EFFECTS OF KARST PROCESSES ON THE MINING OF GYPSM

The mineability of a gypsum deposit is highly dependent upon the presence or absence of groundwater and karst processes. The hydration of anhydrite to gypsum is related to structural and stratigraphic factors, as well as the availability of groundwater. Infiltration of water affects the quality and quantity and mineability of the gypsum. If the permeability is low a deposit may contain a significant proportion of anhydrite that may not be used to produce valuable products. If the permeability and availability of groundwater are high the deposit may have been subjected to karst processes that significantly affects the recoverability of gypsum reserves.

In surface mining, the mineability of gypsum may be affected by solution, erosion and staining. An irregular bedrock surface, varying from hummocks to cutter-and-pinnacles to erosional cutouts, may be formed. Solution enlarged fractures may be filled by soil, mud or organic material. The gypsum may be heavily stained by mud and/or iron oxide. Any of these conditions may significantly reduce the recoverable gypsum reserves.

In underground mining, the hydration pattern, mining geometry, accessibility by mining equipment and the potential for significant water inflows may be affected. In shallow underground gypsum mines, solution-enlarged fractures may intersect the workings allowing direct pathways for surface water inflows. This may lead to soil piping, the formation of surface sinkholes or poor mine roof conditions.

In deeper mines, groundwater may travel from more distant recharge zones through a combination of fractures and porous strata. Where groundwater infiltration has been low, the gypsum deposit may contain significant blocks of gypsum contaminated by anhydrite. Where groundwater is abundant, the gypsum interval may quickly thin to an unmineable thickness or be completely dissolved in the updip direction. Zones of solution collapse breccia may occur within the gypsum seam, reducing reserves and forming potential zones of poor roof and/or water inflows. Changes in mine geometry may result in the disruption of the location of haulageways, conveyor beltlines or other mining utilities as well as the loss of gypsum reserves.

Finally, any combination of the factors described above may be encountered in both surface and underground mining of gypsum.