Understanding Ore Heterogeneity: In-situ and in Broken Ore

1. Understanding Ore Heterogeneity:In-situ and in Broken Ore Dr Geoff Lyman Materials Sampling & Consulting Southport, Queensland Sampling Australia 2013 - May 14 - 16

2. Overview • What is heterogeneity? • How do we quantify heterogeneity? • How does it impact exploration? • How does it impact mining and beneficiation? • How does it impact analytical procedures? • How to control the impact • Tools to evaluate heterogeneity Sampling Australia 2013 - May 14 - 16

3. Heterogeneity • Heterogeneity must be defined with respect to a particular analyte • a mineral phase or an elemental concentration • In a broken ore: • particles vary in size, composition and density • it is the extent of variation in composition from one particle to the next that controls the extent of heterogeneity • we call this type of heterogeneity intrinsic heterogeneity Sampling Australia 2013 - May 14 - 16

4. Intrinsic Heterogeneity pure ‘other’ pure ‘target’ Size 1 Size 2 Size 3 Size 4 Size 5 Size 6 The extent of liberation of the target phase increases as particle size decreases and so does the heterogeneity Sampling Australia 2013 - May 14 - 16

5. Intrinsic Heterogeneity • Quantification of particulate heterogeneity • In statistics, we measure variability using the variance of a probability distribution Sampling Australia 2013 - May 14 - 16

6. Intrinsic Heterogeneity • Quantification of particulate heterogeneity • within a size fraction, we measure intrinsic heterogeneity much the same way, except we include particle density Sampling Australia 2013 - May 14 - 16

7. Intrinsic Heterogeneity • Quantification of particulate heterogeneity • For a sample as a whole, the intrinsic heterogeneity is characterised uniquely by a ‘sampling constant’, • The sampling constant has units of mass (grams) Sampling Australia 2013 - May 14 - 16

8. Intrinsic Heterogeneity • Quantification of particulate heterogeneity IH of the material in the size fraction mass fraction in the size fraction average volume of a particle in the size fraction The particle size effect is very important: it is usually the top 2 or 3 size fractions that control the value of the sampling constant Potential problem: dilution of ore by waste Sampling Australia 2013 - May 14 - 16

9. Intrinsic Heterogeneity • Quantification of particulate heterogeneity • The value of the sampling constant will vary from one analyte to the next • mineral phases present in low concentration generally have larger sampling constants • nuggetty gold is a clear example of this • the mineral phase with the highest sampling constant will usually control sampling protocol design, unless degraded precision for that phase can be tolerated Sampling Australia 2013 - May 14 - 16

10. Intrinsic Heterogeneity • Gy’s Simplified Formula • The simplified formula is • the critical factor is the determination of the liberation factor l. • The plot at the right shows that this can be a complex issue and that simplifications can misleading data of Pedro Carrasco (Codelco) Sampling Australia 2013 - May 14 - 16

11. Heterogeneity • Sampling a process stream or ROM ore • When we sample a process stream or use an on-line analyser on the stream, we see deviations from the average grade for the analyte Sampling Australia 2013 - May 14 - 16

12. Distributional Heterogeneity • Sampling a process stream or ROM ore • The type of heterogeneity exhibited is called distributional heterogeneity as it is a variation in time or position along the conveyor belt • To statistically characterise the variation, we use a variogram or a covariance function • With the variogram, we can calculate how often we need to sample the process stream in order to control the sampling variance due to distributional heterogeneity Sampling Australia 2013 - May 14 - 16

13. Distributional Heterogeneity • Sampling a process stream or ROM ore • The shorter the variogram range, the shorter the time between increments to control the variance Sampling Australia 2013 - May 14 - 16

14. Heterogeneity • Sampling a process stream or ROM ore • When sampling a particulate stream, the total variance is the sum of the distributional heterogeneity variance and all intrinsic heterogeneity variances from the primary sample right down to the selection of the analytical aliquot plus the analysis variance Sampling Australia 2013 - May 14 - 16

15. In-situ Heterogeneity • In-situ Heterogeneity (in the ore body) • The total picture of variability is captured in the variogram and variability is a combination of distributional heterogeneity and intrinsic heterogeneity Sill Nugget Range Sampling Australia 2013 - May 14 - 16

16. In-situ Heterogeneity Nugget • The nugget variance has two parts • The variance of sample preparation and analysis when the core is split and crushed for assaying • if the preparation protocol is inappropriate, this variance can be high and dominate the nugget variance • The ‘geological’ component of nugget variance • this variance can be conceptualised as the variance between assays of the 2 splits of the core, after subtraction of the preparation and analysis variance • this variance depends on the texture of the ore and the size of the core sample Sampling Australia 2013 - May 14 - 16

17. { In-situ Heterogeneity Sill - Nugget • The sill – nugget variance • This difference is the spatially regionalised variance • It is relevant to how densely the ore body should be sampled to define block grades with an acceptable block estimation variance • If the nugget variance is high, it makes capture of the regionalisation difficult and makes estimation of the variogram shape and range difficult Sampling Australia 2013 - May 14 - 16

18. In-situ Heterogeneity • To quantify in-situ heterogeneity, we can consider the following: • choose a standard block size • define an acceptable relative variance for the block estimation variance, • choose a standard sample size (core diameter and length) • determine the total mass of standard samples per block that provides the target relative estimation variance Sampling Australia 2013 - May 14 - 16

19. Exploration Impact • In exploration, there are two objectives: • define the ore body structure/continuity • provide preliminary data for variography which leads to estimates of the value of the resource • For an critical analyte showing a high nugget value (for any reason), both objectives are compromised • There is motivation to establish a geological nugget variance in each domain and reduce it to a reasonable value by choice of sample and core size • Similarly, sample prep protocol can be optimised Sampling Australia 2013 - May 14 - 16

20. Mining and Beneficiation Impact • Failure to solve heterogeneity issues at exploration will lead to poor planning of infill drilling for the resource model and degrade the block model • this will affect the mine plan and definitions of ore and waste • the design of the beneficiation plant and pre-plant ore blending facilities may be adversely affected • project risk increases in a manner that may be difficult to quantify Sampling Australia 2013 - May 14 - 16

21. Impact on Analytical Procedures • If the heterogeneity of the ore from each domain is well-understood, optimal sample prep protocols can be devised for each domain • sample prep variance can be minimised • the variance due to the intrinsic heterogeneity of the analytical aliquot in conjunction with the analysis variance can be minimised • optimal analytical procedures result • optimal aliquot mass • optimal analytical method Sampling Australia 2013 - May 14 - 16

22. Controlling Heterogeneity • For each domain • The intrinsic heterogeneity of the ore is estimated at a series of material top sizes • The impact of dilution of ore by waste is assessed • The sample mass retained at each stage of sample preparation is determined to control the total preparation variance • This work will support exploration as well Sampling Australia 2013 - May 14 - 16

23. Tools for Heterogeneity Evaluation • Until recently, heterogeneity evaluation had to be done by analysis of many replicates • this is expensive and needs to be corrected for preparation and analytical variance to get the correct value of at a given state of comminution • It is now possible to use the Qemscan to determine heterogeneity for particle top sizes less than about 5 mm • three or four points on the curve of vs can be determined first followed by other tests at larger sizes as necessary Sampling Australia 2013 - May 14 - 16

24. Tools for Heterogeneity Evaluation • For low concentrations of analyte: • Qemscan ‘bright phase’ searches can be run • Preconcentration of heavy mineral phases can be used as well • All the work should be guided by the mineralogists and geologists Mineralogy, Texture, Heterogeneity Sampling Australia 2013 - May 14 - 16

25. Summary • Intrinsic heterogeneity impacts all sampling • Distributional heterogeneity impacts exploration work and sampling from process streams • A full set of tools exists to evaluate heterogeneity in a quantitative manner • these are assisted by a careful evaluation of ore textures and domains • Control of data accuracy reduces project risk Sampling Australia 2013 - May 14 - 16