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.
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At least five altered and mineralized porphyry centers related to the cooling of a polyphase Eocene intrusion occur within a 25-km2 "pampa"-type area in the southwestern sector of the Chuquicamata district in northern Chile. These deposits take place 1 to 2 km apart as discrete porphyry "columns" covered by postmineral, poorly consolidated Miocene sedimentary rocks. Such copper oxide and sulfide deposits were discovered and evaluated by drilling done by Codelco from 1996 through 2007 during a brownfield exploration program, driven by the necessity to replace and increase leacheable ore consumed by the Chuquicamata and Radomiro Tomic operations. During this program a resource of more than 20 million metric tons (Mt) Cu was discovered, including 6 Mt Cu of oxide, mixed and secondary sulfide ore, representing one of the largest supergene copper resources discovered worldwide during the last 10 years.
Despite their close location and their genetic relationship to a single, polyphase intrusion mineralization event, the five porphyry centers display contrasting host-rock and structural framework as well as different hypogene alteration and ore mineral assemblages. This picture reaches high levels of complexity because of the different levels of exposure of the mineral systems, resulting from primary emplacement processes and post-mineral faulting. These hypogene features and the effect of landscape and climate evolution controlled supergene alteration, thus generating different profiles in each specific porphyry center. The key controlling factors in the supergene overprint are discussed on the basis of their relationship to ore and gangue mineralogical abundance and occurrence, assemblage distribution, geochemical response, and the broad geologic setting.
As exploration for covered porphyry copper deposits in the southwestern sector of the Chuquicamata district progressed, numerous lessons were learned about the origin of supergene profiles and the analysis and use of supergene effects and their products as a guide for exploration. These lessons, which include geological and geochemical criteria among others, are discussed in the context of the appraisal of the mineral potential of copper oxide-mixed-secondary sulfide blankets and underlying sulfide protore.