SUPERGENE OXIDIZED AND ENRICHED COPPER DEPOSITS: PRODUCTS OF THE INTERPLAY BETWEEN ORE TYPE, TIME, TECTONICS, AND CLIMATE

Supergene leaching, oxidation, and chalcocite enrichment in porphyry and related Cu deposits take place in the weathering environment to depths of several hundred meters. Supergene ores in the central Andes and southwest North America (Southwest) currently account for >50 % of world mined Cu.

Combinations of local and regional controls optimize supergene profile development. Ore bodies should be vertically extensive, and contain steep fault and fracture arrays, high pyrite/Cu-bearing sulfide ratios, and nonreactive advanced argillic and sericitic alteration zones. Mature oxidation and enrichment are promoted by: uninterrupted supergene activity for at least 0.5 m.y., but typically minima of 3-9 m.y.; tectonically induced uplift to depress water tables and expose sulfides to oxidative weathering; and hot, semiarid to pluvial climates so long as erosion rates do not outpace supergene processes. Subplanar erosion surfaces are not required for efficient supergene activity.

Supergene processes are active worldwide, but resulting profiles are commonly immature because of historically high denudation rates. In parts of the North and South American Cordillera and elsewhere, however, fossil supergene profiles exist as a result of either lower erosion rates combined with intermittent concealment beneath volcanosedimentary sequences or, as in northern Chile, of climatic desiccation. Pre-Cenozoic supergene profiles are preserved locally, although the major enriched zones of the central Andes and Southwest date back to only ca. 40 Ma. Copper enrichment in the central Andes spanned several major pulses of contractional deformation, crustal shortening, surface uplift, and erosional exhumation, which gave rise to paleowater table descent and piedmont gravel accumulation. The uplift that stimulated supergene activity in the Southwest, however, was largely a response to extreme crustal extension. Regional variability in supergene profiles is fundamentally attributed to differential uplift of fault blocks. Supergene activity in northern Chile took place under unusually arid conditions, as shown by the prevalence of Cu oxychloride minerals in the oxidized ores, but virtually ceased at ca. 14 Ma because of the onset of hyperaridity. Central Andean Cu deposits act as effective paleoclimatic indicators.