Blast Hole Sampling validation at Mantoverde. Antoni Magri, Eduardo Magri and Cristian Neira

Transcription

1 Blast Hole Sampling validation at Mantoverde Antoni Magri, Eduardo Magri and Cristian Neira

2 Mantoverde Mantoverde is an iron ore, copper and gold deposit. It produces 62,000 tons of fine copper a year by heap/dump leaching material from three open pit operations. They use radial buckets to categorise blasted material.

3 Objectives of this study Validate blast hole sampling, which is currently done using radial buckets, by examining: Recovery of blast holes vs. reverse circulation samples Ore grades (CuT, CuS and CaCO 3 ) Granulometric distributions Ore grades for each granulometric fraction Bias between blast hole and reverse circulation/diamond drill hole samples Variogram nugget effects

4 Methods Drilled 44 blast holes on a 10 x 10 m grid. Prior to drilling, a plastic tarp was placed under the rig to capture all material ejected from the blast hole. The blast hole was sampled in duplicate using 2 radial buckets. Remaining blast hole material (751 kg per hole on average) was recovered, homogenized by alternate shoveling (x4) and split into lots of 18 kg each by fractional shoveling using an 800 g JIS shovel.

5 Methods

6 Methods Drilled 44 RC holes as close to the each blast hole as possible, perforating 2 benches. Sampled at 2 m intervals. Another 43 blast holes were drilled close to the RC holes when the pit advanced to the second bench, and sampled using 1 radial bucket. Radial bucket 1 (top bench): 22 bucket samples were split into granulometric fractions (9.5, 6.3, 4.75). 1/12 of material 4.75 mm was extracted and sieved (1.18, 0.425, 0.15 and mm meshes) g were extracted from each fraction and pulverized. 200 g were sent for analysis. Remaining 22 bucket samples were crushed to 1.7 mm, 1000 g were extracted and pulverized and 200 g were sent for analysis. Radial bucket 2 (& radial bucket 1 on second bench): Prepared the same way (without splitting into fractions).

7 Blast hole material: Methods The material was homogenized and split into kg bags. One 18 kg bag was split into fractions and prepared in the same way as the 22 sieved radial bucket samples. The remaining 18 kg bags were prepared in the same way as the unsieved radial bucket samples. CuT, CuS and CaCO 3 grades: RC: average of the 1 st 5 samples (0 to 10 m) and second 5 samples (10 to 20 m) Radial buckets: mass weighted grades for sieved samples, direct lab results for all other samples. Blast holes: mass weighted grades for all of the 18 kg samples and granulometric fractions.

8 Historical database BH, RC and DDH production database (2009 and 2010): blast holes (7 to 12.3 m in length) 1475 RC and DDH 10 m composites Getpairs (GSLIB) generated 2 sets of pairs of BH and RC/DDH composites with maximum separations of 3 and 5 m. Pairs were analyzed for bias. Variographic analysis: Used the same BH, RC and DDH database. Comparison of nugget effects for omnidirectional variograms (GSLIB)

9 Results Lower recoveries occurred near the edge of the bench, likely due to fractures in the rock. Recovery for blast holes: Average was 91.7%. 11 holes were above 100% because the holes widen as material is ejected.

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11 Results Bench 1 Bench 2 N Mean Std.Dev. COV Median Min Max P25 P75 RC CuT C1 CuT C2 CuT CBH CuT RC CuT C1 CuT Bench 1 Bench 2 RC CuS C1 CuS C2 CuS CBH CuS RC CuS C1 CuS Bench 1 Bench 2 RC CaCO C1 CaCO C2 CaCO CBH CaCO RC CaCO C1 CaCO

12 Results Percent data with absolute Contrasts relative errors < 30 % Mean relative error (%) CuT CuS CaCO 3 CuT CuS CaCO 3 RC vs RB 1, bench RC vs RB 1, bench RC vs RB 2, bench RC vs CBH, bench RB1 vs RB 2, bench RB 1 vs CBH, bench RB 2 vs CBH, bench Note: RB = radial bucket; CBH = complete blast hole material, bench Nº 1 is the upper bench; bench Nº 2 is the lower bench. Radial bucket and complete blast hole samples are more alike than RC samples, as the contrasts have higher percentages of data with absolute relative errors of 30 % or less, and lower mean relative errors.

13 Results Test T for statistically significant Contrasts differences between means CuT CuS CaCO 3 RC vs RB 1, bench 1 No No Yes RC vs RB 1, bench 2 No No Yes RC vs RB 2, bench 1 No No Yes RC vs CBH, bench 1 No No No RB 1 vs RB 2, bench 1 No No No RB 1 vs CBH, bench 1 No No Yes RB 2 vs CBH, bench 1 No No Yes No statistical differences in the means of CuT and CuS for RC, RB and CBH samples. Significant differences for CaCO 3 between RC and RB samples, and between RB and CBH samples. Differences between RC and RB could be due to the hydrocarbon additive used in RC drilling rigs, but this does not explain the lack of difference between RC and CBH samples, or the differences between RB and CBH samples. This suggests that the radial buckets (possibly due to their placement under the drill) may fail to adequately capture lighter minerals.

14 Results

15 Over 90 % correspondence for geological areas long-term resource model and BH mapping results match well. Paired data Bias (%) 3 m separation 5 m separation CuT CuS CaCO3 CuT CuS CaCO3 BH and RC BH and DDH DDH & RC have lower nugget effects than blast holes better sampling protocols.

16 Conclusions Radial buckets samples work well for CuT and CuS, but not for CaCO 3. This may be due to bucket placement, which has limited options due to operator safety and spatial distribution of CaCO 3 under the drill as material is ejected. Higher CuT, CuS and CaCO 3 grades occurred in the fine fractions (0.425 to mm). 17 to 20 % of fine CuT and CuS occurred in the mm fraction imperative to control loss of fines during sampling and sample preparation!

17 Conclusions Variogram nugget effects: BH sampling protocols are inferior to those for RC and DDH samples this could lead to significant hidden economic losses if an important portion of the blasted material is misclassified. Differences between benches 1 and 2 may be due in part to better RC recovery, and to more homogeneous material.

18 What did we recommend? Study the economics of advanced RC sampling to obtain higher quality samples and allow for better short term planning well ahead of blasting and processing the material.