Exotic Deposits - Products of Lateral Migration of Supergene Solutions from Porphyry Copper Deposits
Carlos MÜNchmeyer, 1998. "Exotic Deposits - Products of Lateral Migration of Supergene Solutions from Porphyry Copper Deposits", Andean Copper Deposits: New Discoveries, Mineralization, Styles and Metallogeny, Francisco Camus, Richard M. Sillitoe, Richard Petersen
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Exotic deposits form in conjunction with the supergene oxidation and enrichment of porphyry copper deposits. Supergene alteration primarily involves vertical solution movement, but percolation often involves a horizontal component whereby copper-rich acidic solutions migrate laterally within the vadose zone. Depending on Eh and pH conditions, copper may be transported through paleodrainage systems for distances of up to 8 km from source to produce continuous copper mineralization. The copper is deposited primarily in oxide form, but sulfide copper also may be present close to source.
Favorable conditions for formation of exotic deposits existed between latitudes 12° and 27° Sin northern Chile and southern Perú. In Chile, twelve deposits are recognized, ranging from large deposits like Exotica, El Tesoro, and the recently discovered Damiana (1.2 to 3.5 million metric tons of Cu), through medium-sized ones like Huinquintipa and Sagasca (160,000 to 400,000 metric tons of Cu), to small ones like Quebrada Blanca and La Planada (100 to 10,000 metric tons of Cu).
Geomorphologic conditions of the paleochannels and the chemical reactivity of the rocks and sediments in them influence the shapes of the exotic deposits confined to paleochannels (e.g., Sagasca). Other deposits are fan-shaped and cover large areas on the slopes of topographic highs underlain by the source deposits (e.g., Damiana). A few small deposits are controlled by faulting, the orientation of which controls their form (e.g., Ichuno).
As a result oflateral flow of copper-rich solutions, exotic deposits display zoned alteration and mineralization patterns controlled by the reactivity of the host rocks, the CulFelS ratios of the source deposits, and the evolution of pH and redox state of solutions during deposit formation. Mineralization characteristics and mineral associations enable subdivision of deposits into proximal (0 - 2 km), intermediate (2- 4 km), and distal (4-6 km) zones. The changes in solution pH over time, from acidic to relatively alkaline, may give rise to vertical zoning.
The origin of exotic deposits is linked to the transpressive tectonism and rapid uplift that produced the Andean chain in the late Oligocene-early Miocene interval. The generation of paleorelief caused water tables to fall, favoring formation of supergene enrichment and exotic copper deposits. The process was terminated by the change from semiarid to extremely arid conditions and, locally, by the deposition of extensive ignimbrite flows that sealed off access of supergene solutions.
An empirical model for exotic deposits emphasizes the lateral and vertical zonation of mineralization and alteration. This zoning is interpreted in terms of successive solution fronts acting repeatedly during the period of mineralization. Low-pH solutions percolated through fractured rocks and rendered them impermeable and unreactive. Subsequently, more alkaline groundwater advanced over the impermeable kaolinized horizons, reaching higher levels and greater distances from the source and precipitatingoxide copper minerals in either unaltered gravels or basement rocks.