METAL ION SOIL GEOCHEMISTRY

WHAT IS METAL ION SOIL GEOCHEMISTRY?

The exploration for economic mineral deposits has become increasingly difficult due to the covering of prospective bedrock by glacially transported foreign (distal) till and its exotic deposits. Thick glaciofluvial and glaciolacustrine sediments, topped by organic deposits, make many traditional mineral exploration techniques in these terrains challenging, and in many cases misleading. 

SGS’S MMI (mobile metal ions) and ALS’s IL (Ionic Leach) are innovative exploration geochemical techniques that use selective and partial digestions, coupled with state-of-the-art instrumentation capable of measuring concentrations in the parts per billion (ppb) and sub-parts per billion range. MMI and IL can often provide the explorationist with tools to “see through” overburden and derive useful mineral exploration data for drill targeting of specific multivariate anomalies. This technology was first commercialized over 20 years ago, and we have been using it successfully ever since!

The multi-element digests used in  “MMI-M” and “IL” have been developed to provide analyses for 55 and 60elements at ppb and sub-ppb concentration levels for base and precious metals, pathfinder elements, rare earths and rare metals, and elements useful for mapping bedrock and alteration geology obscured by glacial overburden. The large number of elements in the analysis provides a cost effective opportunity to assess a prospective area of interest for a wide range of potential mineral deposits. 

Both MMI and IL requires properly trained field crews! Sampling is not carried out like the old B-horizon soil surveys. The collection of MMI and IL soil samples require the strict adherence to sampling protocols usually established during an orientation program. Geochemical data generated from properly collected MMI and IL soil samples will generate consistent values that can be correlated using either univariate or multivariate statistical and graphical presentations.

MMI and IL SAMPLE COLLECTION AND ANALYSIS
MMI and IL soil samples, each weighing between 250-500 grams, are collected at recommended intervals along either a cut and picketed grid or a GPS located grid. Sample locations are documented according to line coordinates and/or hand-held GPS readings (usually UTM NAD83) at each station. Samples are collected from a consistent depth according to strict protocols and orientation work parameters established by the field supervisor. Samples are usually collected with a stiff vinyl trowel after the initial sample site is dug with an appropriate sampling spade. The spade is kept clean and without paint or rust. Samples are bagged on site without preparation except for the occasional removal of rock fragments greater than 2 mm in diameter. The samples will be kept secure until the end of the survey, at which time they are delivered to the MMI Analyses Facility at SGS Laboratories in Vancouver, or the IL Analyses Facility at ALS Laboratories in Vancouver or Ireland.

DATA QUALITY and ANALYTICAL DUPLICATES

The samples submitted for analysis usually include field duplicates, replicates and/or standards. However, SGS and ALS also generate internal analytical duplicates and insert their own standards to monitor analytical reproducibility. 

Analytical reproducibility of the duplicate pairs is expected to be excellent over the observed range of concentration levels. Overall, the analytical data will be considered to be of excellent quality and suitable for interpretation when all quality measures meet expectations. 

DATA TREATMENT

Analytical data is examined visually for analyses less than the lower limit of detection (LLD) for ICP-MS analyses. LLD data, as per industry standard practice, is usually replaced with a value 1/2 of the LLD for statistical calculations and graphical representation. The 25th percentile of each element’s data is used to generate a background noise threshold to normalize all data and generate “response ratios” which are utilized in subsequent interpretations. 

Response ratios can also be used to compare MMI or IL data collected from different grids, areas and environments from year to year. Analytical data as received from the labs will be merged into a spreadsheet with sample site observations, calculated response ratios, and grid and/or UTM coordinates (NAD 83). 

METHOD OF INTERPRETATION

Multivariate statistical and graphical imaging techniques are utilized in the interpretation of MMI and IL. Subsequent to the calculation and plotting of response ratios as an image matrix, the plots are examined for anomalous spikes or groups of elevated responses for single and/or coincident elements. 

Generally, a gold response ratio of 20 times background is an initial indication of a low contrast anomalous response, although this “threshold” is not universal. Often a response ratio of between 10 and 30 can be considered to be of moderate contrast gold anomaly and >30, a high contrast response.

RESULTS

Metal Ion Element Image Correlation Matrix

We often present results in an image matrix comprising colour images for each element’s response ratios. The patterns of high response ratios can be easily compared amongst all of the elements for correlations indicative of key signatures, alteration, structures, lithology, etc. This way, we see where anomalous areas of our target mineralization occur and we can also easily see what other elements are elevated. 

It should also be noted that geochemical data is notoriously spiky in nature, and the overall patterns and trends should be observed for litho-geochemical interpretation, rather than any single data result at one specific sample site.

As noted previously, MMI and IL results do not represent the absolute amount of an element in the soil, but rather an indication of the presence of an element’s ions in anomalous amounts.