| Title | GEOTHERMAL SYSTEMS AND EPITHERMAL DEPOSITS OF NEW ZEALAND: A FLUID CHEMISTRY COMPARISON |
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| Authors | M.P. Simpson, S.S. Palinkas, M. Nieuwoudt, R.J. Bodnar, J.L. Mauk |
| Year | 2015 |
| Conference | New Zealand Geothermal Workshop |
| Keywords | Geothermal systems, TVZ, fluid chemistry, epithermal Au-Ag deposits, Hauraki Goldfield, fluid inclusions, LA-ICP-MS, Laser Raman, Synchrotron FTIR. |
| Abstract | Geothermal systems are considered the modern analogs of epithermal deposits. In the North Island of New Zealand there are approximately 23 geothermal systems within the Taupo Volcanic Zone (TVZ) and approximately 50 epithermal Au-Ag deposits in the Hauraki Goldfield. The chemistry of fluids in geothermal systems can be directly measured, whereas those for epithermal deposits are only preserved in fluid inclusions; here the fluids from each are compared. Deep reservoir chloride waters from TVZ geothermal systems typically have ≤1,500 ppm Na, but based on laser ablation-inductively coupled plasma-mass spectrometric analyses fluids trapped in epithermal quartz and sphalerite inclusions have up to ~15,600 ppm and 35,300 ppm Na, respectively. Both geothermal fluids and most inclusions in epithermal quartz have Na > K > Ca, although inclusions in sphalerite have enriched Ca that can exceed K. Concentrations of Sb and As in quartz hosted inclusions are 2 to 3 orders of magnitude greater than in geothermal waters (Sb up to 987 ppm vs 2 ppm and As up to 486 ppm vs 4.8 ppm). Most inclusions have Sb > As, whereas geothermal waters have As > Sb. The main gas in geothermal fluids is CO2, and its concentration can exceed that of dissolved salts in some systems. Laser Raman and limited Synchrotron Fourier Transform Infrared (FTIR) microscopy suggest low amounts of CO2 within fluid inclusions, but the amount cannot be quantified. Overall, the liquid trapped in epithermal fluid inclusions is more saline and has greater concentrations of Na, K, Ca, Sb and As than TVZ chloride waters. Fluids at the Tui deposit are the most saline, and likely consist of exchanged meteoric waters that include a significant magmatic fluid component. Based on these results it appears that chloride waters in TVZ geothermal systems differ in composition from those trapped in epithermal fluid inclusions of the Hauraki Goldfield, and that the latter are interpreted to have a variable to significant magmatic fluid component. |