Tierra Group performed a technical peer review of the Isabella Pearl heap leach facility (HLF) owned by Gold Resource Corporation. Isabella Pearl mine is located at the west end of the Santa Fe mining district in the Walker Lane gold belt about 6 miles northeast of Luning, Nevada, in Mineral County. The HLF will have a nominal throughput of 8,000 tons per day over a period of 15 months following of pre-production development and construction. Tierra Group performed the review with the following objectives:

• Peer review the existing engineering design and drawings to ensure the proposed engineering design is adequate and in conformity with accepted engineering practices;
• Identify opportunities to “optimize” the design to potentially reduce capital or operating costs; and
• Identify associated risks (regulatory, financial, construction schedule, etc.) with implementing optimization.

Tierra Group identified a number of optimization opportunities and provided design enhancement recommendations to potentially reduce capital or operation costs.

Expansion Pit Slope Review

Tierra Group conducted an expansion pit slope review that included a technical memorandum of site observations/investigation, a review of past and new data, a basic slope analyses that developed recommendations for slope angles that determined if the Pit Expansion would be stable, and provided suggestions for a slope monitoring program.

HLF Phase 2A Engineering Design Change (EDC)

Tierra Group prepared an EDC to increase the permitted storage height of the existing HLF. A geotechnical field investigation and stability analysis for the Phase 2A and 3 expansion were also completed. Analysis results guided the civil grading and engineering design for each phase.

Crimson Preliminary Pit Slope Design

Tierra Group recommended a field investigation program and geotechnical modeling for the new Crimson Pit near the existing Isabella Pearl Pit. Tierra Group also provided preliminary pit slope design parameters.

Civit Cat Diversion

Tierra Group completed a diversion design to route stormwater around a planned open pit expansion. Tierra Group conducted hydrologic analyses to determine peak flows due to the 500-year 24‑hour rainfall event and completed the diversion’s civil and hydraulic design for permitting and construction. Challenges included steep topography and limiting rock excavation.