LECTURES ON GEOCHEMICAL !N1 ERPRETATION OF ...

- 31 -THE SILICA MIXING MODELUnde~ some ci~cumstances the dissolved silica eoncent~ation of a mixedwater and a silica- cnthalpy diagram may be used to determine the temperatu~of the hot- water component (Fig. 6). A straight line drawn from apoint representing the non thermal component of the mixed water (point A.Fig. 6) through the mixed-water wa~ spring (point B) to the intersectionwith the quartz solubility curve gives the initial silica concentration andenthalpy of the hot- water component (point C). This procedure assumes thatBny steam that formed adiabatically (as the hot- water component moved up toa more shallow environment) did not separate from the residual liquid waterbefore mixing with the eo Id- water component.The folloving procedure can be used to dete~ine the enthalpy and temperatureof the hot-water component when steam was lost before mixing tookplace. For the situation in vhich steam vas lost at atmospheric pressureprior to mixing (point D, Fig. 6), a horizontal line drawn from point D tothe intersection vith the maximum steam loss curve gives the initialenthalpy of the hot- vater component (point E). The initial dissolvedsilica is shown by point F. If steam had been lost at a higher pressurebefore mixing, point D vould lie above 419 JIg on the extension of line AB,and point E would lie betveen the maximum steam loss and quartz solubilitycurves.In order for the above silica mixing model to give accurate results. itis vital that no conductive cooling occurred after mixing. If the mixedwater cooled conductively after mixing, the calculated temperature of thehot-vater component viII be too high. It is also necessary that no silicadeposition occurred before or after mixing and that quartz controlled thesolubility of silica in the high- temperature vater. Even vith theserestrictions the silica mixing model has been found to give good resultsin many places. In special circumstances. a silica mixing model could beused in vhich chalcedony or another silica phase is assumed to control thedissolved silica in the high- temperature component.A mathematical solution may be obtained for the silica mixing model.In Figure 6, non thermal ground vater, point A. has a silica concentrationA in a solution that has an enthalpy LA' Similarly, the warm spring of"mixed" origin. point B. has a silica concentration B in a solution that