Abstract:

The Jinding ZnPb deposit has been generally considered to have formed from circulating basinal fluids in a relatively passive way, with fluid flow being controlled by structures and sedimentary facies, similar to many other sedimentshosted base metal deposits. However, several recent studies revealed the presence of sand injection structures, intrusive breccias, and hydraulic fractures in the open pit of the Jinding deposit and suggested that the deposit was formed from explosive release of overpressured fluids. This study reports new observations of fluid overpressure related structures from underground workings (Paomaping and Fengzishan), which show clearer crosscutting relationships than in the open pit. The observed structures include: （1) sand (± rock fragment) dikes injecting into fractures in solidified rocks；（2) sand (± rock fragment) bodies intruding into unconsolidated or semiconsolidated sediments；（3) disintegrated semiconsolidated sand bodies； and （4) veins and breccias formed from hydraulic fracturing of solidified rocks followed by cementation of hydrothermal minerals. The development of ore minerals (sphalerite) in the cement of the various clastic injection and hydraulic fractures indicate that these structures were formed at the same time as mineralization. The development of hydraulic fractures and breccias with random orientation indicates small differential stress during mineralization, which is different from the stress field with strong horizontal shortening prior to mineralization. Fluid flow velocity may have been up to more than 11 m/second based on calculations from the size of the fragments in the clastic dikes. The clastic injection and hydraulic fracturing structures are interpreted to have formed from explosive release of overpressured fluids, which may have been related to either magmatic intrusions at depth or seismic activities which episodically tapped an overpressured fluid reservoir. Because the clastic injection and hydraulic structures are genetically linked with the mineralizing fluid source, they can be used as a guide for mineral exploration.

Key words: hydrodynamics, clastic injection, sand dike, hydraulic breccia, hydraulic fractures, Jinding, fluid overpressure