A statistically rigorous approach based on the isocon method (Grant,
1986) to evaluate mass transport is presented. Chemical analyses of
multiple samples of unaltered (parent) and altered rock are used to
calculate the average oxide or element concentration and its standard
deviation for each rock population. In a typical application, the
uncertainties in element concentration associated with the lack of
homogeneity in each population outweigh the analytical uncertainty. An
efficient algorithm is presented to select the immobile elements by
identifying the maximum number of elements that are, within their
uncertainties, compatible with the same isocon. The actual isocon is
constructed by forcing a line through the origin and optimizing its
slope using a weighted least-squares procedure for the selected immobile
elements. Thus, individual uncertainties for each immobile element are
taken into account.
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 of 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 alteration
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.