To characterize the evolution of dioctahedral interstratified clay minerals in the Golden Cross epithermal deposit, New Zealand, hydrothermally altered volcanic rocks containing the sequence smectite through illitesmectite (I–S) to muscovite were examined by optical microscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission and analytical electron microscopies (TEM/AEM).

XRD analyses of 30 oriented clay samples show a broad deposit-wide trend of increasing illite content in I–S with increasing depth and proximity to the central vein system. Six representative samples were selected for SEM/TEM study on the basis of petrographic observations and XRD estimates of I–S inter-stratification. Ca and Na are the dominant interlayer cations in smectite, but as the proportion of illite layers in I–S increases, so does the K content and (IVAl + VIAl)/Si ratio. Layers and packets tend to flatten and form larger arrays, reducing the amount of pore 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 ~300 Å thick, but a limited chemical and structural gap exists between illite, which is intermediate 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 2M1 polytype.

Petrographic, SEM, and TEM data imply that all phyllosilicates formed via neoformation directly from fluids. Relatively mature I–S and micas form simultaneously, without progressing through the series of transformations that are commonly assumed to characterize diagenetic sequences during burial metamorphism in mud-dominated basins. Although the overall distribution of clay minerals is consistent with temperature as a controlling variable, local heterogeneities in the distribution of clay minerals were controlled by water/rock ratio, which varied widely owing to different rock types and fracture control.

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