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Exploration and Mining Geology; January 2006; v. 15; no. 1-2; p. 75-88; DOI: 10.2113/gsemg.15.1-2.75
© 2006 Canadian Institute of Mining, Metallurgy & Petroleum
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Alkali-Alumina and MgO-Alumina Molar Ratios of Altered and Unaltered Rhyolites

J.F. DAVIES1 and R.E. WHITEHEAD1

Department of Earth Sciences, Laurentian University, Sudbury, Ontario P3E 2C6.

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 Na2O and gains in K2O. The extreme loss of Na2O from highly altered rhyolites immediately adjacent to VMS deposits is strikingly illustrated by alkali-alumina plots. MgO/Al2O3 vs. K2O/Al2O3 diagrams display a strong negative correlation between MgO and K2O that results from the conversion of sericite to chlorite. A similar negative correlation between MgO and whole-rock {delta}18O 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.

Key Words: Alkali-alumina molar ratio • MgO-alumina molar ratio • Alteration • Rhyolite • Volcanogenic massive sulfide deposit






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