Normative minerals and alteration indices developed for mineral exploration

NORMAT is a new technique for quantifying hydrothermal alteration in greenschist facies rocks using normative mineral ratios without precursor or mineralogical determination.Using five petrologic rules, the NORMAT calculations first compute normative alteration (paragonite, sericite, chlorite, pyrophyllite) and precursor minerals (albite, orthoclase, anorthite), as well as normative loss on ignition (LOI) and CO2 contents. The results are then computed as alteration indices that quantify the overall alkali element depletion in terms of the paragonitization, sericitization, chloritization and pyrophillitization components. Equations for the alteration indices are ratios between the normative alteration and precursor minerals, thus providing a relative numerical measure of alteration intensity that is independent of protolith. Alteration by carbonatization is quantified by the IPAF index, which represents the position of the sample''s analytical LOI value within a range bounded by CO2-free normative LOI and maximum possible normative LOI-CO2 content. This procedure also allows the CO2 content to be estimated from LOI, which can then be used to calculate normative carbonate minerals.Alteration index (AI) values and normative minerals for 28 selected samples from the Val d''Or Formation illustrate the main properties of normative mineral ratio alteration indices. [Copyright &y& Elsevier]