TY - JOUR
T1 - Fluid-rock interaction processes in the Te Kopia geothermal field (New Zealand) revealed by SEM-CL imaging
AU - Bignall, Greg
AU - Sekine, Kotaro
AU - Tsuchiya, Noriyoshi
N1 - Funding Information:
This study was financially supported by Grant-in-Aid to G.B. by Japan Society for the Promotion of Science (JSPS; Scientific Research B—14404015). Professor K. Nakatsuka (Tohoku University) is acknowledged for his continued encouragement and support of this work. The authors wish to thank Dr. Enrico Barbier, Dr. Giovanni Gianelli, and an anonymous Geothermics reviewer for their comments, and for improving the readability of the paper.
PY - 2004/10
Y1 - 2004/10
N2 - Scanning electron microscopy-cathodoluminescence (SEM-CL) imaging of hydrothermal quartz exposed by weathering in the Te Kopia geothermal field (New Zealand) has revealed a history of crystal growth, dissolution, overprinting and fracturing that cannot be detected using other observational techniques (e.g. transmitted or reflected light microscopy, back-scattered electron imaging or secondary electron imaging). The crystals initially grew as CL-dark quartz, at least 350 m below their present location on the Paeroa Fault scarp, in a neutral pH, 215±10 °C liquid reservoir (inferred from the analysis of primary liquid fluid inclusions: mean Th of 213 °C; 0.2-0.4 wt.% NaCleq.). Relict quartz-adularia-illite alteration occurs at the surface, in the vicinity of the quartz crystals, and in drillcores from the nearby TK-1 exploration well. Repeated movement on the Paeroa Fault uplifted pyroclastic rocks hosting the quartz crystals, but also provided pathways for "pulses" of hot fluids to move through the system. Quartz precipitation occurred at the edge of the crystals as the reservoir fluids cooled, as indicated by micron-scale alternating CL-dark/CL-bright quartz growth bands, which contain fluid inclusions with T h values of 210±40 °C. Pressure fluctuations were the likely cause of dissolution, marked by corroded crystal edges, with subsequent precipitation of quartz into open space. SEM-CL imaging shows that the quartz crystals contain healed fractures, which trapped low salinity fluids with Th values of 201±6 °C. Low-pH fluids in the near-surface setting also rounded the quartz crystals, and coated them with kaolinite and CL-grey amorphous "silica residue".
AB - Scanning electron microscopy-cathodoluminescence (SEM-CL) imaging of hydrothermal quartz exposed by weathering in the Te Kopia geothermal field (New Zealand) has revealed a history of crystal growth, dissolution, overprinting and fracturing that cannot be detected using other observational techniques (e.g. transmitted or reflected light microscopy, back-scattered electron imaging or secondary electron imaging). The crystals initially grew as CL-dark quartz, at least 350 m below their present location on the Paeroa Fault scarp, in a neutral pH, 215±10 °C liquid reservoir (inferred from the analysis of primary liquid fluid inclusions: mean Th of 213 °C; 0.2-0.4 wt.% NaCleq.). Relict quartz-adularia-illite alteration occurs at the surface, in the vicinity of the quartz crystals, and in drillcores from the nearby TK-1 exploration well. Repeated movement on the Paeroa Fault uplifted pyroclastic rocks hosting the quartz crystals, but also provided pathways for "pulses" of hot fluids to move through the system. Quartz precipitation occurred at the edge of the crystals as the reservoir fluids cooled, as indicated by micron-scale alternating CL-dark/CL-bright quartz growth bands, which contain fluid inclusions with T h values of 210±40 °C. Pressure fluctuations were the likely cause of dissolution, marked by corroded crystal edges, with subsequent precipitation of quartz into open space. SEM-CL imaging shows that the quartz crystals contain healed fractures, which trapped low salinity fluids with Th values of 201±6 °C. Low-pH fluids in the near-surface setting also rounded the quartz crystals, and coated them with kaolinite and CL-grey amorphous "silica residue".
KW - Cathodoluminescence
KW - Fluid inclusion geothermometry
KW - Hydrothermal alteration
KW - New Zealand
KW - Quartz microtextures
KW - SEM-CL
KW - Te Kopia geothermal field
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U2 - 10.1016/j.geothermics.2004.03.001
DO - 10.1016/j.geothermics.2004.03.001
M3 - Article
AN - SCOPUS:4143119962
VL - 33
SP - 615
EP - 635
JO - Geothermics
JF - Geothermics
SN - 0375-6505
IS - 5
ER -