Characterising the hydrothermal alteration of the Broadlands-Ohaaki geothermal system, New Zealand, using short-wave infrared spectroscopy

Hydrothermal clay minerals present in the Broadlands-Ohaaki geothermal fiel d were characterised by field portable shortwave infrared spectroscopy. Thr ee major alteration zones, an upper smectite, a middle illite and a lower i llite-chlorite, are spectrally separable. The zoning pattern is generally c onsistent with the thermal structure of the geothermal field, although occa sionally zone boundaries cut present-day isotherms. The data indicate that temperature is the major control on clay zoning and permeability plays a su bordinate role. Both bridellite and montmorillonite are common in the upper, low-temperatur e smectite zone. Kaolinite, mainly of low crystallinity, marks the margin o f the field where cool acidic ground waters inflow. In the middle alteratio n zone, illite, dominantly K-rich, shows a narrow compositional variability . Some highly permeable zones are characterised by illite with low octahedr al Al contents. Ammonium-bearing illite and buddingtonite are present local ly in permeable horizons within the illite zone, where temperatures are abo ve 200 degreesC. Chlorite is most abundant in the lower alteration zone (te mperature >250 degreesC), although it also occurs unevenly in the upper and middle alteration zones. Chlorite varies from Mg- to Fe-rich varieties (bu t mostly with Mg# values <0.5), but no compositional trends with respect to depth are spectrally detectable. <(c)> 2001 Elsevier Science B.V. All righ ts reserved.