Publication Type:

Journal Article

Authors:

Davies, J. F.; Whitehead, R. E.

Source:

Exploration and Mining Geology, Canadian Institute of Mining, Metallurgy and Petroleum, Montreal, QC, Canada, Volume 15, Number 1-2, p.75-88 (2006)

ISBN:

0964-1823

Keywords:

alkalic composition, alteration, aluminum oxides, Cenozoic, chemical composition, chemical ratios, chemical reactions, chlorite, chlorite group, correlation, geochemistry, Hydrothermal alteration, igneous rocks, isotope ratios, Isotopes, magnesium oxides, mass balance, massive deposits, massive sulfide deposits, metasomatism, mineral composition, O-18/O-16, oxides, oxygen, rhyodacites, rhyolites, sericite, sheet silicates, silicates, Stable isotopes, volcanic rocks

Abstract:

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 (sub 2) O and gains in K (sub 2) O. The extreme loss of Na (sub 2) O from highly altered rhyolites immediately adjacent to VMS deposits is strikingly illustrated by alkali-alumina plots. MgO/Al (sub 2) O (sub 3) vs. K (sub 2) O/Al (sub 2) O (sub 3) diagrams display a strong negative correlation between MgO and K (sub 2) O that results from the conversion of sericite to chlorite. A similar negative correlation between MgO and whole-rock delta (super 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.

Notes:

GeoRef, Copyright 2018, American Geological Institute.<br/>2007-008136