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WEBINAR – Geochemical Analyses for Mapping Geological Processes – Scott Halley

Comparison of assay methods How “complete” is a 4 acid digest? What are the advantages and disadvantages of a 4 acid digest compared with a Li-borate fusion?

Scott Halley Consulting Geochemist, Mineral Mapping Pty Ltd, Hawley Beach, Tasmania, 7307. Email: [email protected] Web: www.scotthalley.com.au Quartz-topaz-andalusite-fluorite breccia, Akulla, Sweden

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WEBINAR – Geochemical Analyses for Mapping Geological Processes – Scott Halley

Digest Methods Aqua regia Mixture of nitric acid and hydrochloric acid in a molar ratio of 1:3. Aqua regia readily dissolves many sulfide, oxide and carbonate minerals, and chlorite, as well as retaining mercury, a particularly volatile element.

Four acid digest HNO3, HClO4, HF, HCl. Quantitatively dissolve nearly all minerals.

Li Borate Fusion Flux for melting the sample into a glass which is then acid soluble. Strongest fusion technique in order to fully digest zircons, barite, rare earth oxides, and tin, tungsten, niobium and tantalum minerals.

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WEBINAR – Geochemical Analyses for Mapping Geological Processes – Scott Halley

What is a 4A digestion? Acids used are nitric HNO3, perchloric HClO4, hydrofluoric HF, and hydrochloric HCl Sample is digested in nitric and perchloric first and heated. HF is added, solution is heated and evaporated to some extent. Max temperature over 150°C Lastly, HCl is added. No two lab companies perform their sample decomposition methods exactly the same way.

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WEBINAR – Geochemical Analyses for Mapping Geological Processes – Scott Halley

4A Digestion: Compromises Not fully dissolved: • Zircon! • Monazite, sphene • Cassiterite, rutile, xenotime, barite, corundum, chromite • Scheelite • Columbite-tantalite • Massive/refractory sulfides

Not fully retained: • Si is fumed off as SiF6 • All Hg is lost due to the temperature of digestion • Semi-volatiles (As, Se, Sb, Tl, Ge, etc.) may be lost depending on exact methodology • Some elements may re-precipitate as insoluble salts depending on exact methodology

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WEBINAR – Geochemical Analyses for Mapping Geological Processes – Scott Halley

ICP-MS Engineering advances are incrementally improving ICP-MS, but the fundamental problems of interferences remains. Interferences: polyatomic ions involving O, H, Cl, Ar having very similar masses or mass to charge ratios as important geochemical elements.

Collision Gas: Inert gas collides with large polyatomic ions more frequently than single atom ions, slows them down and allows the ion of interest to reach the detector earlier Reaction Gas: Reacts with ion of interest (or the interferant) and creates a polyatomic ion, changing its mass and negating interferences

5 Agilent 7900

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WEBINAR – Geochemical Analyses for Mapping Geological Processes – Scott Halley

“Research Quality Assays” By combining several methods into one package, a complete sample characterization is obtained. These packages combine whole rock analysis, trace elements by fusion, aqua regia digestion for the volatile trace elements, carbon and sulfur by combustion analysis, and several detection limit options for the base metals.

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Costs are typically around $100/sample This assay package is not routinely used because of the high unit cost.

ALS Schedule of Services and Fees, Australia 2019

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WEBINAR – Geochemical Analyses for Mapping Geological Processes – Scott Halley

Exploration Assays In exploration it is common to use a 4 acid digest and a combination of ICP-AES and MS assay methods. This gives a broad suite of major elements, trace elements and metals, with exceptionally low detection limits Costs are typically around $42/sample; cheap enough to be used routinely.

Resource Definition Assays

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In a resource drill out where the very low detection limits are not important, it is common to use a 4 acid digest with just ICPAES assays. Costs are typically around $22/sample. ALS Schedule of Services and Fees, Australia 2019

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WEBINAR – Geochemical Analyses for Mapping Geological Processes – Scott Halley

Comparison of Methods The following presentation compares 4 acid digest ICP results with LiBorate fusion assays, These plots compare standard immobile trace element scatterplots that are used to categorise rock compositions and plots that are used to track magmatic fractionation. Also presented are plots of 4 acid digest results versus Complete Characterization results for elements that might be volatilised during digestion, or elements that are difficult to dissolve.

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WEBINAR – Geochemical Analyses for Mapping Geological Processes – Scott Halley

Comparison of Methods With a 4 acid digest, SiO2 is not reported because Si forms a volatile fluoride complex. Rock compositions can be classified by plotting immobile trace elements against Sc. Sc substitutes for Fe in silicate minerals, but not in oxides and it is far less mobile than Si during hydrothermal alteration. Rhyolites generally have >10ppm Sc, basalts >30ppm Sc.

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WEBINAR – Geochemical Analyses for Mapping Geological Processes – Scott Halley

Comparison of Methods 4 acid digest ICP-MS These data are from calc-alkaline volcanics in a VMS province. The rocks include organic-rich black shale, siltstone, pumice breccia and a porphyry unit. These plots show a classification of the assay data from plots of Sc vs Th, Ti, V, Zr, Nb and Cr. Note these do not form distinct populations because of the heterogeneous nature of the rocks. These assays are from a 4 acid digest ICP-MS method. The same samples were also assayed with a LiBorate fusion ICP-MS. The same plots are presented in the next slide:

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WEBINAR – Geochemical Analyses for Mapping Geological Processes – Scott Halley SH1

Comparison of Methods “Complete Characterization” These are immobile trace element plots from LiBorate fusion ICP-MS. These assays are more accurate than the 4 acid digest since it is a more complete digestion process, but the precision of the analyses is poorer.

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WEBINAR – Geochemical Analyses for Mapping Geological Processes – Scott Halley

Comparison of Methods There are some specific plots we can use to track fractional crystallization of particular minerals. In this suite of samples, the pumice breccias show evidence of fractional crystallization of plagioclase, magnetite and eventually biotite and zircon. V/Sc vs Sc is a plot designed to show the degree of fractional crystallization of magnetite; decreasing V/Sc with decreasing Sc (increasing SiO2). This is very obvious in the 4 acid digest data, and not at all obvious on the Li Borate data.

4 acid digest ICP-MS

Complete Characterization

Organometallic V

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Magnetite fractionation

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WEBINAR – Geochemical Analyses for Mapping Geological Processes – Scott Halley

Comparison of Methods There are some specific plots we can use to track fractional crystallization of particular minerals. In this suite of samples, the pumice breccias show evidence of fractional crystallization of plagioclase, magnetite and eventually biotite and zircon. Ta/Nb vs Ti is a plot designed to show the degree of fractional crystallization of biotite; Increasing Ta/Nb with decreasing Ti. This is very obvious in the 4 acid digest data, and scattered from the Li Borate data.

4 acid digest ICP-MS

Complete Characterization

Biotite fractionation

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WEBINAR – Geochemical Analyses for Mapping Geological Processes – Scott Halley

Comparison of Methods There are some specific plots we can use to track fractional crystallization of particular minerals. In this suite of samples, the pumice breccias show evidence of fractional crystallization of plagioclase, magnetite and eventually biotite and zircon. Hf vs Zr is a plot designed to show fractional crystallization of Zircons; Increasing fractional crystallization of zircons leads to lower total Zr but higher Hf/Zr. Zircons are usually NOT totally dissolved in the 4 acid digest, especially in younger rocks. However the pattern of zircon fractionation is still very evident in the data .

4 acid digest ICP-MS

Complete Characterization

Zircon fractionation

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WEBINAR – Geochemical Analyses for Mapping Geological Processes – Scott Halley

Comparison of Methods; Aluminium Li-borate fusion produces more accurate analyses of the major elements. 4-acid digest produces higher precision analyses of the trace elements because it results in lower inference effects during analysis. Choose a method that is fit for purpose! Can you accept lower accuracy on the major elements in order to get greater information from the trace elements? If too much liquid is retained during the HF/heating stage, Al is precipitated as a fluoride complex.

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Al_pct 4 acid digest

Al2O3_pct_LiBorate fusion

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WEBINAR – Geochemical Analyses for Mapping Geological Processes – Scott Halley

Comparison of Methods; Potassium Test how “complete” the 4 acid digest is by plotting 4acid vs Li Borate results from the same samples. With a 4 acid digest, some K may reprecipitate as potassium perchlorate.

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K_pct 4 acid digest

K2O_pct_LiBorate fusion

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WEBINAR – Geochemical Analyses for Mapping Geological Processes – Scott Halley

Comparison of Methods; Titanium Ti is a real test of the efficiency of the 4 acid digestion. Zr will usually underreport quite significantly even with the best 4 acid procedure. If a 4 acid digest is done well, >90% of the Ti will dissolve. Ti is more difficult to dissolve in samples that have had advanced argillic alteration. If Al under-reports, then it is likely there will be significant under-reporting of Ti.

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Ti_pct 4 acid digest

TiO2_pct_LiBorate fusion

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WEBINAR – Geochemical Analyses for Mapping Geological Processes – Scott Halley

Comparison of Methods; Arsenic Arsenic under-reporting at higher values; volatilization during digest?

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As_ppm 4 acid digest

As_ppm_aqua regia ICP-MS

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WEBINAR – Geochemical Analyses for Mapping Geological Processes – Scott Halley

Comparison of Methods; Antimony It is common for antimony to be incompletely dissolved in aqua-regia.

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Sb_ppm 4 acid digest

Sb_ppm_aqua regia ICP-MS

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WEBINAR – Geochemical Analyses for Mapping Geological Processes – Scott Halley

Conclusions • Good quality 4 acid digest ICP-MS trace element data has significant advantages over Li Borate fusion “research quality” assays. • With the exception of Zr, in most data sets the digestion is very close to total. • The major elements are measured with less accuracy than a Li Borate fusion, but the results are good enough for quantitative mineralogy calculations. • The calculated mineralogy estimates are significantly improved by adding SiO2 analyses by portable XRF to the assay suite.

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WEBINAR – Geochemical Analyses for Mapping Geological Processes – Scott Halley

Comparison of Methods ALS has developed a proprietary ICP-MS method with a modified sample introduction method and a collision/reaction cell. This method has an order of magnitude reduction in detection limits and increase in precision due to fewer oxide and polyatomic interferences, and low carry over effects. The difference between this and the standard ICP-MS is as great as the difference between ICP-MS and LiBorate fusion ICP-MS. ALS is now working to introduce the same proprietary method for LiBorate samples, which will give complete digest AND high precision. Costs are typically around $50/sample.

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ALS Schedule of Services and Fees, Australia 2019

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WEBINAR – Geochemical Analyses for Mapping Geological Processes – Scott Halley

Comparison of Methods Sample fractional crystallization diagrams from ME-MS61L with ultra-low detection limit 4 acid digest ICP-AES/MS. The effect of the higher precision in these assays is obvious.

Hf versus Zr, ME-MS61L

Hf versus Zr, Li-borate fusion

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WEBINAR – Geochemical Analyses for Mapping Geological Processes – Scott Halley

BUT,…. Not all 4 acid digests are the same!! Each assay company has different procedures for the digestion process. • The following discussion compares data sets from ALS and laboratory X • Both were analysed twice, by Li-borate fusion ICP-AES, and 4 acid digest ICP-AES/MS. • This demonstrates that some laboratories DO get close to a complete dissolution with a 4 acid digest, but some DO NOT.

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WEBINAR – Geochemical Analyses for Mapping Geological Processes – Scott Halley In all the following plots, the black lines represent a 1:1 correlation

Sodium ALS 4 acid digest ICP-AES vs Li-borate fusion

Na_pct 4 acid digest

Na_pct 4 acid digest

Lab X 4 acid digest ICP-AES vs Li-borate fusion

Na2O_pct_LiBorate fusion

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Na2O_pct_LiBorate fusion

Good repeatability of Na by both methods means that all of the feldspar has dissolved in the 4 acid digests from both labs.

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WEBINAR – Geochemical Analyses for Mapping Geological Processes – Scott Halley

Potassium ALS 4 acid digest ICP-AES vs Li-borate fusion

K_pct 4 acid digest

K_pct 4 acid digest

Lab X 4 acid digest ICP-AES vs Li-borate fusion

K2O_pct_LiBorate fusion

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K2O_pct_LiBorate fusion

ALS has good repeatability of K to 6% and above. Lab X has very significant under-reporting of K at 3% and above.

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WEBINAR – Geochemical Analyses for Mapping Geological Processes – Scott Halley

Aluminium ALS 4 acid digest ICP-AES vs Li-borate fusion

Al_pct 4 acid digest

Al_pct 4 acid digest

Lab X 4 acid digest ICP-AES vs Li-borate fusion

Al2O3_pct_LiBorate fusion

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Al2O3_pct_LiBorate fusion

ALS has repeatable Al analyses but with modest precision. Lab X has huge under-reporting of Al. This is a very common outcome where acid digests leave too much water during the heating process before addition of HCl. Al is retained as an insoluble fluoride.

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WEBINAR – Geochemical Analyses for Mapping Geological Processes – Scott Halley

Titanium Lab X 4 acid digest ICP-AES vs Li-borate fusion

Ti_pct 4 acid digest

Ti_pct 4 acid digest

ALS 4 acid digest ICP-AES vs Li-borate fusion

TiO2_pct_LiBorate fusion TiO2_pct_LiBorate fusion From the ALS data, the 4 acid digest values for Ti under-report by around 5 to 10%. In the Lab X assays, where samples are not sufficiently evaporated before adding HCl, the errors don’t just report in Al, but in many other elements, such as Ti. ______________________________________________________________________________________________________________________ ______________________________________________________________________________________________________________________ ______________________________________________________________________________________________________________________ ______________________________________________________________________________________________________________________ ______________________________________________________________________________________________________________________ ______________________________________________________________________________________________________________________

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WEBINAR – Geochemical Analyses for Mapping Geological Processes – Scott Halley

Conclusions • Whether or not 4 acid digest ICP-AES/MS analyses are fit for purpose depends on which laboratory provided the data. In my experience, there are 2 global assay chains that are satisfactory, and the rest are not. • In a 4 acid digest ICP-AES/MS package, the major elements are assayed by ICP-AES. Therefore ICP-AES assays from a reputable lab will be sufficient for the calculated mineralogy estimates (at about 20% of the cost of complete characterization assays). SiO2 by pXRF can be added for around $3.50 sample to significantly improve the estimations. • If/when the Li Borate fusion ultra-low detection limit method is available it will provide superior quality results than the current research quality assay at about half the unit cost.

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WEBINAR – Geochemical Analyses for Mapping Geological Processes – Scott Halley

Multielement Geochemistry data validation Just because your data passes the validation tests in Acquire or Datashed doesn’t mean it is correct! Try these simple tools as an extra check on your data quality.

Scott Halley Consulting Geochemist, Mineral Mapping Pty Ltd, Hawley Beach, Tasmania, 7307. Email: [email protected] Web: www.scotthalley.com.au Native Sulfur in advanced argillic alteration, Puren Sur, La Coipa

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WEBINAR – Geochemical Analyses for Mapping Geological Processes – Scott Halley

Check the Elements and units in your database. ioGAS has useful data validation tools. Set the element name and units (ppb, ppm, %) for all data columns.

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WEBINAR – Geochemical Analyses for Mapping Geological Processes – Scott Halley

Check the Elements and units in your database. Does your data base have a mixture of ppb, ppm and pct values? It is very easy to make a series of scatter plots or cumulative frequency plots. It is obvious when the wrong units are in a table, or when elements have been imported to the wrong columns.

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WEBINAR – Geochemical Analyses for Mapping Geological Processes – Scott Halley

Have some idea of what ranges to expect. When you are using multielement data, you should have an idea about what ranges of analytical results are reasonable, and which are not.

Typical Ranges

Basalt

Rhyolite

Sc

30 - 50

2 - 10

Th

0.5 - 3

10 - 20

Ti

5000 - 10000

1000 - 3000

V

200 - 300

10 - 40

Zr

40 - 80

150 - 250

Nb

4-8

10 - 15

La

4-8

20 - 40

Ce

10 - 20

60 - 80

Cr

100 - 300

5 - 10

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WEBINAR – Geochemical Analyses for Mapping Geological Processes – Scott Halley

ALWAYS check that you have the correct locations in your database First thing; always plot a map of your data. Are the locations correct?

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WEBINAR – Geochemical Analyses for Mapping Geological Processes – Scott Halley

Check for data type Many software systems will have tools that identify non-numeric (text), >,