One of the analytical problems in exploration geochemistry is the choice of a method of sample attack which would be rapid, simple and reliable in terms of precision and discrimination between different concentration levels of elements sought. The wet sample attacks currently used in analysis of silicate rocks vary from total decomposition to leaching by weak extractants. The total decomposition of such materials is usually achieved either by alkaline fusion or hydrofluoric acid attack. The first method requires the use of platinum vessels which is both expensive and slow for batch operation. The second type of attack is more widely applied in rock geochemistry, but it is still relatively slow for exploration purposes and requires extra care (especially when hydrofluoric acid is used in mixture with perchloric acid).
AQUA REGIA EXTRACTION (ARex) – 2 ml of acid are added to 2 g of sample in a Pyrex beaker and evaporated to dryness. The residue is moistened with concentrated HCl and evaporation repeated.
PYROSULFATE FUSION (PSf) – 0.5 g of sample is mixed and fused with 2 g of potassium pyrosulfate in a Pyrex test-tube to a quiescent melt.
HYDROFLUORIC-PERCHIORIC ACID ATTACK – 1 g of sample is slowly evaporated to dryness with 20 ml of HF and 4 ml of HClO4 in a teflon dish.
The precision is usually determined by replicate analysis of samples and expressed as coefficient of variation (C):
C = 100 x s/X
where s is standard deviation and X arithmetic mean of the replicates. Formula (1) gives the precision at 66.6% confidence level, i.e. 1/3 of observations fall outside the precision interval determined. A more reliable estimate of precision is obtained by taking 2s as basis of calculation which gives: the confidence level of 95%.
It is often advisable to use the method of statistical series after logarithmic transformation of data. However, it does not seen to be necessary when the difference in concentration between end members of the series is moderate which is the case in the present study.
The parameters obtained on statistical series of Guichon batholith rocks are summarized below,
Note: exH – proportion of metal extracted from H
exL – proportion of metal extracted from L
Amount extracted by HF-HClO4 attack is taken as 100%. All figures are rounded to whole numbers.
The extraction of zinc is considerably lower than that of copper, particularly at lower concentrations. Unlike copper, a bettor contrast for zinc is achieved by PSf than by ARex. These differences indicate that the two metals differ in their form of occurrence in intrusive rocks studied. It is known that copper often occurs in igneous rocks as sulfide, whereas zinc tends to accumulate in ferromagnesian silicates and magnetite, but is rarely observed in sulfide form.
Note: G – geometric means, X – arithmetic means, R² – coefficient of determination.