Geochemistry of a dry steam geothermal zone formed during rapid uplift of Nanga Parbat, northern Pakistan

Natural dry steam zones (vapour only) are relatively rare; most geothermal systems contain both liquid and vapour and typically follow a boiling point-depth (BPD) relationship. The Nanga Parbat uplift-driven conductive thermal anomaly results in a geothermal system which follows a BPD relationship at shallow levels, but below about 3 km fluid inclusions show that the hydrothermal fluid is dry steam with fluid densities from 0.36 to as low as 0.07 g/cm(3). This dry steam zone may persist down to the brittle-ductile transition. The dry steam has salinities up to 5 wt.% dissolved salts, and up to 22 mole% dissolved CO2. The dry steam originated as meteoric water high on the slopes of Nanga Parbat, with delta(18)O as low as -16 parts per thousand. Oxygen isotopic exchange with the host rock was facilitated by high temperatures (340 degrees to 450 degrees C) and low fluid densities so that the fluid meteoric isotopic signature was completely obliterated. Hence, quartz veins formed by the migrating dry steam have delta(18)O between +9 and +15 parts per thousand, a range which is indistinguishable from quartz in the host rocks. Quartz vein precipitation from dry steam requires 3 to 5 orders of magnitude greater volume of fluid than typical hydrothermal fluids. The dry steam zone at Nanga Parbat has formed due to near-isothermal depressurization of very hot fluid during rapid tectonic uplift at rates > 3-6 mm/year. (C) 1997 Elsevier Science B.V.