SURFACE HYDROTHERMAL ALTERATION AND EVOLUTION OF THE TE-KOPIA THERMALAREA, NEW-ZEALAND

The Te Kopia Thermal Area has surface manifestations that extend over an area of about 3 x 1.2 lan along the Paeroa Fault Zone. Steaming gro und, fumaroles, mud and acid sulphate pools are present on both the up thrown and downthrown blocks of the Paeroa Fault, but near-neutral pH chloride-bicarbonate springs discharge up to 2 km west of the fault tr ace. The host rocks comprise mainly Quaternary ignimbrites that dip ge ntly (similar to 7 degrees) eastwards but are vertically displaced by the fault by several hundred metres. Hydrothermal alteration is widesp read, and the present thermal activity is now producing kaolinite, alu nite, silica residue, hematite and cristobalite. This assemblage occur s both in the otherwise unaltered ignimbrites, as well as overprinting the products of earlier hydrothermal activity. Evidence for changes t hat have occurred in the nature and extent of thermal activity at Te K opia include: (1) The widespread overprint of a kaolinite-alunite-cris tobalite assemblage upon hydrothermal minerals (including adularia, qu artz and mordenite) produced by alkali-chloride, bicarbonate-chloride, and heated groundwaters. (2) The occurrence of silica sinter which de posited from alkali chloride springs that discharged about 3000 years ago. This is now a steam zone. (3) Areas of cold ground that were once hot, as demonstrated by the presence of hydrothermal minerals. (4) Th e presence of euhedral quartz crystals at the surface on the upthrown fault block. Fluid inclusions in these crystals from two places homoge nise at 188 +/-15 degrees C and 196 +/-11 degrees C. The trapped fluid s have apparent salinities between 0.2 and 0.4 wt % NaCl equivalent. T he quartz crystals thus grew at depths of at least 120 m below the wat er table. Vertical movements along the Paeroa Fault totalling at least 300 m have uplifted these crystals to their present positions. The su rface geology and alteration provide evidence that thermal activity at Te Kopia has been long lived, possibly as long at 120 000 years. Howe ver, the hydrology of the system and its thermal regime have changed g reatly during its lifetime, mainly in response to movements along the Paeroa Fault.