Slide 1: SGP: Tailings Storage Facility
S G P : TA I L I N G S S T O R A G E FA CI L I T Y May 2021 TSX :P P TA N A SDA Q:P P TA PERPETUA RESOURCES C r i t i c a l R e s o u r c e s . R e s p o n s i b l e M i n i n g . E n v i r o n m e n t a l R e s t o ra t i o n . www.perpetuaresou rc e s.c om
Slide 2: Presentation Outline
PRESENTATION OUTLINE 1. Stibnite Gold Project site overview. 2. What are tailings, and what does “managing” them mean? 3. How do we understand and manage a tailings facility throughout its lifecycle? 4. Stibnite Gold Project tailings storage facility (TSF) design.
Slide 3: Stibnite Gold Project
STIBNITE GOLD PROJECT Coeur d’Alene Stibnite Gold Project Perpetua Resources Au-Sb 3
Slide 4: Historical Stibnite Mining District
1890s The Thunder Mountain gold rush brings mining to the area 1900-1930 The town of Stibnite is established HISTORICAL STIBNITE 1938 Mining at Yellow Pine pit stops salmon migration upstream 1941-1950 The town of Stibnite booms when antimony & tungsten were declared critical & strategic minerals MI
Slide 5: Legacy Environmental Impacts
YELLOW PINE PIT HISTORICAL The East Fork of the South Fork dumps into a legacy mine pit. Currently, ~80 feet of sediment has collected at the bottom LEGACY FISH PASSAGE Fish migration After 100+ years of mining activity, many environmental legacies remain. is blocked by the Yellow TAILINGS & SPENT O
Slide 6: Tailings Risk Management Introduction
TAILINGS RISK MANAGEMENT INTRODUCTION 6
Slide 7: Tailings Lifecycle
TAILINGS LIFECYCLE TAILINGS: the materials left over after separating the valuable fraction from the uneconomic fraction of an SITE ore – typically a mixture of sand- and silt-size rock TEAM CHARACTERISTICS particles and water. Four areas comprise the lifecycle management of a LIFECYCLE Tailings Sto
Slide 8: Risk Minimization
RISK MINIMIZATION Goal of Tailings Management Team = minimize catastrophic and chronic risk No “one size fits all” in tailings management. The mining industry operates lots of different types of safe tailings storage facilities (TSFs).
Slide 9: Design Types: Methods of Storing Tailings
DESIGN TYPES Other methods include: DISCHARGED TO STREAM / RIVER / OCEAN Methods of Storing Tailings Vary RETENTION BY EMBANKMENT STORAGE IN A LAKE Most common, and proposed for SGP IN-PIT STORAGE SGP: Stibnite Gold Project
Slide 10: Design Types: Liners
DESIGN TYPES Liners Can Help Protect the Environment FULLY SYNTHETICALLY LINED Tailings characteristics: toxic versus non-toxic Lining types: composite, soil, none NO LINING
Slide 11: Design Types: Climate and Topography
DESIGN TYPES Consider Climate and Topography MINIMAL CATCHMENT, PADDOCK CONFIGURATION, COLD / WET CLIMATE LARGE CATCHMENT, VALLEY CONFIGURATION, COLD / DRY CLIMATE
Slide 12: Team and Project Execution
TEAM AND PROJECT EXECUTION 12
Slide 13: Teams Matter
TEAMS MATTER ✓ Internal management team: experienced versus inexperienced ✓ External design team: experienced versus STIBNITE GOLD PROJECT TEAM inexperienced • Experienced internal, external, and 3 rd party ✓ Regulatory environment: experienced and review teams comprehensive versus inexperienced and
Slide 14: Construction & Ops Management
CONSTRUCTION & OPS MANAGEMENT Promote Best Practices and Reduce Risk ✓ Construction execution: owner built versus specialty contractor ✓ Construction management: none versus third-party experienced ✓ Instrumentation, surveys, and monitoring: none, rudimentary, extensive ✓ Reviews and audits: none ve
Slide 15: Site Characteristics and Design Considerations
SITE CHARACTERISTICS and DESIGN CONSIDERATIONS 15
Slide 16: TSF Configuration Considerations
TSF CONFIGURATION CONSIDERATIONS Key Aspects of Facility Design and Construction ✓ Embankment/containment type: downstream, upstream, centerline, no embankment ✓ Geotechnical design criteria: foundation conditions, static/dynamic analyses and factors of safety ✓ Embankment design: filter compatibili
Slide 17: Location Considerations
LOCATION CONSIDERATIONS Identifying the Best Location • Finite number of alternatives to store tailings at a given site • Process involves iterating between pit sizes, infrastructure layouts, and development rock storage facility locations • Many considerations involved in establishing the preferred
Slide 18: Tailings Storage Site
TAILINGS STORAGE SITE Selected TSF Site: • Readily buttressed for stability • Lowest elevation • 2nd least impact on fish access • 2nd smallest embankment • Most prior disturbance • Half the stream disturbance of 2 nd place contender • Reprocess/reuse legacy waste Buttress TSF Site 45% of proposed p
Slide 19: Legacy Spent Ore Mitigation
LEGACY SPENT ORE MITIGATION DISPOSAL AREA Older tailings (beige) were covered with spent heap (1980s) leach ore (brown) after being deposited, unlined, in the Meadow Creek valley. TODAY Revegetation attempts have been made; however, legacy materials continue to degrade water quality and leach metals
Slide 20: Site Characteristics: Geology and Soil Types
SITE CHARACTERISTICS Geology and Soil Types Inform Design Foundation geotechnical characteristics: dense/competent/uniform versus loose/liquefiable/nonuniform Construction material availability: abundant broad range of construction materials versus high degree of effort needed to manufacture materia
Slide 21: Site Characteristics: Well-studied Soils and Geology
SITE CHARACTERISTICS Well-studied Soils and Geology Glacial, fluvial, talus + mostly granitic rock → variety of competent construction materials, modest processing 99 holes, 8 years, 3 drilling methods + lab and modeling work → well understood foundations for TSF/Buttress
Slide 22: Site Characteristics: Seismicity and Geohazards
SITE CHARACTERISTICS Seismicity and Geohazards Recent Idaho quake was ~1/6th the strength of SGP TSF embankment design event. SGP site-specific study update in progress; no design changes anticipated. Seismicity: high seismicity and poorly understood versus low seismicity and well understood Topogra
Slide 23: Site Characteristics
SITE CHARACTERISTICS Climate: wet/dry, cold/hot, windy/calm Hydrology: large catchment versus paddock Hydrogeology: depth to groundwater, flow patterns Vegetation, wetlands, soils, streams, seeps STIBNITE • Seasonally wet; cold snowy winters/dry summers • Potential TSF site catchments a few square m
Slide 24: Water Recovery and Management Elements
At Stibnite DESIGN TYPES Need all listed elements Water Recovery and Management Elements Water supply, recycled water (reclaim) recovery, underdrains, stream diversions Water recovery (reclaim) is a key operational practice, potentially implemented several ways
Slide 25: Water Balance and Sizing
DESIGN CONSIDERATIONS Water Balance and Sizing of Water Management Elements • Hydrologic avg. recurrence intervals for diversions/spillways • Store vs. pass storms • Water supply and disposal of excess • Pool volume and freeboard confirmation
Slide 26: Design Considerations: Tailings Characteristics
DESIGN CONSIDERATIONS Tailings Characteristics Tailings physical and chemical characteristics: coarse, rapidly consolidating, benign versus fine, poorly consolidating, high toxicity
Slide 27: Design Considerations: Consolidation
DESIGN CONSIDERATIONS Tailings Characteristics Consolidation Tailings settle after deposition, increasing in density and strength and expelling water. Some factors affecting consolidation: • Grind (gradation) • Rise rate • Dewatering method • Pool management and additives • Drainage condition • Tran
Slide 28: Slurry Dewatering and Tailings Deposition Method
DESIGN TYPES THICKENED Slurry Dewatering and Tailings Deposition Method CONVENTIONAL LOW-DENSITY TAILINGS FILTERED “DRY STACK” PASTE
Slide 29: Stibnite Tailings Consolidation
STIBNITE TAILINGS CONSOLIDATION ✓ Tested pilot tailings with SICT ✓ Analyzed with SICTA; modeled in both FS Consol and CONDES (steady-state normal consolidation) ✓ Design curve from average of representative LOM tailings, assuming impervious base (i.e., no drainage/wicking – conservative for capacit
Slide 30: Stibnite Tailings Characteristics – A Manageable Risk
STIBNITE Tailings Characteristics – A Manageable Risk Physical: • Approx. 120 million tons over 14.25-year mill life • Grind to P80 ~85 microns / ranging from fine sand through minimal clay • 85-90% flotation tailings, 10-15% pressure oxidation (POX) tailings • Grind + bulk are from flotation + lime
Slide 31: Typical Water Balance Analysis Results
DESIGN CONSIDERATIONS Typical Water Balance Analysis Results • Pool volume • Makeup water requirements • Excess water disposal • Schedule (in)sensitivity to consolidation performance
Slide 32: Geotechnical Factor of Safety (FOS)
TSF CONFIGURATION CONSIDERATIONS Geotechnical Factor of Safety (FOS) FOS = Ratio of design strength to stresses placed on a structure = Ratio of stabilizing forces to destabilizing forces. Evaluated for a variety of potential failure locations/directions and loading conditions Stabilizing: Destabili
Slide 33: Geotechnical Factor of Safety: Chair Example
TSF CONFIGURATION CONSIDERATIONS Geotechnical Factor of Safety (FOS) FOS = Ratio of design strength to stresses placed on a structure. Simpler approach – building a chair. Stabilizing / Strength: Destabilizing / Stress: • Strength of wooden legs (4? 2?) • Self-weight of chair (ignore for now) • Stre
Slide 34: Closure Considerations
DESIGN CONSIDERATIONS Closure Considerations ✓ Proactive/walk-away versus reactive/active long-term management ✓ Can evolve in sequence/phases
Slide 35: Stibnite TSF Design
STIBNITE TSF DESIGN 35
Slide 36: SGP Tailings Facility Design Goes Beyond Idaho State Standards
SGP TAILINGS FACILITY DESIGN Goes Beyond Idaho State Standards Driven by 4 State Rules, and meets or exceeds many others: STABILITY: 1. Cyanidation (IDEQ) – Static FOS > 1.5 [standalone – unbuttressed] 2. Dam Safety (IDWR) 3. Tailings Impoundments (IDWR) – Pseudostatic (seismic) FOS > 1.0 [standalon
Slide 37: SGP TSF Design in Context
SGP TSF DESIGN IN CONTEXT Increasing Factor of Safety Dramatically Reduces Risks FOS FOS is not simply the inverse of probability of failure. Increasing FOS can reduce failure probability by 1,000’s SGP TSF: 1 in 10 million Graphic Source: Oboni & Oboni 2020. The Factor of Safety and Probability of
Slide 38: ModPRO2 Design: Starter Facility Slope Stability
SGP TAILINGS FACILITY DESIGN ModPRO2 Design Slope Stability Analysis – Starter (Stage 1) Facility • Analyzed hypothetical standalone dam (buttress placement begins before tailings deposition) • Approx. 250’ tall • Exceeds minimum static FOS 1.5 • Water level far below crest during limited unbuttress
Slide 39: ModPRO2 Design: Final Facility Slope Stability
SGP TAILINGS FACILITY DESIGN ModPRO2 design: Slope Stability Analysis – Final facility FOS= Pool – Standalone Dam, FOS = 1.8 (hypothetical only) FOS= Pool – Buttressed, FOS = 5.9 (actual condition at full buildout) Meets or Exceeds Regulatory Standards Rock Buttress At Least Doubles Req’d FOS (Stati
Slide 40: TSF/Buttress Design Configuration
TSF/BUTTRESS DESIGN CONFIGURATION ✓ Rockfill with downstream raises to maintain freeboard ✓ Composite-lined ✓ Surface water diverted ✓ Underdrains ✓ Over-liner drainage/pore pressure relief TSF: Tailings Storage Facility 40
Slide 41: TSF/Buttress Expansion Phasing
TSF/BUTTRESS EXPANSION PHASING Stability and Flexibility in Mine Planning and Water Management • Lined TSF expanded to keep pace with ore processing • Design freeboard at “just-full” end of phase. Much more before that. • Buttress expanded to receive rock from pits; far faster than needed for embank
Slide 42: TSF/Buttress Expansion Phasing
TSF/BUTTRESS EXPANSION PHASING Stability and Flexibility in Mine Planning and Water Management • Trivial pool (10’s of feet) relative to embankment height • Bulk of embankment shell essentially complete in year 5 to final elevation, with substantial buttress. • Post-Year 5 expansions of TSF related
Slide 43: SGP Tailings Facility Closure
SGP TAILINGS FACILITY CLOSURE Designed to Become Permanent River and Wetland Habitat ✓ Restore Meadow Creek and wetlands ✓ Allow and accommodate natural processes (migration, avulsion, scour, deposition; including alluvial fans). ✓ Protect water quality via isolation from tailings and active and pas
Slide 44: Stibnite TSF Risk Profile (Pre-Mitigation)
STIBNITE TSF RISK PROFILE (PRE-MITIGATION) Internal Management Team External Design Team Team Regulatory Environment Review Team Team Continuity Climate Hydrology Site Hydrogeology Foundation Characteristics Characteristics Seismicity Risk Mitigation Measures: Geohazards Topography • Freeboard Const
Slide 45: Tailings Safety
TAILINGS SAFETY No Known Failures of Facilities With These Design Characteristics STIBNITE GOLD PROJECT TAILINGS STORAGE FACILITY DESIGN CHARACTERISTICS: ✓ Downstream constructed Best Practice For Tailings Facility ✓ Engineered rockfill Design ✓ Full composite liner ✓ Reviewed by an independent expe
Slide 46: SGP Tailings Storage Facility Design
SGP TAILINGS STORAGE FACILITY DESIGN Designed for Water Quality Protection, Long-Term Stability, and Habitat Restoration ✓ Legacy material reuse and reprocessing ✓ Tailings neutralized and TSF fully lined for water quality protection; water treatment in closure ✓ Substantial rock buttress and robust
Slide 47: Webinars
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Slide 48: Thank You.
THANK YOU. w w w. p e r p e t u a r e s o u r c e s . c o m 48