Stockpile Characterization and Hydrogeochemical Seepage Modeling for Mine Closure

Mining regulations require mine materials characterization and post closure seepage quality and flow prediction.  Material samples can be collected from surface sampling and drilling returns for characterization purposes.  Data on constituent distribution and mobility within existing and future leach ore and waste rock stockpiles from several copper sulfide ore types were collected from two different mine sites.  The analyses conducted included the Meteoric Water Mobility Procedure (MWMP), Acid Base Accounting, whole rock x-ray fluorescence, x-ray diffraction, reflectance spectroscopy, kinetic testing and down hole geophysical and geochemical surveys.

These analyses show that sulfate, acidity, and other constituents are stored in waste piles in the form of jarosite and other sulfate minerals, which occur in significantly greater abundances in the stockpile materials as compared to fresh mine materials as a result of weathering, leaching and oxidation of pyrite.  Static and kinetic testing results show that the potential for constituent release from leach ore and waste rock stockpile materials does not appear to be correlated with stockpile depth and age, but is a function of mine material type.

The weathering and leaching processes that can mobilize and redistribute constituents within the stockpiles and affect water quality and slope stability after closure were also investigated at these mines through hydrogeochemical modeling.  The modeling results show that sulfate and other constituent concentrations and mass loading to surface and groundwater is highly dependent on material type, infiltration rates, and internal stockpile conditions which may be affected by the cover design.  Stockpile slope and cover stability was also investigated.  Acidic degradation stockpile slope stability and cover material is mitigated if alkaline materials are used in the cover design and reaction rates of minerals in the materials is slow.