Characteristics and formation mechanism of the veins in Mengxi porphyry Cu-Mo deposit, eastern Junggar, Xinjiang, China

The Mengxi porphyry Cu-Mo deposit located north of the Qiongheba granite in the Yiwu County, eastern Junggar, was characterized by the abundance of various ore-bearing veins and veinlets, such as quartz vein, quartz-sulfide vein and sulfide vein. On the basis of formation mechanics, veins can be classified into two types; Hydrofracture-filled vein and tectonic fracture-filled vein. According to dislocation along tectonic fractures, sliding fracture and thrusting fracture were recognized. The exposed veins were mainly quartz veins with big dip angles, and their distribution were controlled by faults. Statistic result of volume fraction of veins in three drill holes of the line 14 show that the ore-bearing fluid were concentrated along a low north-dipping zone in a depth between 100m and 400m, which was corresponded to the location of ore-bodies. In this fluid concentrated zone, veins and their corresponding fractures were conjugate, thrusting fracture with small dip angle plus sliding fracture with big dip angle. These conjugate fractures were interpreted as second-order fractures of a thrusting shear zone. This indicated that the ore-bearing fluids were filled along these second-order fractures. Veins in shallow (<100m in depth) and deep area (>400m in depth) had big dip angles and were characterized by low Cu-content. It was suggested that ore-forming fluids in the Mengxi porphyry Cu-Mo deposit possibly migrated upwards through hydrofracture in deep, and flowed laterally and upwards along the second-order fractures within the shear zone where sulfide precipitated to form orebody. Evidence that some veins containing minerals formed at relative higher temperature were cut by those formed at relative lower temperature suggested that it was possibly in uplifting setting when these veins were formed.