Da. Tillick et al., Genesis of dioctahedral phyllosilicates during hydrothermal, alteration ofvolcanic rocks: I. The golden cross epithermal ore deposit, New Zealand, CLAY CLAY M, 49(2), 2001, pp. 126-140
To characterize the evolution of dioctahedral interstratified clay minerals
in the Golden Cross epithermal deposit. New Zealand, hydrothermally altere
d volcanic rocks containing the sequence smectite through illite-smectite (
IS) to muscovite were examined by optical microscopy, X-ray diffraction (XR
D), scanning electron microscopy (SEM), and transmission and analytical ele
ctron microscopics (TEM/AEM). XRD analyses of 30 oriented clay samples show
a broad deposit-wide trend of increasing illite content in IS with increas
ing depth and proximity to the central vein system. Six representative samp
les were selected for SEM/TEM study on the basis of petrographic observatio
ns and XRD estimates of I-S inter-stratification. Ca and Na are the dominan
t interlayer cations in smectite, but as the proportion of illite layers in
IS increases, so does the K content and (Al-IV + Al-VI)/Si ratio. Layers a
nd packets tend to flatten and form larger arrays, reducing the amount of p
ore space, Smectite coexists with (R = 1) I-S, rather than being (R = 0) I-
S where R is the Reichweite parameter. The highest alteration rank samples
contain discrete packets of mica to similar to 300 Angstrom thick, but a Li
mited chemical and structural gap exists between illite, which is intermedi
ate in composition between common illite and muscovite, and illite-rich I-S
. Selected-area electron diffraction (SAED) patterns of mica show that the
1M polytype dominates, rather than the common 2M(1) polytype.
Petrographic, SEM, and TEM data imply that all phyllosilicates formed via n
eoformation directly from fluids. Relatively mature IS and micas form simul
taneously, without progressing through the series of transformations that a
re commonly assumed to characterize diagenetic sequences during burial meta
morphism in mud-dominated basins. Although the overall distribution of clay
minerals is consistent with temperature as a controlling variable, local h
eterogeneities in the distribution of clay minerals were controlled by wate
r/rock ratio, which varied widely owing to different rock types and fractur
e control.