Supergene Oxidized and Enriched Porphyry Copper and Related Deposits

Abstract Supergene leaching, oxidation, and chalcocite enrichment in porphyry and related Cu deposits take place in the weathering environment to depths of several hundred meters. The fundamental chemical principles of su-pergene processes were elucidated during the early decades of the twentieth century, mainly from studies in the western United States. The products of oxidation and enrichment continue to have a major economic impact on Cu mining in the central Andes and southwestern North America, currently accounting for >50 percent of world-mined Cu, and have sustained these two premier Cu provinces for the past 100 years. Enriched grades may attain 1.5 to >2 percent Cu, commonly two or three times the hypogene tenor. Deep oxidation also transforms low-grade refractory Au mineralization into bulk-mineable ore. The mechanisms of oxidative weathering are well understood because of studies in support of heap leaching of supergene Cu ores and amelioration of acid mine drainage. Sulfide oxidation takes place above the water table as an electrochemical process mediated by acidophilic, Fe- and S-oxidizing bacteria. Where acidic conditions prevail, Cu is efficiently leached and transferred downward to the reduced environment, beneath the water table, where sulfide enrichment takes place. Enrichment appears to be mainly the product of abiotic cation-exchange reactions involving substitution of Cu for more electronegative metals. Much of the required S is inherited from the replaced sulfide minerals, although sulfate-reducing bacteria may generate a minor proportion. Bacteria may also contribute to the enrichment process by facilitating metal adsorption. Where hydraulic conditions permit lateral flow of Cu-charged solutions from porphyry Cu deposits into contiguous drainage channels, exotic oxide Cu deposits form at the base of, or within, coevally accumulating piedmont gravel sequences. Several local and regional controls optimize supergene profile development. Orebodies should be vertically extensive and contain a well-developed array of steep faults and fractures, elevated pyrite/Cu-bearing sulfide ratios to maximize acidity of the supergene solutions, and nonreactive advanced argillic and sericitic alteration assemblages to minimize neutralization of the acidity. Porphyry Cu deposits pass through a natural supergene cycle in which leaching and mature enrichment in advanced argillic and sericitic zones give way during eventual exposure of deeper potassic zones to in situ oxidation without attendant enrichment. Mature oxidation and enrichment are promoted by the following: uninterrupted supergene activity for at least 0.5 m.y. but typically minima of 3 to 9 m.y.; tectonically or isostatically induced surface uplift responsible for depression of water tables and exposure of sulfides to oxidative weathering; and hot, semiarid to pluvial climates so long as erosion rates remain in balance with rather than outpacing supergene processes. Subplanar erosion surfaces, such as pediplains, are not considered to be a requirement for efficient supergene activity. Leached cappings are traditionally subdivided on the basis of their dominant limonite component into hematitic above mature enrichment, goethitic above hypogene ore or protore where Cu leaching is limited, and jarositic above pyrite-rich mineralization. Major oxidized Cu orebodies are developed either in situ where pyrite contents and leaching are minimal or as exotic accumulations located lateral to enriched zones. Oxidized ore comprises Cu minerals and mineraloids of both green and black color, with the latter, such as Cu wad, Cu pitch, and neotocite, being poorly characterized and tending to typify low-grade rock volumes. Among the green Cu species, chrysocolla dominates most high-grade ores of both in situ and exotic origin, hydrated sul-fates and hydroxysulfates typify oxidation of pyrite-bearing enriched zones, and prevalence of hydroxychlorides in northern Chile testifies to exceptionally arid supergene conditions. Enrichment, by factors of three or even more, generates chalcocite and other Cu-rich sulfides in proximity to the overlying water table but lower grade covellite mineralization at depth. Gold and Mo do not normally undergo significant enrichment. Oxidized and enriched zones undergo pervasive supergene argillic alteration, with kaolinite, accompanied under arid to semiarid conditions by alunite, dominating leached and enriched deposits; smectite is more common in zones of in situ sulfide oxidation. Dissolution of hypogene anhydrite typifies the supergene profiles of porphyry Cu deposits and extends deeper than all significant enrichment. Geologic context and mesoscopic textural criteria facilitate distinction between hypogene and supergene Cu sulfides, limonite, clay, and alunite. Supergene oxidation and enrichment are active throughout much of the world, although supergene profiles are commonly immature because historical denudation rates are high. In parts of the North and South American Cordillera and elsewhere, however, fossil supergene profiles exist as a result of lower erosion rates combined with intermittent concealment beneath volcanic or sedimentary sequences or, as in northern Chile, of climatic desiccation commencing at ~14 Ma. Oxidation and enrichment of Paleozoic and Mesozoic age are preserved locally, although the major enriched zones of the central Andes and southwestern North America date back to only ~40 Ma. Copper enrichment in the central Andes coincided with several major periods of con-tractional deformation, crustal shortening, surface uplift, and erosional exhumation, which gave rise to descent of paleowater tables and thick piedmont gravel accumulations. The uplift that stimulated supergene activity in southwestern North America, however, was largely an accompaniment to extreme crustal extension. Variability in supergene profiles is fundamentally attributed to differential uplift of fault blocks. Supergene transformations of the upper parts of many Cu and related Au deposits are beneficial because the ores are either upgraded or can be more easily and cheaply processed, commonly using heap-leaching technology. Nevertheless, heap-leaching efficiency is highly dependent on the oxide and sulfide mineralogy. An exception to this general rule is provided by porphyry Cu-Au deposits in which supergene oxidation may introduce metallurgical complexity. Moreover, some supergene products, in particular clays, may negatively affect processing. Exploration of exposed porphyry Cu systems remains highly dependent on leached-capping appraisal for interpretation of the nature and grade of subjacent sulfide mineralization. In contrast, the search for concealed supergene profiles, which may be stunted where piedmont gravel sequences are excessively thick, relies heavily on geologic and geochemical vectoring and geophysical techniques. Exotic oxide Cu accumulations constitute targets in their own right besides acting as guides to potentially undiscovered porphyry Cu sources, in which the corresponding enriched zones may or may not be preserved.