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-1 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) CO2-bearing inclusions. Microthermometric measurements reveal that from stage I to III, the homogenization temperatures range from 317 to 262 degrees C, from 297 to 192 degrees C, and from 248 to 151 degrees 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 CO2-NaCl-H2O +/- N-2. Metallogenic fluorites yielded a Sm-Nd isochron age of 158 +/- 35 Ma. The delta S-34(V-CDT) values of sulfides range from -1.3 parts per thousand to 6.3 parts per thousand, 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.