| Abstract |
Andesites are minor rock types in the Taupo Volcanic Zone (TVZ) and most occur near the Tongariro Volcanic Centre (TVC), White Island, Putuaki and Rolles Peak near Rotokawa. However, several subsurface andesites have also been intersected by wells drilled at the Ngatamariki, Rotokawa, Wairakei, Waiotapu, Kawerau and Tauhara geothermal fields. The Spa Andesite at Tauhara, at least 201 meters thick, comprises interbedded lavas and breccias between 390 and 591 m depth. It was intersected only in well THM18 in the Spa Bowl area. Some of rocks are intensely altered. This study characterizes the Spa Andesite and compares it with other andesites within the TVZ. Twelve cores were chosen to cover the vertical extent of the andesite as well as differences in its hydrothermal alteration. The samples were examined by petrography, X-Ray Diffraction (XRD), X-Ray Fluorescence (XRF) and fluid inclusion geothermometry. The Spa Andesite cores have silica contents ranging from basalt to dacite (52.2 to 63.4 wt.%). They have the characteristics of orogenic andesites, as described by Gill (1981), with Ba/La, Th/La, and La/Nb ratios ranging from 15-80, 2-7, and 2-5 respectively. The Spa Andesite has a composition, particularly its iron content, similar to other andesites in the central TVZ (Rotokawa, Ngatamariki, and Rolles Peak). The hydrothermal alteration of the Spa Andesite is dominated by quartz, calcite and chlorite with subordinate wairakite, two unknown zeolites, titanite, and pyrite. The clays indicate a slight temperature reversal as a chlorite + illite zone retrogrades towards an interlayered chlorite (0.9)-smectite + illite assemblage near the base of the andesite. The degree of clay crystallinity also decreases with depth. Fluid inclusion homogenization temperatures (Th) are generally consistent with those indicated by the secondary minerals, including clays, in the temperature ranges of 235-250 degC and 220-235 degC at 435m and 587m depths respectively. The altering fluid was of near neutral pH with an apparent salinity of 1.4 % wt. |