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.
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.
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).
|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
Average ash yields are 2-4% for species commonly used in exploration surveys. Minimum recommended sample weight is 100g.
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.
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)|
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.
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)|
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.
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.
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.
A method for non-quantitative halogen analysis of vegetation is offered for exploration applications where relative concentrations can be used for vectoring.MORE INFORMATION
Geochemical exploration in regions of transported cover may also benefit from hydrogeochemistry sampling.MORE INFORMATION