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 geothe rmal systems contain both liquid and vapour and typically follow a boi ling point-depth (BPD) relationship. The Nanga Parbat uplift-driven co nductive thermal anomaly results in a geothermal system which follows a BPD relationship at shallow levels, but below about 3 km fluid inclu sions show that the hydrothermal fluid is dry steam with fluid densiti es from 0.36 to as low as 0.07 g/cm(3). This dry steam zone may persis t 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 d ry steam originated as meteoric water high on the slopes of Nanga Parb at, 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 me teoric isotopic signature was completely obliterated. Hence, quartz ve ins 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 hydroth ermal fluids. The dry steam zone at Nanga Parbat has formed due to nea r-isothermal depressurization of very hot fluid during rapid tectonic uplift at rates > 3-6 mm/year. (C) 1997 Elsevier Science B.V.