November 2011    Print this article

New analytical results of lake sediments following four acid digestion in the Churchill Province: implications for mineral exploration

Charles Maurice,
Ministère des Ressources naturelles et de la Faune (MRNF)

Background

Québec’s Ministère des Ressources naturelles et de la Faune (MRNF) announces the publication of new geochemical data from the re-analysis of archived lake sediments using four-acid near-total digestion. The project was launched following renewed interest in exploration for rare earths (lanthanides) and other rare metals (Zr-Y-Nb-Be) associated with peralkaline granitic rocks in the Churchill geological Province (Figure 1). The region covered by the Rivière George survey1 includes at least three rare earth and rare metal mineral deposits, namely Strange Lake-Lac Brisson, Lac Misery and Ytterby 2 (Salvi and Williams-Jones, 2006; Petrella, 2011).

The objective of the current project is to compare the analytical results obtained following partial digestion in aqua regia with the results obtained using near-total digestion in four types of acid.

Click on image to enlarge

Figure 1 : Location of the Rivière George survey and rare earths and rare metals mineralizations in the Churchill geological Province.

The comparison will help identify and distinguish environmental chemical signatures from the chemical signatures of the basement rock. Since some of the minerals containing rare earth and rare metal mineralizations are refractory (such as zircon, pyrochlore, monazite, and xenotime), their signature in lake sediments could be enhanced.

Analytical method

The lake sediment samples from the Rivière George survey were re-analyzed using the ICP-MS method (Maurice and Labbé, 2009) following partial digestion in aqua regia,2 the digestion generally used for lake sediments in Québec. The new data, made available from today,3 were obtained using the same ICP-MS method, at the same laboratory (ACMELabs, Vancouver), but the samples were prepared by near-total digestion at high temperature in four types of acid (nitric, perchloric, fluorhydric and chlorhydric). The results are close to total contents for several of the elements analyzed, except when the samples contain highly resistant phases such as barite (Ba), cassiterite (Sn), chromite (Cr), monazite (La, Ce, Nd), titanite (Ti), xenotime (Y) and zircon (Zr). Of the original 1902 samples from the Rivière George survey, 1865 were analyzed using both digestion methods. The new results present analytical data for all the rare earth elements, while the 2009 data contained only the results for La, Ce and Y.

The digestion of the samples in the four types of acid may lead to volatilization of the elements arsenic, antimony and gold during fuming, giving lower concentration readings than the actual concentrations. For this reason, unless the analysis for these three elements is considered unnecessary, four-acid digestion cannot be used for routine analyses.

Preliminary results

The rank correlation coefficients4 for the 41 elements analyzed with both digestion methods were calculated (Table 1). Almost half of the elements had coefficients over 0.80, demonstrating an excellent match between the two methods. These elements include the transitional elements copper (Cu; 0.98), molybdenum (Mo; 0.99) and zinc (Zn; 0.94), for which near-total digestion appears to have no effect. Similarly, the light rare earth elements lanthanum (La; 0.97) and cerium (Ce; 0.96) have extremely high correlation coefficients (Figure 2a). Yttrium (Y), an element chemically close to the heavy rare earth elements, also has a relatively high correlation coefficient of 0.85. Since the anomalous samples obtained using the two digestion methods are the same, the use of a near-total digestion to explore for several metals and rare earth elements does not appear to be justified.

Alcall-earth metals

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Transition metals

Ba

0.20

Ag

0.93

Be

0.35

Cd

0.88

Ca

0.10

Co

0.86

Mg

0.77

Cr

0.71

Sr

-0.56

Cu

0.98

Lanthanides

Fe

0.85

Ce

0.96

Hf

0.40

La

0.97

Mn

0.75

Post-transition metals

Mo

0.99

Al

0.07

Nb

0.50

Bi

0.81

Ni

0.87

Ga

0.58

Sc

0.47

Pb

0.65

Ti

0.73

Sb

0.72

V

0.71

Sn

0.75

W

0.83

Actinides

y

0.85

Th

0.77

Zn

0.94

U

0.99

Zr

0.41

Alkali metals

Non-Metals

Cs

0.98

As

0.38

K

0.64

P

0.96

Li

0.97

S

0.99

Na

0.36

 

 

Rb

0.83

 

 

Table 1 : Rank correlation coefficients for the elements analyzed using two digestion methods. The chemical series in the periodic table of the elements were used to group the results. The major elements are shown in bold italics.

Almost one quarter of the chemical elements analyzed, however, have correlation coefficients below 0.60, and strontium (Sr) even have a negative correlation of -0.56 (table 1). In these cases, the results using the two digestion methods are significantly different, since the anomalous samples obtained from partial digestion are often different from those obtained using near-total digestion (Figures 2b, 2c and 2d). The use of the analytical results obtained through partial digestion for these elements should, therefore, be used with caution when drawing up exploration models.

Click on image to enlarge

Figure 2 : Diagrams showing concentrations from ICP-MS analyses of the elements cerium (a), niobium (b), aluminum (c) and titanium (d), following partial digestion in aqua regia, and near-total four-acid digestion. The lines in figures (c) and (d) show the digestion rates for aluminum and titanium in common minerals following partial digestion in aqua regia (Snäll and Liljefors, 2000).

Among the elements with the lowest correlation coefficients are the major elements calcium (Ca; 0.10), sodium (Na; 0.36) and aluminum (Al; 0.07). If we assume that the analytical results following four-acid digestion represent the total concentrations, then it is possible to obtain an idea of the mineral phases that influenced the dispersion of data.5 Figure 2c, for example, shows the digestion rates for aluminum in common minerals following digestion in aqua regia (Snäll and Liljefors, 2000). The samples that give similar concentrations for both digestion methods may contain a predominance of easily-dissolved minerals such as biotite and/or chlorite (and possibly clays).

On the other hand, the samples showing higher concentrations following four-acid digestion, but lower concentrations following aqua regia digestion, may contain a higher proportion of feldspar and/or muscovite (Figure 2c). Similarly, Figure 2d shows the concentrations of titanium, an element that has a strong correlation in several samples, but for which systematically higher values are observed following four-acid digestion. This suggests that the titanium content of lake sediments is not due solely to biotite, in which titanium is almost totally dissolved following partial digestion, but rather to an amalgam of other, more resistant mineral phases such as titanite, ilmenite and rutile.

Future work

A study is currently under way to analyze the behaviour of these chemical elements using a spatial statistical approach. The results of this study will not only help to apply partial analyses more effectively for exploration purposes, but also to better understand the links between the bedrock and the concentrations of chemical elements in lake sediments.

Acknowledgement

A discussion with Sylvain Trépanier (CONSOREM) helped define the implications of the results more clearly and direct the treatment of the analyses.

References

PETRELLA, L., 2011. Caractérisation lithologique et pétrographique de l’intrusion syénitique de Misery. Québec: Ministère des Ressources naturelles et de la Faune, GM 65518. 34 pages.

SALVI, S. and A. E.WILLIAMS-JONES, 2006. “Alteration, HFSE mineralisation and hydrocarbon formation in peralkaline igneous systems: Insights from the Strange Lake Pluton, Canada”. Lithos, Volume 91, p. 19-34.

SNÄLL, S. and T. LILJEFORS, 2000. “Leachability of major elements from minerals in strong acids”. Journal of Geochemical Exploration, Volume 71, p. 1-12.

TRÉPANIER, S., 2007. Identification de domaines géochimiques à partir des levés régionaux de sédiments de fond de lacs. Québec: Ministère des Ressources naturelles et de la Faune, GM62922. 95 pages.

TURCOTTE, J., GIRARD, R. and C. MAURICE, 2011. Bonifier les techniques de prospection en précisant la source des métaux dans les sédiments de fond de lac. Québec: Ministère des Ressources naturelles et de la Faune, GM65640. 44 pages.


1 : Project 1982055 in the SIGEOM database.

2 : Aqua regia: a mixture of nitric and hydrochloric acid in the ratio  1:3.

3 : Under the heading "Geochemistry– Sediment sample "

4 : The analytical values for each chemical element were ranked according to their rank in the population as a whole. The lowest value was rank 1, while the highest was the same as the total number of samples. Ranks were used rather than actual values to calculate the correlation coefficients, in order to attenuate the effect of the most widely divergent samples.

5 : Environmental factors, the percentage of organic material, water quality and even micro-organisms can influence the content of certain elements in lake sediments (Trépanier, 2007; Turcotte et autres, 2011). These factors are not considered here.

 

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© Gouvernement du Québec, 2011