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Geochemical Characterization and Source Loading Predictions

Geochemical Characterization and Source Loading Predictions

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Geochemical Characterization and Source Loading Predictions

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  1. Geochemical Characterization and Source Loading Predictions Bruce Mattson Lorax Environmental

  2. GEOCHEMICAL CHARACTERIZATION • Geochemical Studies conducted to minimize impact to the aquatic environment through characterization and Management • Evaluated Geochemical Characteristics of • Mine Rock, Tailings, and Road • Characterized Leaching behaviour of mine rock • Predicted source concentrations from exposed mine rock

  3. STUDY OBJECTIVES • Rationale is to obtain necessary geochemical information to develop mitigation and management plans • Identify materials with a high ML/ARD potential that require special management and identify the elemental constituents that may be leached and affect water quality • Detailed characterization of materials that are used for construction and mitigation purposes to ensure that these materials, that will be exposed in the weathering profile, do not contribute unacceptable elemental loads to mine drainage • Characterization designed to drive management • Iterative process • Management drives characterization

  4. WASTE ROCK SAMPLING RATIONALE • Static test samples selected to evaluate ARD characteristics of different lithologies, spatially through the AB and D Zone • Sample frequency was selected to delineate sufficient material with low ML/ARD potential to implement the proposed management plan • AB Zone – Hanging wall n=52, Footwall n=54, Ore Zone n=5 • D Zone – Hanging wall n = 35, Footwall n = 15 • 50 Additional samples in D Zone and AB Zone hanging wall • A lower sampling frequency was used to characterize materials in the West Zone underground as management plans were developed to handle this quantity of material • Placement in core of AB Dump (AB PAG) • Temporary storage and rehandling underground (D Zone – West Zone) • It is acknowledged that non-PAG material will comprise a portion of the material defined as PAG in the management plan

  5. STATIC TEST METHODS • ARD analytical methods provided Geochemical Characterization and Source Loading Study - Appendix E. • Total S, HCl sulphate S, HNO3 sulphide S, Inorganic C, NP, paste pH, and leachable metals (Aqua regia leach) • Aqua regia leach method is widely applied method in environmental studies (noted in BC ARD Guidelines) to examine extractable rather than total element concentrations, to evaluate the mobility of heavy toxic metals and their bioavailability. • Provides the appropriate precision for inter-sample comparison. • Residual element fractions that are not released by aqua regia digestion would not weather. • “MEND NP” refers to modified Sobek NP method with addition of hydrogen peroxide following filtration of HCl leachate

  6. SULPHUR CONTENT AB Zone • Sulphur content of hanging wall waste rock < 1%S • Acid potential calculated by difference of S total and HCl sulphate • Provides a conservative estimate of acid potential • Over estimates for sediments D Zone West Zone

  7. ARD POTENTIAL AB ZONE • Non Acid Generating Granodiorite with low leaching potential in hanging wall of AB Zone • Material in 10 – 30 m interval above AB zone included as PAG for waste management calculations (includes mafic subvolcanic)

  8. ARD POTENTIAL D ZONE • Non-acid generating mafic subvolcanic and granodiorite in the D Zone hanging wall • Predominance of potentially acid generating lithologies in footwall (east) • Clearly defined zones of waste rock in pit for effective operational segregation • Sampling maps of drill collars do not illustrate true sample location • Inclined boreholes • Review sample sections

  9. NEUTRALIZATIONPOTENTIAL (NP) AB Zone • Carbonate NP consistently lower than MEND NP indicates non-carbonate minerals contribute to measured NP • NP difference most pronounced in granodiorite samples • Kinetic test program indicates a portion of the non-carbonate NP source is available in granodiorite • Use of CaNP is considered conservative, does not include non-carbonate NP sources D Zone West Zone

  10. MINERALOGY • Mineral identification and quantification conducted using XRD with Rietveld refinement on major waste rock lithologies. • Optical examination on granodiorite, sediments and diabase samples • Calcite was the predominant carbonate mineral in all lithologies although dolomite identified in diabase • Pyrite is the predominant sulphide mineral • Chalcopyrite and pyrrhotite identified in granodiorite • Sphalerite and chalcopyrite observed in sediments • Additional optical mineralogy on carbonate and sulphides

  11. MINE WASTE MANAGEMENT • Lithology and ABA results were used to demarcate PAG from non-PAG rock in the hanging wall of AB and D Zones • Demarcation line typically lies 10 to 30 m above the mineralized zones – Easy Segregation • Non acid generating waste rock from hanging wall of AB Pit and D Pit account for ~ 40% of total waste rock • Waste rock mined from underground accounts for 4% of total waste rock and will be managed as potentially acid generating

  12. MINE WASTE MANAGEMENT • Non acid generating waste rock from hanging wall of AB Pit and D Pit - foundations, roads, dams and shell of AB Dump • Potentially acid generating waste rock will be stored: • In a frozen condition in the core of the AB Dump • Temporary storage on surface and final placement as backfill in underground mine workings • Collection and treatment of ore and temporary waste rock drainage • Cover reactive pit walls – tailings, waste rock and water • Large WMF (High Lake) for flow and loading attenuation

  13. HUMIDITY CELL TESTING • Eleven humidity cell tests were conducted to evaluate metal loading rates • Sample selection near or above 50 percentile for solid phase sulphur and metal concentrations • Metal loading predictions updated with leaching rates following at least 40 test cycles • Updated metal leaching rates consistent with original rates, Zn and Cu show increase

  14. HUMIDITY CELL TESTING • pH is variable during final weeks (consistent signature for cells) • Sulphate and Alkalinity do not have a corresponding increase • Granodiorite, mafic subvolcanic, mafic volcanic and diabase have low sulphate release • Felsic volcanic and sediment samples are the most reactive

  15. FIELD BIN TESTING • Five field bins constructed September 2005 • Leachate sample September 2007 following two years of leaching • Leachate concentrations used to increase predicted source concentrations of Co, Cu, Mo and Ni

  16. FIELD BIN SAMPLE COMPOSITION • Granodiorite and felsic volcanic field bin samples enriched in As, Cu, Ni and Se • Sediment field bin sample enriched in As and Ni • These anomalies are noted when evaluating leachate concentrations (As and Se not increased)

  17. SOURCE TERM CONCENTRATION PREDICTION • Source terms revised with loading data reported in Humidity Cell Test Results report (Lorax, 2007) • Source term base case – Waste Rock • loading derived from 5 m active freeze/thaw zone rather than 3 m reported in the DEIS • Incremental waste rock source terms were calculated for active layers of 10, 15 and 20 m • Predicted source term scrutinized with respect to 36 parameters field bin leachate concentrations

  18. WASTE ROCK SOURCE TERM • Source loads weighted using waste rock tonnage from each major lithologic unit • Apply loading rates (mg/kg/wk) from humidity cells for the major stratigraphic units • Adjust for the particle size and the effect of incomplete flushing in dump (next slide) • Calculate porewater concentrations from monthly net infiltration into the dumps applied to the total available loads • Equilibrium with atmospheric CO2(g) and O2(g); • If solutions are supersaturated, allow precipitation of: Al4(OH)10SO4, barite, calcite, ferrihydrite, gypsum and rhodochrosite; and adsorption of trace metals in solution onto freshly precipitated ferrihydrite • Adjust predictions based on field bin data

  19. Coarse and fine, horizontal and inclined layers Unsaturated flow preferentially occurs in finest grained layers (< % of dump) Under extreme precipitation conditions, preferential flow channelling in coarse conduits (<<<% of dump) Complete rinsing of waste rock does not occur in arid environments WASTE DUMP FLUSHING

  20. PIT WALL SOURCE TERM • Loading calculated for: scenario 1 – total pit scenario 2- pit above discharge elevation • AB Pit rapid fill with tailings • D Pit natural fill with contingency for tailings fill • Calculate rinse-water concentrations from monthly net infiltration • Pit wall source terms assume 80% of load rinses from wall • Secondary mineral controls on the calculated water chemistry

  21. ROAD SAMPLING • Initial sampling of potential rock quarries for road construction completed for EIS • It is recognized that additional characterization and risk assessment will be required along road alignment • Quarry site selection will include geochemical considerations and be sited accordingly to avoid ML/ARD issues • Due to the ability to move quarry locations detailed sample characterization associated with road would be appropriate as a Water and/or Land Use Licence requirement

  22. OPERATIONAL MONITORING • Objective • gather additional data to guide management of mine waste during mining and post-closure • confirm the original characterization work completed at the site. • ML/ARD operational monitoring of the EB Pit waste rock will include sampling on dumps during deposition. • ML/ARD sampling of ultimate pit walls as the final pit walls are exposed. • A subset of samples for QA/QC • Field leach bins will continue to operate at site • Leachate collected and reported on an annual basis • Seepage surveys from the toe of waste rock dumps and pit waters

  23. SUMMARY • Used conservative methods for determining both acid generating potential (NP/AP) • Used conservative methods for predicting drainage chemistry (sample selection and methods) • Established that there are sufficient quantities with low metal leaching potential to implement appropriate management and mitigation