to a H2O–CO2–NaCl system. Hydrogen (δD = –6.5 to 9.1‰) and oxygen (δ18O = –139.4 to –90.1‰) isotopes of the hydrothermal quartz indicate that the early-stage ore-forming fluids were mainly magmatic sourced. The δ34S values (–12.5 to 18.7‰) of the ore sulfides overlap with the Paleoproterozoic ore host (δ34S = –0.5 to 13.2‰) and the Late Triassic granites (δ34S = 5.6–7.6‰). Lead isotope compositions of the ore sulfides (206Pb/204Pb = 17.269–18.334; 207Pb/204Pb = 15.335–15.883; 208Pb/204Pb = 37.254–38.784) also support a metallogenic link with the granites and wall-rocks. Therefore, we suggest that the Qingchengzi Au–Ag deposits are best classified as magmatic-related medium-temperature hydrothermal lode type, albeit spatial correlations with magmatic intrusions are not ubiquitous. Closer to the hydrothermal centers, the higher-temperature, magmatic-dominated fluids were responsible to generate the Au(-Ag) mineralization, whereas the more distal, lower-temperature magmatic-meteoric-mixed fluids may have formed the Ag and Pb–Zn mineralization. The ore-forming fluids and materials were likely sourced mainly from the Triassic granitoids, although significant contributions of ore metals and sulfur from the Au-/Ag-rich Dashiqiao and Gaixian Formations should not be neglected. Considering the regional tectonic setting, the Late Triassic Qingchengzi Au–Ag polymetallic mineralization may have occurred during the post-collisional extension after the Yangtze-North China continent–continent collision.