practical applications of portable xrf in mineral exploration

Practical Applications of Portable XRF in Mineral Exploration:Confessions of a Geochemist

Dennis ArnePrincipal Consultant ‐ Geochemistry

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• Introduction to analytical approaches• Case study of what not to do• Public reporting codes• Sampling is the key• Analytical aspects to getting it right


Outline

Two philosophical approaches to the use of portable XRF in mineral exploration:

• Laboratory in the field• Representative sampling procedures• Calibration of relevant elements

• Rigorous quality control program• Allows for public reporting of results

• Relative variations• Sampling not representative• Accuracy not essential• Focus is on consistency of data• For internal use only

Introduction


Case Study

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Absolute values reported with insufficient supporting information (ahead of JORC 2012)

4Denver X‐ray Conference, August 3, 2017

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Share Price chart

HoleDepth

(m)

Cu (%)

pXRF

Ag (g/t)

pXRF

Au (g/t) pXRF

1 26‐27 3.1 85 10.1

2 12‐13 2.3 82 14.0

2 13‐14 0.4 32 9.1

2 18‐19 0.5 8 10.0

Company releases lab results

20% price drop

Company releases pXRF Cu, Au, Ag

results

10% price jump

Company retracts Au & AgHole

Depth

(m)

Cu (%)

pXRF

Cu (%) Lab

Ag (g/t)

pXRF

Ag (g/t) Lab

Au (g/t) pXRF

Au (g/t) Lab

1 26‐27 3.1 1.02 85 23 10.1 0.01

2 12‐13 2.3 1.26 82 63 14.0 <0.01

2 13‐14 0.4 0.45 32 36 9.1 0.01

2 18‐19 0.5 0.22 8 9 10.0 <0.01

Whoops!


“Nature and quality of sampling (eg cut channels, random chips, or specific specialised industry standard measurement tools appropriate to the minerals under investigation, such as down hole gamma sondes, or handheld XRF instruments, etc).”

“For geophysical tools, spectrometers, handheld XRF instruments, etc, the parameters used in determining the analysis including instrument make and model, reading times, calibration factors applied and their derivation, etc.”

JORC 2012 Table 1


What Else Does JORC Say?

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• “For all sample types, the nature, quality and appropriatenessof the sample preparation technique.”

• “Quality control procedures adopted for all sub‐sampling stages to maximise representivity of samples.”

• “Measures taken to ensure that the sampling is representativeof the in situ material collected, including for instance results for field duplicate/second‐half sampling.

• Whether sample sizes are appropriate to the grain size of the material being sampled.

• Nature of quality control procedures adopted (eg standards, blanks, duplicates, external laboratory checks) and whether acceptable levels of accuracy (ie lack of bias) and precision have been established.


“Item 11: Sample Preparation, Analyses, and Security – Describe• (a) sample preparation methods and quality control measures employed before dispatch of samples to an analytical or testing laboratory, the method or process of sample splitting and reduction, and the security measures taken to ensure the validity and integrity of samples taken;

• (b) relevant information regarding sample preparation, assaying and analytical procedures used, …..;

• (c) a summary of the nature, extent, and results of quality control procedures employed and quality assurance actions taken or recommended to provide adequate confidence in the data collection and processing; and

• (d) the author's opinion on the adequacy of sample preparation, security, and analytical procedures.”

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NI43‐101


Sampling

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Representative Sampling

• Addressed by sampling theory ‐ data precision improved by: Smaller grain size (pulverise) Larger sample mass

More particles (i.e. higher grade)

• No instrumental advance, calibration or QAQC during measurement can compensate for a non‐representative sample to begin with.


Representative

sample

Non‐representativesample

Single spot analysis 1 cm2 by 0.05 cm* = 0.05 cm3 x 3.0 g/cm3 = 0.15 g

Average of 10 spot analyses over 1 m

0.15 g x 10 = 1.5 g

Drag analysis over 1 m 0.15 g x 100 = 15 g

Filet sample over 1 m 0.1 cm x 1 cm x 100 cm = 10 cm3 * 3.0 g/cm3 = 30 g

Half core saw cuttings (NQ) 4.76 cm x 0.2 cm x 100 cm = 95.2 g

Half core crushed and pulverised (NQ) 0.5 x 5,336 g= 2,668 g

How big is a pXRF sample?

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Approximate sample weight for acid digestion & ICP

analysis, after homogenization

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*Assumes a critical depth of

penetration of 50 microns

Homogenized

Non

‐homogenized

Fine Chalcopyrite in Drill Core

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Increasing

accuracy

Multiple drag analyses produce the best data in terms of both accuracy and precision. Multiple spot analyses produce a reasonably accurate estimate, but the precision is poor.


Incre

asin

g precisio

n

Relative

Standard

Deviation

(%)

Incre

asin

g precisio

n

Relative

Standard

Deviation

(%)

Coarse Chalcopyrite in Drill Core

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Increasing

accuracy

The average of three multiple drag analyses again performed best on diamond drill core in terms of both precision and accuracy, although

data for 20 spot analyses were reasonably accurate.However the precision is beginning to deteriorate for the drag analyses.


Spot analyses

Drag analyses

Relative Standard Deviation

Relative

Standard

Deviation

(%)/Bias (%)

Bias

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Molybdenite in High Angle Veins

Accuracy (% bias)

Precision (% RSD)

No sampling approach gave good data in this case, with a significant negative bias, but the drag analyses had the best precision.



pXRF‐Lithogeochemistry

The correlation between uncalibrated individual spot analyses by pXRF taken every meter in drill core and 1 metre lithogeochemical samples collected every 5 m is poor.


However, comparison of portable XRF spot analyses every metre down‐hole versus 1 m lithogeochemical

sample every 5 m down‐hole is informative.

Down‐hole Plots


The benefits to be gained from sample preparation are dependent on the element in question and how close the

data are to the lower limit of detection.Are the improvements in data precision worth the extra cost? Are we trying to replace laboratory analyses?

Soil Sample Preparation


Preparation RMS CV

(%)

Handling

time

Total Extra time

Cu Mn Ti(minutes/

sample)(min) (hours)

Data range (ppm) 35-65 300-1200 3000-6500

Shot through cloth bag 13 7 6 2 12,000 0

Split in plastic bag 15 8 8 5 30,000 300

Sieved in plastic bag 12 8 3 10 60,000 800

*Based on 10 repeat analyses for each preparation method and a sample through‐put of 6,000 samples in a season


RSD

No Preparation/Calibration

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• Suitable for determining relative differences between samples, rather than absolute abundances.

• Need to be aware of the influence of sample bag material on elements of interest (attenuation and/or contamination).


• Early stage Au exploration project• Intensive grid sampling program in 2011• 1 kg C‐horizon soils collected• 14,651 un‐sieved, dry soil samples analyzed by 2 different pXRFs• Analyzed by pXRF through Hubco sample bags• Also aqua regia digestion/ICP‐MS analysis of a 30 g, ‐100 micron

prepared sample to obtain Au + multi‐element data• Results (with no calibration):

– Good agreement for As & Cu from both machines with trends in ICP data– Pb, Mo & Ni data needed leveling and then give acceptable results– Data for low level pathfinder elements (W, Sb, Ag, Bi, Te) pick highs

Whiskey Project, Yukon


Arsenic by ICP‐MS & pXRF

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From Arne, Mackie & Jones, 2014, The use of property‐scale portable x‐ray fluorescence data in gold exploration: advantages and disadvantages,Geochemistry: Exploration Environment Analysis, vol. 14, pp. 233‐244.


Lead by ICP‐MS & pXRF





Antimony by ICP‐MS & pXRF



Analytical Aspects


Instrument Selection

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• Have you got the correct anode (eg. Ta, Rh, Au) for the elements of most interest to your project?

• Is the instrument set up correctly for the elements you are interested in?

• Are the detection limits suitable for the concentrations you will measure?


Lower Limits of Detection

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Limits o

f Detection

(ppm)

Element LLD Value (ppm)% of Samples Above LLD

Cu 2.5 99.6

Mo 0.6 54

Sb 6 30

Pb 1.2 100

Zn 1.4 100

W 2 4

Good

Okay

Poor

Good

Good

Poor

Actual Delta Premium LLDs calculated for contaminated soil samples


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Counting Time

26

Standard LKSD‐4 Analyzed in Soil Mode

Handheld 1 Handheld 2 Handheld 3 Benchtop 1 Benchtop 2

Time (seconds)

Cr (ppm)

Ca

(pct)

As (ppm)

Poor precision/poor accuracy

Good precision/poor accuracy

Good precision/good accuracy (from Hall et al., 2011)

LR: 5 pXRF 10 replicates 8 beam times (20120sec)Denver X‐ray Conference, August 3, 2017

Accuracy

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• Certified reference materials (CRM) should be matrix‐appropriate.

• CRM should span a range of expected values.

• CRM data should be collected on a regular basis to monitor drift.

• Calibrate with either matrix‐appropriate CRM or project‐specific samples analyzed by appropriate methods.


Summary

• Sampling is one of the most important aspects of using portable XRF data in mineral exploration.• Must be representative if data are to used quantitatively• Non‐representative samples can also be used qualitatively

• The collection of quantitative data requires appropriate choice of instrument, adequate count times and calibration for the elements of interest.• However, even inaccurate and imprecise data may be useful

to reveal relative differences (just not publicly reported!)www.csaglobal.com 28Denver X‐ray Conference, August 3, 2017

practical applications of portable xrf in mineral exploration

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