Importance of sample preparation in handheld XRF technique: examples from Madero Pb-Zn (Mexico) and Carmax Cu-Mo (Canada) deposits

A.K. SomarinThermo Fisher Scientific, Tewksbury, MA, USA

Conclusion and RecommendationsField-portable x-ray florescence is a technique that is gaining momentum and acceptancein addressing applications in various fields in geology, mining and even oil and gasexploration and production.

• Portable XRF can be a very effective method in running any type ofexploration/mining project.

• The concentrations can be from a few ppm to almost pure elements.• It can analyze elements from Mg to U with detection limits from 5 to 50 ppm.• Geochemical maps and strip logs can be prepared in real time using this

technique.• Although use of portable XRF is very simple, user must be aware of limitations

of this method.• Sample surface should be clean and as flat as possible. X-ray penetration is

minimal (maximum a few millimeters).• To minimize heterogeneity issue and increase quality of data, it is better to

prepare (pulverize) samples. Some companies (such as ThermoFisher) offerportable sample preparation kits.

• Factory calibration can provide accurate data for specific type of samples (andmatrix types) and for specific concentration ranges. As matrix of samples mayvary, it is better to adjust factory calibration. This can be done internally in someportable XRF analyzers such as Thermo Scientific Niton XL3t GOLDD+ analyzerused in this study.

AcknowledgementsStaff and field assistants at the Peñoles exploration division and management of Carmaxproject are thanked for their great help with fieldwork and logistics.

OverviewX-ray Fluorescence (XRF) is described as a surface technique because of minimalbeam penetration (commonly much less than 5 mm). As a result, a flat finelydisseminated and homogeneous sample is preferred for XRF analysis. This explainsthe routine procedure to prepare samples in any laboratory analysis. However, mosthandheld XRF (HHXRF) users prefer to save time and not to prepare samples. Thisstudy compares lab results with HHXRF assays of prepared and unprepared samplesfrom two deposits.

The investigations were carried out on drill core samples from two deposits: MaderoPb-Zn (Mexico) and Carmax Cu-Mo (Canada). The pulverized samples were analyzedby laboratory methods including atomic absorption (AAS) and inductively coupledplasma emission spectroscopy (ICP-ES) as well as portable XRF instruments includingThermo Scientific Niton XL3t GOLDD+ analyzer and Niton FXL field x-ray lab. Alsocore samples were directly analyzed by HHXRF.

This investigation shows high correlation between data from portable XRF and labmethods for all metals of interest in the Madero deposit (Zn-Pb-Cu-Ag) as well as theCarmax mineralization (Cu-Mo). In the prepared samples, the correlation is as high as98% for Pb, 97% for Zn, 96% for Cu, 80% for Ag in the Madero deposit and %86 forCu, %95 for Mo and %82 for Zn in the Carmax deposit. Sample preparation has asignificant role on the quality of data. For example, the correlation between lab assaysof core samples and their direct shot data (i.e. unprepared samples) obtained fromHHXRF is only 72% for Zn. This correlation jumps to 96% on prepared samples.

The results of this study indicate that sample preparation can be excluded only if semiquantitative results are sufficient to run and manage the exploration/mining project.However if high quality data and high accuracy is needed, samples should be prepared(pulverized). This is more crucial for light elements such as Al, Si, S, P and Mg as theirsecondary X-ray is weak.

Introduction

FIGURE 5. A) Niton FXL B) Direct shot of core samples using Niton XL3t GOLDD+ C) Direct rock/core sample.

FIGURE 9. Representative strip log from the Carmax deposit showing efficiency of portable XRF in detecting anomalous zones of Cu and Mo.

0 4 km2

N

Morelos

Francisco I. Madero

El Maguey

Cieneguillas

102 45’ 102 40’ 102 35’

22 4

5’22

50’

Tuff and basaltic lava

Micritic limestone

Black shaleGranitoid (dike and stock)Diorite (dike and stock)

Alluvial deposit

Rhyolitic ignimberiteMexico

FIGURE 1. Geological map of the Francisco I. Madero Zn–Cu–Pb–(Ag) deposit, Mexico.

FIGURE 2. Cross section of the Francisco I. Madero Zn–Cu–Pb–(Ag)deposit, Mexico

A DCB

FIGURE 3. A-B) Pyrite-chalcopyrite mineralization C) Mineralized breccia along a normal fault D) Propylitic (epidote-chlorite) alteration.

FIGURE 3. Geometry of the ore zone is indicated by drill holes. Note the localization of this zone between a shale unit at the bottom and limestone units at the top.

MethodTwo types of analyses were carried out on drill core samples from both Madero andCarmax deposits (Figure 4): laboratory assays (AAS and ICP-ES) and portable XRF(Niton FXL on pulp and also Niton XL3t GOLDD+ on pulp as well as direct shot of thecore samples). Pulp samples were collected from drill cores using a direct rock/coresampler (Fig. 5C).

Results• There is a high correlation between data from portable XRF and lab methods in drill core

samples from both deposits (Figures 6 and 7).• Sample preparation significantly improves quality of data. For example, correlation

between Zn data from portable XRF and lab jumps from 72% in the unprepared samplesto 96% in the prepared samples.

• Although portable XRF provides reliable and consistent data, calibration adjustment maybe needed to increase accuracy further.

y = 0.926x - 132.4R² = 0.974

0

20000

40000

60000

80000

0 20000 40000 60000 80000

Pb (p

pm) -

Nito

n X

L3t

-Pu

lp

Pb (ppm) - Lab (ICP-ES)

y = 0.950x - 124.5R² = 0.977

0

20000

40000

60000

80000

0 20000 40000 60000 80000

Pb (p

pm) -

Nito

n FX

L -

Pulp

Pb (ppm) - Lab (ICP-ES)

y = 0.744x + 108.2R² = 0.916

0

2000

4000

6000

8000

10000

12000

14000

0 2000 4000 6000 8000 10000 12000 14000

Cu

(ppm

) -N

iton

XL

3t -

Pulp

Cu (ppm) - Lab (ICP-ES)

y = 0.724x + 110.3R² = 0.919

0

2000

4000

6000

8000

10000

12000

14000

0 2000 4000 6000 8000 10000 12000 14000

Cu

(ppm

) -N

iton

FXL

-Pu

lp

Cu (ppm) - Lab (ICP-ES)

FIGURE 6a. Representative correlation graphs for Pb, Cu and Fe analyzed by the Niton FXL, Niton XL3t GOLDD+, and lab (AAS and ICP-ES) methods.

• Systematic analyses of drill core samples by portable XRF can help field geologiststo prepare real time and reliable strip logs (Figures 8 and 9) that can help them tomanage drill programs effectively.

y = 0.946x - 2.193R² = 0.889

0

5

10

15

20

25

30

35

0 5 10 15 20 25 30 35

Fe (%

) -N

iton

FXL

-Pu

lp

Fe (%) - Lab (AAS)

y = 0.960x - 1.787R² = 0.881

0

5

10

15

20

25

30

35

0 5 10 15 20 25 30 35

Fe (%

) -N

iton

XL

3t -

Pulp

Fe (%) - Lab (AAS)

y = 1.143x - 485.5R² = 0.966

0

40000

80000

120000

160000

0 40000 80000 120000 160000

Zn (p

pm) -

Nito

n FX

L -

Pulp

Zn (ppm) - Lab (ICP-ES)

y = 0.976x - 465.5R² = 0.963

0

40000

80000

120000

160000

0 40000 80000 120000 160000

Zn (p

pm) -

Nito

n X

L3t

-Pu

lp

Zn (ppm) - Lab (ICP-ES)

FIGURE 6b. Representative correlation graphs for Zn and Ag analyzed by the Niton FXL, Niton XL3t GOLDD+ and lab methods.

y = 0.746x + 7175.R² = 0.722

0

40000

80000

120000

160000

0 50000 100000 150000 200000 250000Zn (ppm) - Lab (AAS)

Zn (p

pm) -

Nito

n X

L3t

-Dir

ect s

hot

y = 0.621x + 10.14R² = 0.800

0

20

40

60

80

100

120

140

160

0 50 100 150 200 250

Ag

(ppm

) -N

iton

XL

3t -

Pulp

Ag (ppm) - Lab (ICP-ES)

FIGURE 8. Representative strip log for drill hole C49-11 from the Madero deposit showing efficiency of portable XRF in detecting anomalous zones of Zn, Pb, Cu, Fe, As and S.

Features Francisco I. Madero Carmax

Country Mexico CanadaType of Project Mining/Production ExplorationOre Zn–Cu–Pb–(Ag) Cu-MoGenetic Type SEDEX/VMS Porphyry

Production (2011) 12.8 MOz Ag, 92.09 kt of Pb, and 623.9 kt of Zn NA

Lithology

Stratabound ore bodies are hosted by the Mesozoic back-arc marine sedimentary rocks.

Ore is hosted by granitic intrusions as well as andesitic/dacitic volcanic rocks.

Hydrothermal Alteration Local skarn, propylitic, argillic

Classic porphyry alteration (phyllic, potassic, argillic, propylitic)

Ore Mineralogy

Sphalerite, galena, chalcopyrite, pyrite, cubanite, enargite, tetrahedrite

Chalcopyrite, molybdenite, bornite, pyrite, covelite, chalcocite

Hydrothermal Breccia Locally Locally

Common features of the studied Madero and Carmax deposits are shown in Table 1.Also geology maps and representative hand samples are shown in figures 1 to 5.

A CBFIGURE 4. Drill core of the Carmax deposit. A) Carmax granite, B) Quartz-chalcopyrite-bornite vein, C) Mineralized hydrothermal breccia.

A B C

Table 1. Geological features of the Madero and Carmax deposits.

y = 0.804x - 612.1R² = 0.860

0

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0 5000 10000 15000

Cu

(ppm

) -La

b

Cu (ppm) – Niton

y = 1.126x + 4.390R² = 0.954

0100200300400500600700800900

0 200 400 600 800

Mo

(ppm

) -La

b

Mo (ppm) - NitonFIGURE 7. Representative correlation graphs for Cu and Mo from the Carmax drill core samples analyzed by the Niton XL3t GOLDD+ and lab methods.