A graben with existing and historic vein-hosted gold production on its west side has similar style veins that have hosted historic silver production on the east side. Are there regional controls on the mineralization of epithermal systems that would control Au/Ag ratio vertically and/or horizontally across a graben? Could it simply be a matter of lighter elements, such as silver, precipitating from cooling epithermal fluids at lower temperatures or pressures than heavier elements, such as gold, do?
Yes, it is possible. The major role in causing this discrimination from east to west, could be due to the difference in the nature of the anions complexed with Au and Ag, as suggested by Yan-Jing. Appropriate conditions for the precipitation of Au-complexed and Ag-complexed species might have a small-scale temporal difference as well. Also the overall release of metals, could be controlled by the formation of proper weak-zones ( shear zone or any weak-plane) with needful hosting lithologies, where fluid-rock interaction processes controlled the discrimination for precipitating of Au and Ag. Notably, the chloride complexes of Ag are stable up to much lower temperatures. So the advent of the Au- and Ag-laden fluids in the graben could have occurred in two separate phases, depending upon their upward release from the primary magmatic regime.
Ag/Au ratios are controlled by fS2 and or fO2 in sulfide bearing hydrothermal systems, as well as temperature and other variables. A detailed description of the geological relations, the occurrence and nature of the veins and the vein mineralogy would probably lead to a better understanding. How are is the Ag/Au ratio arrived at? Are the veins truly epithermal and cross-cutting or might they be silica crusts formed on the seafloor from vents or seeps.......what is the age of the system...........?
I have noticed a relationship of higher silver with the presence of organic matter in the host rocks or the basement (coal, black shale, bitumen, etc. reductants). Agree with Ulrich that it may be a function of oxygen fugacity and availability of a sulfur. Worth asking the question.
what are AU/Ag ration in high sulfida and phorpyry? maybe, We cauld see thermodinamic law in mineralisasi process
First of all, metal solubility is, naturally, equal to ore mineral precipitation.
Ligands (Cl-, HS-, SO4 2- OH-, F-, CH3COO) for stable complexes increasing the solubility of ore metals. Au is complexed predominantly with sulfur species at epithermal conditions, however, sometimes can be seen CL- as well as OH-. So the present of S- species is the key, as I know.
(Stefansson and Seward, 2003, 2004)
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