WHAT PRESENT-DAY FORMATION WATERS CAN TELL US ABOUT THE SEDIMENTARY GENESIS OF ANCIENT ORE-FORMING FLUIDS

A large data base on the composition of formation waters exists for the northern Gulf of Mexico sedimentary basin, including many analyses of dissolved Pb and Zn. It can be concluded from these data that a threshold chlorinity of approximately 100 g/L (salinity = 150 g/L) exists, below which sedimentary waters are typically poor solvents for Pb and Zn in the presence of dissolved sulfide and above which metals in concentrations of interest to problems of sedimentary ore genesis can be accommodated in solution. On the basis of thermodynamic analysis, this threshold corresponds to the onset of the predominance of the metal-tetrachloro complexes (MeCl₄^2-) with increasing chloride concentrations. Additional properties of basinal fluids which facilitate metal solubilization include the decrease in pH with increasing salinity and the strongly non-ideal behavior of dissolved chloride in very saline waters. There is no evidence for significant metal complexing by bisulfide or organic ligands. It is thus reasonable to conclude that sedimentary venues in which Pb & Zn-rich brines can form are limited to a significant degree by natural variations in chlorinity. The lack of correlation between metal concentrations and Br/Cl ratios indicates that it is less important whether a brine acquired high chloride as a result of subaerial evaporation of seawater or dissolution of halite than the hydrodynamic setting of brine formation. Formation water chlorinities in the south Louisiana salt dome basin are typically less than 100 g/L as a result of dynamic mixing of halite-saturated waters with ambient waters of much lower salinities. A more likely venue for the accumulation of large volumes of metal-rich brines is above the subhorizontally-bedded evaporites of the Gulf rim, where density stratification of fluids has resulted in the accumulation of large volumes of brines having chlorinities well in excess of 100 g/L.