13.—Zoning of Ore Deposits: Types and Causes

Synopsis The zoning found in ore deposits is dominantly of two distinctive types: that in syngenetic sedimentary deposits, like the Kupferschiefer, and that in common hydrothermal deposits of either epigenetic or syngenetic characteristics. Here, epigenetic hydrothermal deposits include those commonly classified as vein, porphyry copper, contact metasomatic, and Mississippi Valley deposits; the syngenetic hydrothermal deposits are conformable, massive ores such as at Rammelsberg. The two zoning sequences, beginning nearest the source of the metals, are: in hydrothermal deposits, Fe—Ni—Sn—Cu—Zn—Pb—Ag—Au—Sb—Hg, and in sedimentary deposits, Cu—Ag—Pb—Zn. A zoning sequence represents the natural order of increasing solubilities of the metallic sulphides and other minerals in ore-forming solutions. Comparison of zoning sequences with relative solubilities in proposed ore solutions provides a rigorous test of the efficacy of such solutions. When corrected for relative metal concentrations (mass-action effect), then both theoretically predicted, and experimental relative solubilities of sulphide complexes match precisely the order of hydrothermal zoning. The order in sedimentary zoning is identical to the mass-action-corrected calculation of the sequence in which sulphides must precipitate from solutions containing metallic ions or weak chloride or hydroxyl complexes. The consanguinity of these correlations imply (1) that hydrothermal zoning is the product of deposition from sulphide complexes, and (2) that chloride complexes may be the metal-transporting agent in sedimentary deposits.