Supergene silver sulfide enrichment has been widely accepted for the last 100 years, but has warranted little or no mention in descriptions of several silver-rich, bulk-tonnage orebodies defined over the last three decades. This dichotomy is addressed by reassessing the importance of enrichment in 40 of the world’s premier silver dominated and other silver-rich deposits, including several of historical significance. The deposits are of high-grade vein and low-grade, bulk-tonnage styles and varied genetic types, but are dominated by representatives of the intermediate-sulfidation epithermal and carbonate-replacement, chimney-manto classes.
The results of this preliminary analysis show that only 12 (30%) of the deposits contain(ed) appreciable amounts of silver ore generated by silver sulfide enrichment, mainly in the form of acanthite and argentian chalcocite-group minerals in the cases where its mineralogic characteristics are recorded. Silver-rich oxidized zones are, however, well developed in 60 percent of the deposits and, locally, display silver enrichment of either residual or chemical origin. Irrespective of whether oxidative weathering takes place under acidic or alkaline conditions, a factor controlled mainly by hypogene iron sulfide and carbonate contents, silver tends to be retained in oxidized zones, with comparatively little remaining available in solution to generate underlying silver sulfide enrichment. The extreme insolubility of the silver halides (chlorargyrite, embolite, bromargyrite, iodargyrite) over broad pH and climatic ranges, besides efficient silver fixation as native silver, argentojarosite, or silver-bearing manganese oxides under the appropriate chemical conditions, explains the metal’s relative supergene immobility.
The efficient dissolution and downward transport of copper under acidic supergene conditions, as exemplified by porphyry copper leached cappings and underlying multicyclic enrichment blankets, appears to have no counterpart in either silver-only or other silver-rich deposits. Nor are the silver equivalents of exotic oxide copper deposits, the products of lateral metal transport in the acidic supergene environment, considered likely to exist. Furthermore, the processing benefits accruing from supergene oxidation and enrichment of copper deposits are not as evident in the silver environment, in which the main supergene oxidation products, especially the silver-bearing manganese oxides and argentojarosite, commonly present metallurgical difficulties.