A simple and accurate method for Ore Reserve estimation and Mineral Resource depletion in caving mines. Alex Campbell

Transcription

1 A simple and accurate method for Ore Reserve estimation and Mineral Resource depletion in caving mines Alex Campbell

2 Introduction Mineral Resource and Ore Reserve estimation is an important part of corporate reporting Estimation requires a number of assumptions and Modifying Factors Caving mines have a number of unique challenges (i.e. cave flow and mixing, high dilution and ore loss) The purpose of the paper is to provide- A method for Ore Reserve estimation in caving mines A system for Mineral Resource depletion An approach to handle dilution and inferred material A worked example 2

3 Presentation Overview This presentation will discuss- The Ernest Henry Mine Estimation requirements and JORC Estimation challenges in caving mines Tracking cave flow and recovered material for reporting purposes Mineral Resource depletion 3

4 Ernest Henry Mine Location Located 38km North East of Cloncurry Copper and gold deposit Open pit completed in 2011 First underground production in December 2012 Mine life to 2026 SLC

5 Ernest Henry Mine Overview Pit is 1.5km x 1.3km across and 530m deep Underground has a planned depth of 1km Ore body inclined at 45 degrees Sublevel cave mining method Complex transition mining around the pit Transverse retreat from hanging wall to footwall 3 Mt / pa when trucking during mine build 6 Mt / pa once hoisting in 2014 Increase to 6.8 Mt / pa in 2017 Open pit Transition zone Planned SLC Shaft access Hoisting Shaft Crusher precinct Conveyor incline Shaft load out

6 Definitions and JORC (2012) A Mineral Resource is a concentration or occurrence of solid material of economic interest.. in such form, grade (or quality), and quantity that there are reasonable prospects for eventual economic extraction (JORC, 2012) Ore Reserves are the economically exploitable portions of the Mineral Resource based on a set of assumptions (Baldwin et al, 2014) Dilution is the material drawn from a SLC drawpoint which has originated from outside of a blasted ring and may or may not comprise of mineralised material (Power, 2004) 6

7 Ore Reserve estimation challenges in caving mines Material flow and mixing Long travel distances Tracking of cave stocks Ore recovery over multiple levels Dilution reporting Quantifying ore loss in the cave Resource depletion Difficulty attributing Mineral Resource properties to recovered ore to estimate the Ore Reserve This forces some mines to use of broad brush assumptions for dilution and recovery 7

8 What do other mines do? Flow models used onsite or by consultants (PGCA, PCBC, ReBOP, CaveSim) Flow models to determine Modifying Factors and estimate Ore Reserves Models validated using mill reconciliation and sampling data Most mines do not track Mineral Resource categories of cave material Each mine surveyed used slightly different methods Limited published information North Parkes block cave (House and Secis, 1997) Ridgeway sublevel cave (Smart and O Sullivan, 2007) 8

9 Requirements of the Ore Reserve estimation methodology A single system for tonnes and grade reporting, Ore Reserve estimation and Mineral Resource depletion Avoid use of broad brush Modifying Factors for dilution and ore loss Track and report diluting material and ore loss in the cave Calculate Mineral Resource classification of material recovered in each drawpoint Calculation of cave stocks Repeatable and not a black box Accurate, simple, transparent 9

10 Flow Model Overview Flow modelling conducted using PGCA software Cellular automata solver Capable of simulating different particle properties / flow rates Tonnes and grade forecasting Used for draw strategy optimisation Facing West

11 Flow model results Recovered copper grade (%) Pit Net smelter return per tonne* High Low Lower half of the SLC not shown * Values are relative and not indicative of actual mine economics 11

12 Flow Model Reconciliation Metal and grade forecasts +/- 10% month on month and within ~2% on an annual basis 12

13 How does the method work? Development is pre-depleted from the model to prevent double counting Ore Reserves from development are estimated separately Recovered ore is reported for each ring (or drawpoint) Mineral Resource classification tracked for each block (Inferred, Indicated, Measured) Grade tracked for each Mineral Resource classification for each block 13

14 Example Block model set-up X Y Z Cu Grade (%) Resource Classification (Binary code) Cu Grade (%) by Classification External Inferred Indicated Measured External Inferred Indicated Measured

15 X-section Facing West Facing North 15

16 Example Block model set-up X Y Z Cu Grade (%) Resource Classification (Binary code) Cu Grade (%) by Classification External Inferred Indicated Measured External Inferred Indicated Measured Flow model output (Recovered material) Ring ID Tonnes Cu Metal Tonnes Cu Grade (%) External Tonnage (%) Inferred Tonnage (%) Indicated Tonnage (%) Measured Tonnage (%) External Tonnes Inferred Tonnes Indicated Tonnes Measured Tonnes Ring % 0% 25% 59% Ring % 0% 79% 7% Similar process for grade / metal Probable Reserve Proved Reserve 16

17 Reporting dilution and inferred material Section 29 of JORC (2012) states that diluting material should be included as part of the Ore Reserve estimate (regardless of Mineral Resource classification) Mine design targets recovery of Indicated and Measured Resource Dilution is unavoidable and mixes with ore Dilution may contain Inferred material (and metal) Two parts of JORC could be conflicting in the context of caving? Not all peer reviewers agreed Q- Should Inferred (diluting) material be included in the Ore Reserve? 17

18 Reporting dilution and inferred material The Ernest Henry Resource does not contain Inferred material in proximity to the SLC The following conclusions were drawn- Inferred portion of the cave dilution should be included in the Ore Reserve estimate This material recovery is unavoidable and mixes with Indicated and Measured material Inclusion or exclusion of Inferred diluting material should be specified The mine should not be designed to target Inferred material The tonnes and grade of each Mineral Resource classification should be specified in the Reserve statement Transparency is critical 18

19 Example - Reporting dilution and inferred material Material recovered as ore may contain all Mineral Resource classifications Previous methods No method to track the Resource classification of recovered ore Unable to (accurately) specify the Mineral Resource classification of the Ore Reserve Easy to unknowingly include Inferred material in the Ore Reserve estimate Cut-off grade boundary Fictitious orebody 0.7% cut-off grade Flow model shutoff grade of 0.6% Copper Grade 1% 0.75% 0.5% 0.25% Waste Orebody X-section 19

20 Example - Reporting dilution and inferred material Material recovered as ore may contain all Mineral Resource classifications Previous methods No method to track the Resource classification of recovered ore Unable to (accurately) specify the Mineral Resource classification of the Ore Reserve Easy to unknowingly include Inferred material in the Ore Reserve estimate Mineral Resource classification Cut-off grade boundary Measured Indicated Inferred Unclassified Surface /pit fines Orebody X-section 20

21 Mineral Resource depletion Flow model simulation follows the mine schedule Can estimate depletion / cave stocks at any stage of the mine life Can generate a residual block model at any mining step Insitu ore + Cave stocks Planned recovery = + Dilution Ore loss 21

22 Conclusions A simple method for Mineral Resource depletion Transparent and repeatable method for Ore Reserve estimation Method can be used in any flow model software (PGCA, PCBC, ReBOP, CaveSIM) Can be applied to any caving mine (block cave, sublevel cave, panel cave) Used as part of the A$880 million transaction with Evolution 22

23 Acknowledgements Anthony Allman Alice Clark Geoff Dunstan Lino Manca Gavin Power Peter Stoker Mike Thomas Glencore Copper 23

24 Thank you