Upper Mississippi Valley district ore fluid model: the role of organic complexes

Abstract

Ligands responsible for zinc and lead mobility in solutions forming Mississippi Valley-type (MVT) deposits have not yet been identified and this deficiency precludes formulation of detailed geochemical models of the genesis of MVT deposits. A suitable ligand must be identified that is capable of complexing a minimum of 10 ppm of zinc and of lead. Nevertheless, many constraints on the geochemical environment during formation of the Upper Mississippi Valley (UMV) district in Illinois, Wisconsin, and Iowa, have been identified. Ore characteristics, such as sphalerite color-banding, require that deposition occur in a reducing environment at near-equilibrium conditions. Conditions for the ore-transporting fluid are T = 125 ± 25°C, pH = neutral ± 1, log aO 2 = −55 to −43, and total dissolved reduced sulfur about 0.001 to 0.01 m. Models for MVT ore formation fall into three classes, labeled herein as the sulfur addition, sulfate reduction, and reduced sulfur models. Comparison of these three models for MVT ore formation with geochemical constraints on the ore-forming fluids of the Upper Mississippi Valley district leads to the conclusion that the reduced sulfur model applies best to this district. Experiments were carried out to test the chemical compatibility of a variety of organic ligands with the geochemical constraints of the reduced sulfur model. Solubility tests were conducted in sealed quartz tubes with a series of short-chain mono- and dicarboxylic acids at 125°C, pH 6.1 (near neutral), total dissolved reduced sulfur = 0.002 m, and ligand concentrations of 0.1 or 0.05 m. Formic, acetic, propionic, n-butyric, iso-butyric, n-valeric, oxalic, malonic, succinic, methyl-succinic, glutaric, and adipic acids were tested, in addition to six thiocarboxylic acids (thiolactic, 3-mercaptopropionic, mercaptosuccinic, 3,3′-thiodipropionic, thioglycolic, and thiodiglycolic acids). Results show that the monocarboxylic acids and the thiocarboxylic acids are weak ligands for zinc and cannot play a role in MVT deposit genesis. Several dicarboxylic acids are moderately strong complexing ligands. Oxalic acid produced solutions with more than 13.6 to 22.0 ppm Zn dissolved under experimental conditions. Average values for formation constants for 1:1 Zn(II) organic complexes ( I = 0 m) at 125°C were determined for malonic acid (log β 1 = 8.30); succinic acid (log β 1 = 9.05); glutaric acid (log β 1 = 10.70); methyl-succinic acid (log β 1 = 10.85); and adipic acid (log β 1 = 10.67). For zinic oxalate, the average values of log β 1 ( I = 0 m) = 11.95 ± 0.40 at 125°C. Oxalate, although a strong ligand for Zn(II), is precluded from contributing to MVT formation because it readily precipitates as calcium oxalate in the presence of calcium ion. In general, none of the short-chain mono- or dicarboxylic acids nor the thiols tested form zinc complexes strong enough to contribute to MVT deposit genesis. Other longer-chain and thiocarboxylic acids as well as humic and fulvic acids remain to be investigated as transporting agents in forming these deposits.