A LEAST-SQUARES APPROACH TO MASS-TRANSPORT CALCULATIONS USING THE ISOCON METHOD

A statistically rigorous approach based on the isocon method (Grant, 1 986) to evaluate mass transport is presented. Chemical analyses of mul tiple samples of unaltered (parent) and altered rock are used to calcu late the average oxide or element concentration and its standard devia tion for each rock population. In a typical application, the uncertain ties in element concentration associated with the lack of homogeneity in each population outweigh the analytical uncertainty. An efficient a lgorithm is presented to select the immobile elements by identifying t he maximum number of elements that are, within their uncertainties, co mpatible with the same isocon. The actual isocon is constructed by for cing a line through the origin and optimizing its slope using a weight ed least-squares procedure for the selected immobile elements. Thus, i ndividual uncertainties for each immobile element are taken into accou nt. As an illustration, the procedure is applied to a published set of data on potassic and sericitic alterations of quartz monzonite of the Bingham porphyry copper system. The surprising result indicates that the apparent increase of potassic alteration phases is a consequence o f acid leaching of calcium and sodium. No mass transport of potassium is indicated. Silica, water, and sulfur were added during alteration. The best at isocon requires a small mass increase during each alterati on process which, however, is within the uncertainty obtained for the slope of the isocon. A software package is available from the authors in the form of a FORTRAN 77 source code.