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/cm3. 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 CO,, The dry steam originated as meteoric water high on the slopes of Nanga Parbat, with 6”O as low as 16%0. Oxygen isotopic exchange with the host rock was facilitated by high temperatures (340” to 450°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 6r80 between +9 and + 15%0, 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.