Experimental Observation of the Ore-Forming Fluid NaCl-H_2O System in the Earth Interior

Abstract The NaCl-H₂O binary system is a major component of solutions coexisting with ores. Observation of saturated solutions of NaCl-H₂O by using the method of hydrothermal diamond anvil cell (HDAC) is a new approach to the study of ore-forming fluids. The salinities of NaCl-H₂O solutions in experimental observation are in a range of 32–55%. The observed temperature range is 25°C–850°C, and the pressure range 1 atm-10 kb. In this temperature-pressure range, the supercritical single phase, two phases (L,V) close to the critical state and two-phased (L+V) immiscible region were observed. And for the salinity of 35% the two phase L+V immiscible region of NaCl-H₂O solution was observed in a range of 253–720°C. Another temperature range, 400–817°C, was observed for the immiscible two-phased region of 50% salinity solution. In the high-temperature part of the two-phased immiscible region, the phase nature is very unstable. A “critical phenomenon” was observed when the heating path was very close to the critical state. It is possible to observe a ‘critical phenomenon’: an “explosion” occurred almost constantly at the interface between the liquid and vapour and the interface is rather obscure. A continuous transition between phases L and V could be found in the immiscible L+V phase while heating continuously. Moreover, as the NaCl-H₂O solution was separated into liquid and vapour phases, static charges surrounding each vapour bubble could be seen, and these bubbles were attracted together by the static charges to form a special solution structure. Besides, critical states of different salinities of NaCl-H₂O were observed in order to study the properties of the fluids occurring in the rocks in the earth interior, the origin of ore-bearing fluids and the significance of supercritical fluid with respect to the ore formation. The comparison of the salinity data of the fluid inclusions in the minerals of ore deposits with observations of NaCl-H₂O under HDAC in the conditions of high temperatures and pressures, combined with further thermodynamic analysis of ore-formation conditions would explain in depth the factors determining the ore formation.