How Much Do We Need to Drill?
C Taylor and E Retz
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How Much Do We Need to Drill?
Overview
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Introduction
Location and Exploration History
Geology
Methodology
– Study area and domain selected
– Sequential Gaussian Conditional Simulation
• Parameters
• Validation
• Selection of simulations
– Drill holes, variography and estimation
Results
– Grade tonnage curves
– Histograms
Discussion
Conclusion
Introduction
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Fortescue currently has approximately 11.9 Bt of hematite Mineral Resources.
Approximately 63% of these are Inferred Mineral Resources.
Selection of drill grids is challenging in greenfields areas.
Sequential Gaussian Conditional Simulation has been used to maximise productivity at
Fortescue’s Eliwana deposit.
Location
Exploration History
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In 2011, Fortescue released an Inferred Mineral Resource on the Eliwana and Flying Fish
deposits.
Tonnes (Mt)
624
Fe %
58.7
SiO2 % Al2O3 % P %
5.44
3.06
0.091
LOI %
6.61
JORC (2004) Inferred Mineral Resource as at 23 March 2011, refer to ASX announcement of same date for further details
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An updated Inferred Mineral Resource for the two deposits was also completed in 2014.
Tonnes (Mt)
740
Fe %
59.1
SiO2 % Al2O3 % P %
5.21
2.88
0.091
LOI %
6.5
JORC (2012) Inferred Mineral Resource as at 20 March 2014, refer to ASX announcement of same date for further details
Geology of Eliwana
Methodology for this Study
Exploratory
data analysis
Creation of
simulations
Selection of
simulations
Creation of
theoretical
drilling grids
Creation of
block models
Estimation into
block models
Analyse
estimates
Study Area and Domain Selected
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An area within the J2 mineralised domain was selected.
The area was tested to find if the local distribution was representative of the global
distribution.
The area was tested for stationarity (statistical homogeneity).
Sequential Gaussian Conditional Simulation
Parameters
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Variogram models were created using data from the entire domain.
Simulations were completed on 5m × 5m × 1m points.
Simple kriging was used.
Neighbourhood parameters were based on a kriging neighbourhood analysis.
Sequential Gaussian Conditional Simulation
Validation of Parameters
Sequential Gaussian Conditional Simulation
Selection of simulations
Theoretical Drill Holes
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Drill spacings created on an offset and grid pattern were:
200m × 100m
50m × 50m
100m × 100m
50m × 25m
100m × 50m
25m × 25m
Variography
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Variography was completed on the flagged drill holes from the best simulation.
The quality of the experimental variograms was ranked as either good, medium or poor.
Estimation
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Two sets of models with different block sizes were created:
– 12.5m × 12.5m × 1m and 50m × 25m × 1m
Estimates were completed using ordinary kriging.
The ‘best’ simulation was regularised to act as the ‘truth.’
Results – Grade Tonnage Curves
200m × 100m – SiO2
Results – Grade Tonnage Curves
100m × 100m – SiO2
Results – Grade Tonnage Curves
50m × 50m – SiO2
Results – Grade Tonnage Curves
25m × 25m – SiO2
Results – Grade Tonnage Curves
Cut-off Plots – 5th to 95th Percentile Range
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Al2O3 at a 3% cut-off
Results – Grade Tonnage Curves
Cut-off Plots – Relative Difference
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Al2O3 at a 3% cut-off
Results - Histograms
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Al2O3 histograms
Discussion
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100m × 50m is required for reasonable variograms.
Ranges stabilise at 100m × 50m for SiO2/Al2O3 and 50m × 50m for Fe.
The largest relative difference is seen at 200m × 100m or 100m × 100m.
50m × 50m or 100m × 50m is required for the histograms to be similar to the truth.
Discussion
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Variation between the two drill patterns is due to the difficulty in representative
sampling.
Little variability was noted between the drill patterns at the closer spacings.
The smaller block size estimates’ grade tonnage curves and histograms became more
similar to the truth.
Conclusion
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The most significant improvement in
results is noted at 100m × 50m or 50m
× 50m.
100m × 50m should first be targeted.
The block size should be optimised with
drill spacing.
Either a grid or offset pattern is
acceptable.
Additional domains are planned to be
tested at Eliwana.
Thanks to:
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My co-author and everyone who reviewed the paper and presentation.
All Fortescue Exploration staff and contractors that have been involved in the Eliwana
deposit.
www.fmgl.com.au
@FortescueNews
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