Metal Transport in Ore Fluids by Organic Ligand Complexation

Abstract

The assocn. of org. matter with ore minerals, gangue, and host rock in many low- to moderate-temp. ore deposits has been known for many years. As a reductant or oxidant, org. matter may function as an active ore-forming agent up to magmatic temps., however, most other roles for org. matter, including metal complexation, are limited to less than about 200 DegC. Metal-humate and metal-fulvate complexes probably contribute significantly to metal transport and speciation in interstitial waters of subaq. sediments, in shallow sediments undergoing early diagenesis, and in the supergene environment. It is, therefore, likely that these complexes are involved in ore-forming processes responsible for syngenetic fixation of metals in young sediment as well as epigenetic transport and deposition of metals in shallow subsurface environments. Because concns. of amino acids are typically an order of magnitude lower than concns. of humic and fulvic acids in interstitial waters, metal-amino acid complexes are probably less important than humate or fulvate complexes in most low temp. ore-forming environments. Organosulfur ligands may contribute to the speciation of metals and sulfur in ore-forming processes responsible for syngenetic deposition of metal sulfides in low-temp. subaq. environments. If they are sufficiently stable at elevated temps., thiols and other organosulfur ligands could conceivably contribute to both metal and sulfur transport in moderate-temp. (50-250 Deg) hydrothermal ore fluids. The concns. of carboxylate ions in surface and shallow subsurface interstitial waters are sufficiently low to preclude the importance of metal-carboxylate complexes as significant metal-transporting agents in low-temp. ore fluids responsible for supergene mineralization, syngenetic deposition in anoxic environments, and low-temp. epigenetic ores. However, the obsd. elevated concns. of org. acids in oil-field brines strongly suggest that metal-carboxylate complexes may contribute significantly to metal transport in ore fluids with temps. ranging from roughly 75 to 150 Deg. To evaluate the ore transport potential of a particular metal-org. complex, the theor. estd. concn. of that species in the ore soln. of interest must be detd. by calcn. Concns. of Ca, Mg, Na, Pb, Zn, Fe, and Al as acetate, oxalate, malonate, succinate, and catechol complexes were calcd. for three reconstructed Mississippi Valley-type (MVT) ore solns. and a model composite ore fluid for red bed-related base metal (RBRBM) deposits. Based on these calcns., some important inferences can be made regarding metal transport by org. complexes in ore fluids of sedimentary origin. Significant amts. of lead and zinc cannot be mobilized as metal-org. complexes involving acetate or other carboxylate ligands in ore fluids contg. greater than 10-5 m reduced sulfur as hydrogen sulfide or bisulfide. However, in reduced ore fluids sufficient metal and reduced sulfur may be transported as complexes involving organosulfur ligands. On the other hand, significant quantities of dissolved lead and zinc can be transported by carboxylate complexes in oxidized ore fluids contg. less than 10-9 m reduced sulfur. It is hoped that these results will encourage investigators to evaluate the role of metal-org. complexing in the genesis of all those deposits with a clear genetic link to org. processes.