In the orogenic belts of western North and South America, metal provinces are aligned approximately parallel to the continental margins, and, despite irregularities, a general pattern of provinces comprises the following sequence from west to east: Fe; Cu (with some Au and Mo); Pb, Zn, and Ag; and in some regions Sn or Mo. The genesis of these metal provinces is attributed to the release of metals or associations of metals from basaltic oceanic crust and pelagic sediments during partial melting at progressively deeper levels on subduction zones which dipped eastward beneath the continent; the metals subsequently ascended as components of calc-alkaline magma. Initially, the metals were released from the mantle at the East Pacific Rise, transported to the margins of the Pacific basin and thrust beneath the continental margins by the process of sea-floor spreading. This model surmounts the problem of envisaging the existence in the crust or upper mantle of long, narrow zones characterized by a concentration of an individual metal or association of metals.
In the context of this model, possible explanations may be advanced for several features of the distribution in both space and time of metal provinces in western America, including: the occurrence of multiple metallogenic epochs within a given metal province; the difference in age of the dominant metallogenic epoch from one region to another; and the concentration or scarcity of metal deposits in certain regions.