THE INFLUENCE OF PH AND OXYGEN FUGACITY ON THE HYDROTHERMAL TRANSPORT OF NA, K, CA, MG, FE AND AL BY CARBOXYLATE COMPLEXES

A chemical model was developed to evaluate the influence of log oxygen fugacity – pH conditions on the complexation of Fe, A1, Ca, Mg, Na, and K by acetate and malonate in 100 degree C basinal brines with bulk chemical compositions similar to diagenetic ore fluids and modern petroleum – field brines of moderate salinity (TDS=180 g/L). In the model composite brine, acetate with a concentration of 7,700 mg/L (0.13m) represents the major monocarboxylate ligands detected in basinal brines, while malonate with a concentration of 300 mg/L (0.0029m) represents the dominant dicarboxylate ligands. The model fluids are constrained by quartz, galena and sphalenite saturation ; saturation with respect to pyrite, magnetite, or hematite, depending on the pH –log oxygen fugacity conditions; and K-feldspar, muscovite, or kaolinite, depending on the K+ to H+ ratio. Total Na+, K+, Mg2+, and Ca2+ are fixed, and hence the fluids may be undersaturated or in equilibrium with calcite and dolomite, depending on the pH of the model fluid. In the model, total acetate and total malonate concentrations are assumed to be independent of total carbonate and, thus, a metastable link between carboxylate species and carbonate is not assumed. Future models will employ this link.

Isopleths, in log oxygen fugacity – pH space, of Na and K in organic complexes and of total concentrations of Na and K in the model fluids show that organic complexes account for several % of the total Na and K in solution and that this % is little affected by the pH above values of 4.5 and by oxygen fugacity. Isopleths of A1, Ca and Mg show a much greater pH dependence and indicate that organic complexes can significantly affect the solubility of aluminosilicate phases, calcite, and dolomite in diagenetic systems. Isopleths of Fe show a strong dependence on both pH and oxygen fugacity and illustrate a significant influence of carboxylate complexing on the mobility of Fe in diagenetic systems.