Quantitative assessment of actual metasomatic changes in mass and volume requires knowledge of (1) chemical compositions and densities of actual protoliths and altered products and (2) the actual bulk volume change of the rock or the actual absolute mass change of any chemical component. Assumed values for (1) and (2) produce metasomatic models that do not necessarily reflect actual metasomatism.
Logarithmic ratio diagrams of the form log10 (X/Z) versus log10 (Y/Z) portray metasomatism in an elegantly simple fashion; metasomatism in a three component system where only one of X, Y, or Z is conserved produces a collinear array of points with predictable slope of 0, infinity, or 1, respectively, independent of protolith composition. Such relationships can be used to identify conserved components and to discriminate altered from unaltered rocks.
Any two conserved components can be identified on the basis of slope and collinearity. Actual conserved components thus identified enable precise calculation of actual rather than model metasomatic changes.
Analyses of 1133 Cenozoic volcanic rocks from a wide variety of tectonomagmatic environments plot as single, well defined trends on certain logarithmic ratio diagrams. By assuming that unaltered pre-Cenozoic volcanic rocks should conform to Cenozoic trends, it is possible to discriminate altered from unaltered volcanic rocks.
Published wall-rock alteration data for the Millenbach mine are evaluated using these new techniques. SiO2, TiO2, and Al2O3 are found to be the most frequently conserved components. Averaged rocks show volume changes of -7.15% to +7.20%, whereas actual samples show volume changes of -22.6% to +32.4%. Alteration is found to be spatially inhomogeneous but more areally extensive with composition changes generally more extreme than previously recognized.
Improved ability to detect and quantify metasomatism results in better definition of its nature and extent. Lithogeochemical exploration, geochemical modeling of metasomatism, and petrogenetic studies will be enhanced by these new techniques.