Biogeochemistry

Why biogeochemistry?

There is a wide range of economic and pathfinder elements that are absorbed by plant systems, which can be effectively measured by the chemical analysis of various plant parts. Once species selection, geographical distribution, tissue type, and sample size have been considered in the context of an exploration program, sample collection is easy and may cover a large geographical area more quickly than a soil survey. Because of the wide lateral reach of many root systems, anomalous targets that could be missed in a widely spaced soil grid have a better chance of biogeochemical detection and may be quickly followed up with more detailed soil sampling or trenching.

Biogeoquímica

Vegetation preparation methods

Sample preparation may include sorting different tissue types, drying, and maceration in specialised milling equipment. Gentle washing to remove dust and pollen is also offered although it is not usually required; if needed, should be used with caution due to the potential to remove surface tissues that can contain important element concentrations. Exact sample preparation specifications will vary according to the plant species, type of tissue collected, and the analytical method of choice.

To ash or not to ash?

When using biogeochemistry for exploration, one of the main decisions during planning is whether to ash samples before analysis or not. Ashing is the process by which biogeochemical samples are heated at 475°C for 24 hours to reduce their weight and pre-concentrate elements of interest (ALS code VEG-ASH01). The alternative preparation method is to mill a dry biogeochemical sample so that it can be sub-sampled for analysis (ALS code VEG-MILL01). Each has their advantages; non-ashed samples represent the concentrations of all elements, even those that are volatile during heating, whereas ashed samples pre-concentrate most elements resulting in fewer or no samples with values below detection. By ashing biogeochemical samples before analysis, the detection level for most elements is effectively lowered. ALS reports results as both a raw analysis result and a calculated concentration based on the pre- and post-ash weights (VEG41a-FACTM).

Code Description
VEG-MILL01 Milling of dry plant tissue to 100% passing 1mm. Produces a homogenous and representative pulp that can be subsampled for analysis.
VEG-ASH01 Vegetation sample is ashed at 475°C for 24 hours. Pre- and post-ashing weights are reported.
Average ash yields are 2-4% for species commonly used in exploration surveys. Minimum recommended sample weight is 100g.

Multi-element analysis of vegetation

Numerous studies in multiple regions have shown that vegetation is an effective sampling media for the identification of geochemical anomalies associated with mineralisation. The ecological environment greatly affects the role that plants have in geochemical dispersion and concentration. Plant roots may sample groundwater, soil or rock through the production of organic acids at the root tips, dissolving minerals and allowing the plant to uptake nutrient elements as well as waste elements that are subsequently segregated into various plant parts. Boreal and deciduous forests, grasslands and highlands, hot and cold arid regions, and all other climate zones will have different species that are more suitable for geochemical sampling than others and different key elements by which the plants reflect geochemical anomalies.

Methods - ME-VEG41™ & ME-VEG41a™

ALS offers a multi-element vegetation analysis either after drying and maceration (ME-VEG41™) or after ashing (ME-VEG41a™). The methods use an aqua regia digestion to produce 50+ elements which are reported with the industry's lowest detection levels. When ME-VEG41a™ is chosen results can either be reported uncorrected to original sample weight or by VEG41a-FAC™ which corrects concentrations back to pre-ash concentrations based on the amount of ash derived from the original vegetation.

Code Analytes & lower limits (ppm)
ME-VEG41™
unashed
ME-VEG41a™
ashed 		Ag 0.001 Cu 0.01 Nb 0.002 Ta 0.001
Al 0.01% Fe 1 Ni 0.04 Te 0.005
As 0.01 Ga 0.004 P 0.00% Th 0.002
Au 0.0002 Ge 0.005 Pb 0.01 Ti 0.001%
B 1 Hf 0.002 Pd 0.001 Tl 0.002
Ba 0.1 Hg 0.001 Pt 0.001 U 0.005
Be 0.01 In 0.005 Rb 0.01 V 0.05
Bi 0.001 K 0.01% Re 0.001 W 0.01
Ca 0.01% La 0.002 S 0.01% Y 0.003
Cd 0.001 Li 0.1 Sb 0.01 Zn 0.1
Ce 0.003 Mg 0.001% Sc 0.01 Zr 0.02
Co 0.002 Mn 0.1 Se 0.005
Cr 0.01 Mo 0.01 Sn 0.01
Cs 0.005 Na 0.001% Sr 0.02
VEG41a-FAC™
Detection
limits when
backcalculated
using the
original preash
weight of
the sample 		Ag 0.00005 Cu 0.0005 Nb 0.0001 Ta 0.00005
Al 0.0005% Fe 0.05 Ni 0.002 Te 0.0003
As 0.0005 Ga 0.0002 P 0.00005% Th 0.0001
Au 0.00001 Ge 0.0003 Pb 0.0005 Ti 0.00005%
B 0.05 Hf 0.0001 Pd 0.00005 Tl 0.0001
Ba 0.005 Hg 0.00005 Pt 0.0001 U 0.0003
Be 0.0005 In 0.0003 Rb 0.0005 V 0.003
Bi 0.00005 K 0.0005% Re 0.00005 W 0.0005
Ca 0.0005% La 0.0001 S 0.0005% Y 0.0002
Cd 0.00005 Li 0.005 Sb 0.0005 Zn 0.005
Ce 0.0002 Mg 0.00005% Sc 0.0005 Zr 0.001
Co 0.0001 Mn 0.005 Se 0.0003
Cr 0.0005 Mo 0.0005 Sn 0.0005
Cs 0.0003 Na 0.00005% Sr 0.001

Rare earth element analysis

Rare earth elements can be important when exploring for deposit types that are enriched in these elements. Several rare earth elements are routinely analysed in the multi-element packages offered at ALS but to obtain the concentration of all REEs add-on methods VEG41-REE™ and VEG41a-REE™ can be used.

VEG41-REE™ & VEG41a-REE™

ALS offers add-on methods for rare earth analysis of vegetation samples that have been ashed or non-ashed. The analysis uses the same digestion as the multi-element packages.

 
Code  Analytes & lower limits (ppm)
VEG41-REE™
unashed
VEG41a-REE™
ashed
Add-on only 		Dy 0.002 Gd 0.002 Nd 0.001 Tb 0.001
Er 0.002 Ho 0.001 Pr 0.002 Tm 0.001
Eu 0.002 Lu 0.001 Sm 0.003 Yb 0.003
VEGFAC-REE™
Add-on only 		Dy 0.0001 Gd 0.0001 Nd 0.00005 Tb 0.00005
Er 0.0001 Ho 0.00005 Pr 0.0001 Tm 0.00005
Eu 0.0001 Lu 0.00005 Sm 0.0002 Yb 0.0002

References and further reading

Dunn, C.E. 2007. Biogeochemistry in Mineral Exploration. Handbook of Exploration and Environmental Geochemistry 9. Series Ed. M. Hale. Elsevier, Amsterdam, The Netherlands, 462 pp + CD.


Smith, S.C., and Vance, R.B., 2005. Discovery using metal concentrations in plants, Rosebud Mine, Pershing County, Nevada, in Rhoden, H.N., Steininger, R.C., and Vikre, P.G., eds., Geological Society of Nevada Symposium 2005; Window to the World, Reno, Nevada, May 2005, p. 1225-1240.

Frequently asked questions

  • How should my vegetation samples be stored for transport to the lab?

    It is important that vegetation samples are stored and transported in either paper or cloth sample bags. Paper and cloth bags allow moisture to escape and the samples to dry out rather than becoming moldy.

  • Should I insert QA/QC samples with biogeochemical samples?

    Yes, it is important to include blind QA/QC samples with all geochemistry samples submitted to the lab. Often rice is used as a blank for biogeochemistry samples. Duplicates can be taken as a second sample from the same location and/or a second aliquot of a prepared sample. There are several sources of vegetation standards that can be purchased.

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