Geochronology and Ore Genesis of the Niujuan-Yingfang Pb-Zn-Ag Deposit in Fengning, Northern North China Craton: Constraints from Fluid Inclusions, H-O-S Isotopes and Fluorite Sr-Nd Isotopes

The Niujuan-Yingfang Pb-Zn-Ag deposit in northern North China Craton (NCC) is hosted at the contact zone between Permian biotite monzogranite and Hongqiyingzi Group migmatitic gneiss. The orebodies are structurally controlled by NE-trending F fault. Mineralization can be divided into three stages: (1) siliceous-chlorite-pyrite stage, (2) quartz-Ag-base metal stage, and (3) fluoritecalcite stage. Four types of fluid inclusions were identified, including: (1) liquid-rich aqueous inclusions, (2) vapor-rich inclusions, (3) liquid-rich, solid-bearing inclusions, and (4) CO-bearing inclusions. Microthermometric measurements reveal that from stage I to III, the homogenization temperatures range from 317 to 262 °C, from 297 to 192 °C, and from 248 to 151 °C, respectively, and the fluid salinities are in the ranges from 1.1 wt.% to 6.5 wt.%, 1.2 wt.% to 6.0 wt.% and 0.7 wt.% to 4.0 wt.% NaCl equivalents, respectively. Fluid boiling and cooling are the two important mechanisms for ore precipitation according to microthermometric data, and fluid-rock interaction is also indispensable. Laser Raman spectroscopic analyses indicate the fluid system of the deposit is composed of CO-NaCl-HO±N. Metallogenic fluorites yielded a Sm-Nd isochron age of 158±35 Ma. The δS values of sulfides range from −1.3‰ to 6.3‰, suggesting that the sulfur may be inherited from the basement metamorphic igneous rocks. Hydrogen and oxygen isotopic compositions of quartz indicate a metamorphic origin for the ore-forming fluid, and the proportion of meteoric water increased during the ore-forming processes. Sr-Nd isotopes of fluorites show a crustal source for the ore-forming fluid, with primary metamorphic fluid mixed with meteoric water during ascent to lower crustal levels. Combined with the geological, metallogenic epoch, fluid inclusions, H-O-S and Sr-Nd isotopes characteristics of the deposit, we suggest that the Niujuan-Yingfang deposit belongs to the medium-low temperature hydrothermal vein-type Pb-Zn-Ag polymetallic deposit, with ore-forming fluids dominantly originated from metamorphic fluids.