Genesis of Dioctahedral Phyllosilicates During Hydrothermal Alteration of Volcanic Rocks: I. The Golden Cross Epithermal Ore Deposit, New Zealand

Abstract

Abstract--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 illite-smectite (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 SEMFFEM study on the basis of petrographic observations and XRD estimates of I-S interstratification. 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 (WAl + VlAl)/Si ratio. Layers and packets tend to flatten and form larger arrays, reducing the amount of pore space. Smectite coexists with (R = l) 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 A 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. Selectedarea electron diffraction (SAED) patterns of mica show that the 1M polytype dominates, rather than the common 2M~ 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.

Genesis of Dioctahedral Phyllosilicates During Hydrothermal Alteration of Volcanic Rocks: II. The Broadlands-Ohaaki Hydrothermal System, New Zealand

“…However, Figure 14 shows that IvSi decreases, when plotted against the proportion of illite-like layers. This trend is expected for dioctahedral, interstratified clays, and was also observed by Tillick et al (2001) for dioctahedral clays of the Golden Cross deposit.…”

“…Several significant differences were o b s e r v e d in the samples o f Broadlands-Ohaaki, as well as those described by Tillick et al (2001). These involve (1) the absence of interstratification of smectite and illite in (R = 0) I -S , (2) a gap between I -S with high proportions of illite-like layers and illite, and (3) the presence of 1M polytypism.…”

“…Air-dried XRD data for bulk samples were obtained with a Philips automated diffractometer with a graphite monochromator and CuKct radiation (35 kV and 15 mA), using a step size of 0.020~ counting time of 10 s/ step and scanning range from 2 to 32~ Data for oriented separates were determined by using a Philips 1050/25 diffractometer with CuKo~ radiation (40 kV and 20 mA) and a graphite monochromater. Methods of preparation of samples for SEM and TEM observations, as well as conditions and methods of observation, were described by Tillick et al (2001). Analytical electron microscopy (AEM) analyses were obtained by rastering over areas as large as possible, in part to minimize alkali diffusion.…”

“…Sample GX4 of Tillick et al (2001) is typical of illite-rich I-S. This material was expected to consist, or have dominant units, of In with n equal to large numbers (dominant illite-like layers with random smectite-like layers).…”

“…The system is hosted by volcanic rocks and contains a dioctahedral clay-mineral series from smectite through I-S to muscovite, similar to that found in sedimentary sequences such as the Texas Gulf Coast shales. A unique opportunity exists to correlate temperature, fluid composition, and other variables with the assemblages of clay minerals for comparison with ancient systems, such as the Golden Cross gold-silver epithermal deposit (Tillick et al, 2001). This is the second of a two-part series, the first being a description of the clay mineral relations at the Golden Cross mine.…”