Discrimination of productive and nonproductive porphyritic intrusions in the Chilean Andes

Abstract

A geochemical comparison between productive intrusions (those associated with mineralization) and nonproductive intrusions (those unrelated to mineralization) has been made on the basis of over 100 fresh granitoid rocks from the Chilean porphyry copper belt. The study was carried out on the scale of a mining district (El Salvador-Potrerillos), then on the scale of a single traverse (El Salvador-Chilean coast), and finally on the scale of the Andes itself. The productive intrusions are best characterized by large negative anomalies in Y, Mn, Th, and the heavy rare earth elements. A discriminant diagram of Y against MnO is recommended for predicting whether an apparently fresh intrusion has associated peripheral mineralization.The low values of Y and Mn in productive intrusions can be partly explained by the early crystallization of Y- and Mn-rich hydrous phases such as hornblende and by the loss of Mn-rich fluids from the magma. The same elements make useful discriminants in other porphyry copper provinces, although the exact position of the discriminant boundary varies from island arcs through continental margin to continental settings.