The environment of base metal transport in Mississippi Valley-type ore solutions has been evaluated thermodynamically. The average ore solution has at 100 degrees C: Sigma m S = 10 (super -) to 10 (super -3) , Sigma m C = 10 (super -2) to 10 (super -4) , log a (sub O 2 ) = -54 to -58 and pH = 6.8 to 7.8; and at 200 degrees C: Sigma m S = 10 (super -1) to 10 (super -3) , Sigma m C = 10 (super -2) to 10 (super -4) , log a (sub O 2 ) = -41 to -46 and pH = 6.2 to 7.2. Under these conditions, chloride and bisulfide complexing together provide maximum galena solubilities of only 10 (super -3) and 10 (super -2) ppm lead at 100 degrees and 200 degrees C, respectively. Bisulfide complexes are predominant over chloride complexes. However, these concentrations are three to four orders of magnitude too low to form an ore deposit. Additional complexes involving the ligands SO (super -2) 4 , CO (super -2) 3 , HCO (super -) 3 , NH 3 , Br (super -) , and S 2 O (super -2) 3 , and the ion pairs NaPbCl 03 and NaPbCl (super -) 4 are also insufficiently stable to account for minimum concentrations of 10 ppm lead in the average ore solution. Thus, inorganic complexes are apparently inadequate mechanisms of leadtransport in Mississippi Valley-type ore solutions.As an alternative to inorganic complexing, the intimate association of organic material with Mississippi Valley-type mineralization suggests that organometallic complexes of lead were important in Mississippi Valley-type ore solutions. In the averge ore solution at maximum feasible ligand concentrations, complexes with acetate, citrate, and oxalate cannot account for significant lead transport. However, lead in salicylate complexes could reach concentrations of roughly 3 to 30,000 ppm at 100 degrees and at 200 degrees C, respectively. Concentrations of salicylate as small as 10 (super -4) to 10 (super -5) m may result in sufficient lead transport to form a Mississippi Valley-type ore deposit.