Constraints of Hydrothermal Zircon U-Pb Ages on the Metallogenic Epoch of the Xishimen Skarn Iron Deposit, Sourthern Hebei Province

Xishimen iron deposit is one of the typical skarn iron deposits in northern China. The new ⁴⁰Ar/^<sup>39Ar mineralization ages of phlogopite in Xishimen iron deposit are 133.1±1.3 Ma and 137.1±1.5 Ma, slightly older than the ages (126-136 Ma) of mineralization parent rocks (Wu'an complex), which is unreasonable. This paper deals with the analyses of geochronology of the metallogenic event on the basis of LA-ICP-MS zircons U-Pb dating for hornblende syenite and skarn dikes developed in this area. The zircons of hornblende syenite are light yellow-colorless, transparent, angular and irregular with weak oscillatory zonings, and a few zircons have irregular inherited zircon core. LA-ICP-MS U-Pb analyses of these zircons yields a weighted mean ²⁰⁶Pb/²³⁸U concordia age of 135.6±1.5 Ma, which represents the crystallization age of the Wu'an complex. Zircons of skarn dikes are brown, transparent to translucent. In contrast to magmatic zircons, hydrothermal zircons have low Th, U, total REE contents (Th=222.37×10^-6-1 541.11×10^-6, U=218.44×10^-6-989.17×10^-6) and Th/U value (Th/U=0.90-1.56). They display a strong positive Ce anomaly(δCe=4.46-196.22) and negative Eu anomaly(δEu=0.59-0.80). Zircons from skarn dikes yielded ²⁰⁶Pb/²³⁸U ages ranging from 124.9 Ma to 133.0 Ma with a weighted mean age of 129.4±2.6 Ma, which is interpreted as the age of crystallization of hydrothermal zircons, showing that 129 Ma is the chief metallogenic period in Xishimen area.