Field observations in areas of hydrothermal alteration in the Chilean and Peruvian Andes have permitted definition of four distinct, independent stages of hydrothermal activity--transformation stage, during which saturated alkaline solutions emanating from a cooling magma caused mild alteration (propylitization); reorganizing stage, involving the activity of high-volatile acid aqueous solutions produced as a result of retrograde boiling of magmatic residual fluids; replacement stage, during which considerable quantities of silica were introduced by low-volatile aqueous solutions and with whose initial phases copper mineralization was contemporaneous; late magmatic stage, during which strongly siliceous residual fluids, perhaps already in a semisolid state, were injected into the previously altered rocks.

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