RATIO ANALYSIS OF BULK GEOCHEMICAL DATA: TRACKING ORE-RELATED CRYPTIC ALTERATION BY MODELLING MINERAL CHANGES

Many economic base metal deposits are the product of metal sulphide precipitation from hydrothermal fluids. The chemical reactions between these mineralising fluids and the rocks hosting the deposits commonly produce 'haloes ' of distinctive mineralogical and geochemical character, zoned about the deposits. The extent of the alteration depends on how extensive the fluid pathway is and the importance of diffusive processes but is generally much larger than the deposits themselves. Exploration companies commonly rely on geochemistry to identify alteration haloes. However, examination of raw data or even simple ratio trends (e.g. empirical alteration indices) is prone to error due to closure effects and/or pre-existing background variation. Closure can be avoided by using ratios, or by utilising mass balance approaches based on fixing volume, mass or concentration changes between samples of parent and daughter lithologies. Using a parent-daughter approach is limiting because only pairs of samples can be compared at any one time and also because an unaltered equivalent must be produced for each sample examined in this way. Ratio methods are not restricted in this fashion and are more amenable to interrogation of large data sets. Furthermore, the ratio methods outlined below are readily applied to commercially derived lithogeochemical assays. Pearce Element Ratios (PERs) are molar ratios formulated using a conserved element as the denominator