Structural and lithological controls on the evolution of ore-forming fluids during the formation of the Chaihulanzi gold deposit, NE China

Abstract

The Chaihulanzi gold deposit, located along the northern margin of the North China Craton, is an example of detecting the controls on the complexity of orogenic gold systems. This deposit consists of a series of gold lodes hosted in graphite-schist, gneiss, and porphyrite and is controlled by NW- to NWW-directed faults. The graphite-schist-hosted gold lodes contain pyrite-arsenopyrite-dominated polymetallic sulfides, whereas the gneiss- and porphyrite-hosted lodes are characterized by pyrite-pyrrhotite-dominated polymetallic sulfides. Despite variations in ore-minerals, these lodes exhibit similar quartz-sericite ± carbonate-chlorite alteration. In the dilatational faults, the δ³⁴S values of pyrite from graphite-schist-hosted lodes from + 0.4 to + 6.5 ‰, whereas pyrite from gneiss-hosted lodes have δ³⁴S values ranging from −3.5 to + 8.5 ‰. In the compressional faults, pyrite from porphyrite-hosted lodes shows δ³⁴S values between + 2.3 to + 4.4 ‰. The broader range of δ³⁴S values from dilatational-lodes compared to compressional-lodes can be attributed to varying fluid-rock interactions and fluid pressure fluctuations. The contrasting ore minerals and sulfur isotope are linked to the interaction of ore-forming fluids with different host rocks in complex structural and lithological settings. Specifically, the relatively positive δ³⁴S in pyrite from graphite-schist-hosted lodes may result from the fluid reduction due to reaction with graphite-schist. The Pb isotope of pyrite with ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb, and ²⁰⁸Pb/²⁰⁴Pb ratios of 18.51–22.13, 15.69–16.54, and 37.92–43.59, respectively, support the hypothesis of Pb contamination of the host rocks. This study underscores the detailed influence of structural settings and lithologies on the complexity of ore-forming fluid compositions and provides valuable insights for guiding regional exploration.