THE SIGNIFICANCE OF CLATHRATES IN FLUID INCLUSIONS AND THE EVIDENCE FOR OVERPRESSURING IN THE BROADLANDS-OHAAKI GEOTHERMAL SYSTEM, NEW ZEALAND

摘要

The Broadlands-Ohaaki geothermal system is host to an epithermal environment where gold-silver transport and deposition involves deeply derived fluids containing up to 3.3 wt percent CO_2 and 0.1 wt percent Cl. Earlier fluid inclusion investigations show that most microthermometric data reflect boiling and mixing in the upper 2 km of the system under modern P-T-X fluid conditions. Here we report the microthermometric results for a single quartz crystal from 1,258-m depth, well Br 25 in the upflow zone of the geothermal system that reveal the presence of clathrates upon freezing due to anomalous concentrations of CO_2 (7.9 to 14.3 wt percent). These inclusions occupy the core of the quartz crystal and have homogenization temperatures from 291 deg to >365 deg C, but the anomalous concentrations of CO_2 are likely artifacts of deep boiling and heterogeneous trapping of the resulting coexisting gas and liquid phases. Only a few fluid inclusions (T_h approx 300 deg, T_m of -1.4 deg to -1.6 deg C), which lack clathrate, reveal more realistic conditions of early quartz precipitation from a modestly overpressured fluid (approx 140 bars) relative to the prevailing hydrodynamic boiling conditions (110 bars). Microthermometric data (T_h approx 300 deg, T_m of-0.2 deg to -0.8 deg C) for fluid inclusions in a later formed overgrowth of the quartz crystal match the modern P-T-X conditions at 1,258-m depth in the well. The overall results of the study show that the clathrates are artifacts of two-phase trapping of steam and liquid and that the deep liquid became overpressured locally, probably due to mineral deposition and sealing of a permeable channel.