Alkali-alumina and MgO-alumina plots are an effective instrument for displaying chemical and mineralogical compositions of altered and unaltered rhyolites and rhyodacites. They reveal that most supposedly unaltered rhyolites, apart from those of Cenozoic age, have undergone losses in Na_O and gains in K_2O. The extreme loss of Na_2O from highly altered rhyolites immediately adjacent to VMS deposits is strikingly illustrated by alkali-alumina plots. MgO/Al_2O_3 vs. K2O/Al_2O_3 diagrams display a strong negative correlation between MgO and K_2O that results from the conversion of sericite to chlorite. A similar negative correlation between MgO and whole-rock delta~(18)O is apparent on plots of these two parameters. Alkali-alumina plots underscore problems in selecting appropriate precursor compositions required for mass-balance calculations. Molar ratio plots are a viable alternative to such calculations.
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