Epochs of intrusion-related copper mineralization in the Andes

Abstract

Seventy-four copper deposits and prospects related intimately to intrusive activity in the Andes have been dated radiometrically during the last 18 years by many different investigators, most of whom used the KAr method. The results are summarized and some of their local and regional implications are reviewed. A number of copper deposits, mainly of the porphyry type, were emplaced in, or near to, premineral volcanic sequences and (or) equigranular plutons. Such precursor volcanism lasted for as long as 9 Ma, and preceded mineralization by intervals of from less than 1 Ma to as much as 9 Ma. Precursor plutons were emplaced no more than 2 to 3 Ma prior to mineralization at several localities in Chile, but possibly as long as 10 to 30 Ma earlier in parts of Colombia and Peru. The time separating emplacement of progenitor stocks and hydrothermal alteration and accompanying copper mineralization, and the duration of alteration-mineralization sequences generally are both less than the analytical uncertainty of the KAr method. However, on the basis of a detailed study of the Julcani vein system in Peru and less clearcut evidence from elsewhere, it may be concluded that alteration and copper mineralization followed stock or dome emplacement by substantially less than 1 Ma and lasted for 0.5 to 2 Ma and, locally, possibly as long as 3 Ma. At several localities, post-mineral magmatic activity could not be separated by the KAr method from the preceding alteration-mineralization events. As many as nine epochs of copper mineralization, ranging in age from late Paleozoic to late Pliocene-Pleistocene, are recognizable in the central Andes of Chile, Peru, Bolivia, and Argentina, and at least four somewhat different epochs characterize the northern Andes of Colombia. Each epoch coincides with a discrete linear sub-belt, some of which extend for more than 2000 km along the length of the orogen. More than 90% of Andean copper resources, mainly as porphyry deposits, are confined to three Cenozoic sub-belts of Paleocene-early Eocene (66-52 Ma), late Eocene-early Oligocene (42-31 Ma), and middle Miocene-early Pliocene (16-5 Ma) ages in southern Peru and Chile. The scarcity of porphyry copper deposits and the increase of pluton-related copper veins in Jurassic and Cretaceous sub-belts farther west in the central Andes is believed to be due to deeper levels of erosion. In the central Andes, copper sub-belts became progressively younger eastward in response to the effects of a compressional tectonic regime imposed on the overriding South American plate, following mid-Cretaceous opening of the South Atlantic Ocean basin. A marked eastward breakout of middle Miocene-early Pliocene copper mineralization up to 300 km into western Argentina and Bolivia is attributed to rapid subduction of young buoyant oceanic lithosphere during the Miocene. A contrasting migration pattern of copper sub-belts characterized the northern Andes, where an early Cretaceous to early Eocene trenchward shift is thought to have been caused by accretion of oceanic terranes to the continental edge. The observed distribution of copper sub-belts reflects the tectonic segmentation of the Andes. Three first-order segments are defined by the well-known Huancabamba and Abancay Deflections, at latitudes 5°S and 14°S, respectively. Each first-order segment is characterized by a profoundly different distribution, timing and, in part, style of copper mineralization, and by distinct copper endowments. Anomalous patterns of sub-belts abut both deflections. Several second-order boundaries appear to have caused more subtle changes along individual copper sub-belts. Additional programs of radiometric dating, on both local and regional scales, are required before the migration and segmentation of copper (and associated metallogenic) sub-belts in the Andes are properly defined and understood.