Genesis of the Erdaohe skarn Pb-Zn-Ag deposit in the Great Hinggan Range, NE China: Evidence from geology, fluid inclusions, and H–O–S isotope systematics

Abstract

The large Erdaohe Pb-Zn-Ag deposit is an important and representative skarn deposit in the middle segment of the Great Hinggan Range. The Pb-Zn ore bodies are mainly hosted in and among different lithological layers of volcaniclastic and carbonate rocks. The Si–Ca interface with abrupt changes in rock chemical properties and the bedding detachment faults played important roles in controlling mineralization. The mineralization of the deposit can be divided into three stages: (1) pre-ore skarn stage (I); (2) syn-ore quartz–sulfide stage (II); and (3) post-ore quartz–carbonate stage (III). Here, we present a combined study of geology, fluid inclusions (FIs), laser Raman analysis and H–O–S isotopes to constrain the origin and evolution of ore-forming fluids and the source of ore-forming materials. Four types of FIs are identified in the Erdaohe deposit, including liquid-rich (L-type), vapor-rich (V-type), CO2 (C-type) and daughter mineral-bearing (S-type) FIs. The L-type, V-type, C-type and S-type FIs appear simultaneously in stages I and II, and their homogenization temperatures (Th) are similar, indicating that boiling of the ore-forming fluids occurred. From stages I to III, Th varies from 338 to 475 °C, 194 to 383 °C and 157 to 323 °C, with salinities ranging from 3.87 to 50.72 wt% NaCl equiv., 0.35 to 39.05 wt% NaCl equiv. and 0.18 to 6.88 wt% NaCl equiv., respectively. The main ore-forming fluid was a H2O–NaCl ± CO2 system. From stages I to III, the variations in δ18Ofluid range from −2.3 to 7.2‰, −8.8 to 1.0‰, and −13.6 to −3.8‰; the ranges of δD are −141 to −208‰, −133 to −161‰, and −126 to −142‰, respectively. H–O isotope studies reveal that the initial ore-forming fluid was mainly derived from magma and underwent intense degassing, followed by mixing with meteoric water. The δ34S values of sulfide minerals vary in a small range, are concentrated from 5.2 to 10.0‰, and are characteristic of deep-sourced magmatic sulfur. The high δ34S values in the Erdaohe deposit are probably the result of magmatic sulfur mixing with sulfur from the surrounding Duobaoshan Formation. The alteration style related to mineralization, combined with FIs, laser Raman analysis, and H–O–S isotopes, suggests that the Erdaohe Pb-Zn deposit is a typical skarn deposit associated with magmatic activity. The boiling of ore-forming fluids and mixing with meteoric water played crucial roles in ore precipitation.