Genesis of dioctahedral phyllosilicates during hydrothermal, alteration ofvolcanic rocks: I. The golden cross epithermal ore deposit, New Zealand

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.