Characteristics of karst development and water abundance of the Beiya Gold Deposit in Heqing county of Yunnan

The Beiya Gold Deposit is a typical karst water-filled deposit. As the hydrogeological conditions and water environment are expected to be changed by future deep mining, the water inrush risk will increase if the roadway is exposed or connected with the karst zone, the underground river and Guochang river. In this study, comparing the field investigation of regional hydrogeology and the geological exploration data with the actual production, we analyze the karst development and water abundance characteristics from the aspects of lithology, structure, landform, etc. The study results show as follows. (1) The karst forms mainly include stone forest, drainage cave, karst depression, solubilization trough, solubilization gap, karst cave and underground river. (2) Lithologically, karst in the second segment of the Middle Triassic Beiya Formation is the most developed, with the purest fine crystalline limestone containing 41.52%–54.77% of CaO. Karst development in the first segment of the Beiya Formation is restricted by limestone and siltstone. Karst is developed in the contact zone on the side of the second segment, while not developed on the side of the first segment. (3) Karst development is mainly controlled by structure, and the direction of karst development is consistent with the NS, EW and NW tectonic lines. Karst is more developed in the fold axis and the warped end, and is relatively developed in the zone affected by fault and the contact zone of rock mass. (4) Karst development shows the characteristics of vertical zonation. Funnels and sink holes are developed at the level of 2,500 m to 3,500 m, grikes and solution grooves at 1,800 m to 2,500 m. Slope valleys, karst caves and underground rivers are developed at the level of 1,694 m, while karst is still relatively developed below the base level of erosion. The lowest control elevation of the cave is 1,479.54 m, and the depth of karst development below the base level of erosion is more than 150 m. (5) The karst water abundance shows its synclinal confluence pattern. The Beiya synclinal water storage structure plays a key role in karst water enrichment. The zone affected by fault and the contact zone between rock mass and its surrounding rock are also conducive to groundwater migration and enrichment. (6) The spatial variation of water abundance in karst aquifer is large. From rock mass as the center to its outward direction, radial water abundance and permeability can be listed in the order of weak-medium-strong or extremely strong. The rock mass and its proximal annular ore-bearing alteration zone shows weak-medium water abundance, while the distal end shows medium-strong water abundance. Some parts of the area are extremely strong in water abundance. (7) The underground river is developed along the axis of the Beiya syncline at the level of 1,740 m to 1,694 m. The Beiya Formation is composed of sandy limestone. The possibility of karst cave or water inrush of the underground river will increase if the mining passes through the fault zone and the syncline core.