Molybdenite Re-Os isotope age of the Dahu Au-Mo deposit,Xiaoqinling and the Indosinian mineralization.

The northeastern Qinling Orogen is the largest Mo belt in the world,which hosts the well-known Xiaoqinling orogenictype gold field.The age and tectonic setting of the Mo and Au deposits in the northeast Qinling Orogen have been a long-time debated issue.Recently,Mo orebodies of quartz-vein type,with total Mo reserve being ranked as medium size,have been discovered in the deep extension of the Dahu orogenic-type Au deposit,Xiaoqinling gold field.Six molybdenite samples from the deposit yield Re-Os isotopic model ages ranging from 215.4±5.4 Ma to 255.6±9.6 Ms,an isochron age of 218±41Ma(2σ,MSWD=38),and a weighted average age of 234±18 Ma(2σ,MSWD=23).This shows that the Mo mineralization occurred in Indosinian Orogeny or Triassic period.Based on summarizing and synthesizing the available isotope ages,this paper suggests that in the Qinling Orogen a significant magmatism-mineralization event occurred during Indosinian Orogeny.This event,however,was modified or destroyed during Yanshanian Orogeny,and could still be survived or recorded in the northernmost area of the Qinling Orogen.