Porphyry copper deposits in Puerto Rico and their relation to arc-trench tectonics

Abstract

The study of porphyry copper deposits in islands arcs provides an opportunity to observe closely the direct relationship between tectonics and ore deposition, and to avoid such complications as multiple orogenies and contamination of magmas that are common to deposits on continental margins. Puerto Rico has proved to be an ideal laboratory to make these observations. In the west central part of the island 240 million tons of .7 to .8 percent copper ore has been proved up in three deposits (Lutjen, 1971). Although deep saprolite covers most of the mineralized area, two marginal deposits, Helecho and Sapo Alegre, are partly exposed by deep stream valleys. The U. S. Geological Survey, in cooperation with the Puerto Rico Department of Natural Resources is in the middle of a five year geologic and geochemical study of copper deposits in the island. Drill core was made available through the courtesy of Kennecott Copper Corporation, American Metals Climax Corporation and the Puerto Rico Mining Commission. I wish to acknowledge the help of the Puerto Rico Laboratory of Geology for the chemical work, and Richard Marvin and J. Thomas Nash of the USGS for geochronologic and fluid inclusion data, respectively. This report will briefly describe the porphyry copper deposits of Puerto Rico and their tectonic-chronologic position in the development of the Greater Antilles. By extrapolation from this data it will be suggested that these deposits and several others are related to a process of subduction zone deactivation.