GSA Annual Meeting Abstract


PICCOLI, P.M., CANDELA, P.A., and WILLIAMS, T.J., Laboratory for Mineral Deposits Research, Department of Geology, University of Maryland at College Park, MD 20742.

Estimation of the concentration of Cl and HCl in the magmatic volatile phase is critical to the evaluation of the deep input to geothermal systems, estimation of ore potential, and the chemistry of volcanic volatiles. We have calculated these parameters in two ways. The first method uses the ASI of an aplite melt at the aplite intrusion P & T, together with our recent experimental data relating HCl/NaCl and HCl/KCl to melt ASI, to calculate a maximum HCl/(total Cl) in the magmatic vapor phase. Further, if brine saturation is assumed, we can calculate a model HCl concentration in brine-saturated magmatic vapor. The second method uses apatite (Cl/OH) in conjunction with thermodynamic data to calculate the fugacity ratio of HCl/H2O; if the ASI of the melt is known, then HCl/(total Cl) can be used with apatite chemistry to calculate the total Cl concentration of the magmatic vapor. Each method requires an aplite intrusion P & T; P can be estimated from the aplite bulk composition relative to the pressure-dependent H2O-saturated granite minima compositions, and T (apatite saturation temperature ~ aplite intrusion temperature) estimated from the P2O5 and SiO2 concentrations in the aplite: see Piccoli & Candela, 1994, AJS, v. 294, p. 92). In this study, we examined the aplites and apatites of the Rush Creek (RC) quartz monzodiorite (Sierra Nevada Batholith, USA) given P = 50 MPa (from aplite composition) and 850oC (close to the apatite saturation temperature of 820oC). The ASI of the residual melt, calculated from the bulk composition of the RC aplites, was 1.02. From our experimental data (Williams, 1995, UMCP dissertation; Williams et al., in prep), we calculate log KCl/HCl (-0.022) and log NaCl/HCl (-0.056) in the magmatic vapor, yielding HCl/(total Cl) = 0.35 for the vapor in equilibrium with RC aplite. Assuming a total Cl = 0.07 molal (brine-saturated vapor), we can calculate a maximum HCl concentration of 0.02 molal. Alternatively, we can calculate HCl/H2O directly from the composition of apatite at P & T (see Flowchart 1 and Flowchart 2 and Equations 1-3 and 4-5). Rush Creek aplites contain apatite with 3.26 wt.% F (XFAp = 0.865) and 0.05 wt.% Cl (XCAp = 0.007). At 820oC and 50 MPa, these apatites would be in equilibrium with a vapor with HCl ~ 0.02 molal. If both apatite composition and melt ASI are available, they can be combined to yield a total chloride concentration; in this case, we calculate a total Cl concentration in the vapor phase of 0.06 (0.35 wt.% NaCl eq.). Note that this is the total Cl concentration of a vapor that is at or near saturation with a brine in the system NaCl-H2O. That is, given apatite data and aplite ASI, we can demonstrate that a vapor in equilibrium with the RC aplite melt was at, or close to, saturation with brine. We suggest that these techniques be used to estimate the concentration of HCl and Cl in the vapor in magmatic hydrothermal systems, given melt ASI and/or apatite composition data.

apatite, aplites, HCl, experiments, hydrothermal