Dioctahedral clay minerals in the volcanic-hosted Golden
Cross epithermal Au deposit, New Zealand,
have been investigated by petrography, X-ray diffraction (XRD), scanning
electron microscopy (SEM) and transmission electron microscopy/analytical
electron microscopy (TEM/AEM) to determine the nature of these minerals in a
hydrothermal ore deposit. Heterogeneity in the distribution of dioctahedral
clays is observed from the hand-specimen scale to the TEM scale, indicating that
primary rock textures and fluid pathways (related to rock type,
porosity/permeability and fluid/rock ratios), in addition to temperature, are
important factors controlling the distribution. As the proportion of illite in
mixed-layer illite/smectite (I/S) decreases, TEM observations show decreasing
layer stacking order, parallelism of packets, and packet size, and increasing
apparent porosity. AEM structural data show a corresponding decrease in
AlIV, AlVI, and K+, and an increase in Si,
Ca2+, and Fetotal. The observations of this study indicate
that local dioctahedral clay mineral reaction progress is controlled mainly by
permeability and structure, but that this is superimposed on a predictable,
system-wide clay mineral distribution that is controlled by temperature and
fluid/rock ratio.