PARTITIONING BEHAVIOR OF SODIUM AND POTASSIUM BETWEEN IMMISCIBLE FLUIDS IN THE H2O-NACL-KCL SYSTEM AT MAGMATIC CONDITIONS

Analyses have shown that magmatic-hydrothermal fluids responsible for the transport of ore forming metals such as copper, iron, lead, zinc, etc. in porphyry copper deposits (PCD) are dominantly (Na,K)Cl brines. As such, the H₂O-NaCl-KCl system has been used to approximate conditions of alteration and mineralization in these deposits. Boiling, or phase separation, whereby a single fluid phase separates into a high salinity brine coexisting with a low salinity vapor is also a commonly observed phenomenon in PCD. Metals and other fluid components are distributed between the two immiscible fluids; the metal content of the two fluid phases controls ore deposition and major element chemistry controls the alteration processes. Thus, understanding the partitioning behavior of Na and K in the H₂O-NaCl-KCl system at temperatures and pressures associated with PCD will advance our understanding of conditions associated with formation of this important class of ore deposit.

In this study, the partitioning behavior of Na and K between two immiscible fluid phases in the H₂O-NaCl-KCl system at 600-800^oC, and 500-1000 bars, and for Na:K molar ratios of 3:1, 2:1, 1:1, 1:2 and 1:3 were determined using the synthetic fluid inclusion technique. At room temperature, inclusions that trapped the vapor phase contain a vapor bubble and a liquid with the composition of the vapor that was trapped at the experimental P & T, and the inclusions that trapped the brine contain a liquid, vapor bubble and daughter minerals of halite and sylvite. The salinities of the inclusions that trapped the brine were determined by measuring the dissolution temperatures of the daughter minerals. The salinities of the inclusions that trapped the vapor phase were determined by measuring the dissolution temperature of sylvite and the melting temperature of ice as the inclusions were heated from low temperature. The compositions of the inclusions were calculated from the microthermometric data using the FORTRAN program “SALTY.”The resulting compositions were used to calculate partition coefficients for Na and K between the liquid and vapor phases ( and ) and found to be nearly unity over the experimental P & T range. The data also show that as the mole fraction of K increases, Na increasingly partitions into the vapor phase.