Genesis of dioctahedral phyllosilicates during hydrothermal alteration of volcanic rocks: II. The Broadlands-Ohaaki hydrothermal system, New Zealand

The clay mineral textures, assemblages, formation mechanisms, and controlli ng geological parameters relating to alteration of silicic volcanic rocks b y hydrothermal solutions, in core samples from the Broadlands-Ohaaki hydrot hermal system, New Zealand, were investigated using X-ray diffraction (XRD) , scanning electron microscopy (SEM), and transmission and analytical elect ron microscopy (TEM/AEM). Mineralogical and textural relations of this acti ve hydrothermal system, for which temperatures and fluid relations are well known, are equivalent to those in the Golden Cross hydrothermal gold depos it as described in Part 1. XRD data show a sequence of clay minerals from smectite to a range of inter stratified I-S to mica with increasing depth and temperature, on average. T EM observations are in general agreement with XRD data, especially with res pect to relative proportions of illite (I)- and smectite (S)- like layers. TEM data also show that: (1) Smectite packets contain no discrete illite-li ke layers in samples identified as (Reich-weite, R = 0) I-S by XRD. They co exist with separate packets of (R = 1) I-S. (2) A continuous range in IS oc curs from (R = 1) I-S with increasing proportion of illite-like layers, but at high illite-like layer contents there is a gap between I-S and illite. (3) 1M and 2M, polytypes of mica coexist in separate packets, but the rare 1M polytype has a larger Mg-VI content. The data imply that clay minerals formed by dissolution and neocrystallizat ion directly from volcanic phases, although multiple reaction events can no t be ruled out. Such "episodic" alteration produces a sequence of clay mine rals identical to those of prograde diagenesis of pelitic sediments. This r esult implies that the presence of a continuous sequence is not definitive proof of continuous sequences of transformation as a function of time and c ontinuous burial. Reaction progress of the clay-mineral sequence is in gene ral accord with the known temperature gradient, but with significant and co mmon exceptions. High porosity and permeability, both inherent in rock text ure and local structure, an inferred to foster local reaction progress, as consistent with metastability of phases and the Ostwald step rule.