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The Geochemical Abundance Index (GAI)

One measure of enrichment of elements in whole rock samples is the Geochemical Abundance Index (GAI). The GAI compares the actual concentration of an element in a sample with the median abundance for that element in the most relevant media (such as crustal abundance, soils, or a particular rock type). The main purpose of the GAI is to provide an indication of any elemental enrichments that may be of environmental importance. The GAI for an element is calculated as follows:

GAI = log2 [ C / (1.5*S) ]

where C is the concentration of the element in the sample and S is the median content for that element in the reference material (mean world soil, crustal abundance, etc). The GAI values are truncated to integer increments (0 through to 6, respectively) where a GAI of 0 indicates the element is present at a concentration similar to, or less than, median abundance and a GAI of 6 indicates approximately a 100-fold, or greater, enrichment above median abundance. The actual enrichment ranges for the GAI values are as follow:

GAI=0 represents <3 times median soil content
GAI=1 represents 3 to 6 times median soil content
GAI=2 represents 6 to 12 times median soil content
GAI=3 represents 12 to 24 times median soil content
GAI=4 represents 24 to 48 times median soil content
GAI=5 represents 48 to 96 times median soil content
GAI=6 represents more than 96 times median soil content

As a general guide, a GAI of 3 or above is considered significant and such an enrichment may warrant further examination.

Table 1 presents an example of the elemental composition of mineralized mine rock and corresponding GAI values compared to median world soil concentrations.

Elements that were significantly enriched (i.e. GAI ≥3) in all three rock types included sulphur (8.1 to 13.4 %), copper (0.35 to 1.34%), molybdenum (96 to 154 mg/kg), and selenium (16 to 51 mg/kg). The phyllic rock was also significant enriched with arsenic (332 mg/kg), whilst the metasediment was significantly enriched with arsenic (480 mg/kg), cadmium (5.2 mg/kg), and zinc (1,721 mg/kg).

Table 1. Elemental composition of mineralized rocks