Geology and Mineralization of La Caridad Porphyry Copper Deposit, Sonora, Mexico

Abstract

La Caridad, located in northeastern Sonora, Mexico, is a recently discovered major porphyry copper deposit. As presently developed, this important orebody has a proven reserve in excess of 600 million metric tons, averaging 0.8 percent copper (including molybdenum as copper equivalent). The deposit was discovered in 1967 by the Mexican Government9s Consejo de Recursos Naturales No Renovables. In 1968, an exploration contract was awarded to ASARCO Mexicana, which subsequently carried out an extensive exploration program.The La Caridad porphyry copper deposit occurs exclusively in felsic to intermediate intrusive igneous rocks and associated breccias. Host rocks include diorite and granodiorite. These rocks are intruded by a quartz monzonite porphyry stock and by numerous breccia masses which contain fragments of all the older rock types. Subsequent hydrothermal alteration and mineralization exhibit a close genetic reltaionship to the quartz monzonite porphyry and one of the breccia types.Alteration in the central core of the deposit is pervasive and is predominantly phyllic (quartz-sericite-pyrite). Alteration grades outward from the center, first into a poorly defined, irregular argillic band and finally into a narrow propylitic halo. Significant amounts of potassic alteration have not been recognized to the depths explored by drilling (approximately 500 meters).Hypogene mineralization consists of pyrite, chalcopyrite, and molybdenite, in order of decreasing abundance, together with minor amounts of sphalerite, galena, and bornite. The total sulfide content averages 2 to 3 percent by volume, with pyrite-chalcopyrite ratios ranging from 2:1 in the central portion of the deposit to as much as 10:1 in the margin areas. The hypogene copper grade in the deposit averages less than 0.3 percent copper. Molybdenum is most abundant in the eastern half of the deposit, where the molybdenum content averages 0.04 percent. In the western portion of the deposit, molybdenum averages less than 0.01 percent.Supergene enrichment, consisting of complete to partial chalcocite replacement of chalcopyrite and pyrite, occurs beneath the leached capping. The zone of supergene enrichment occurs as a flat, tabular blanket which has been modified to approximate the shape of an inverted saucer. The chalcocite blanket has an average diameter of 1,700 meters and an average thickness of 90 meters. Threefold enrichment, relative to hypogene mineralization, has resulted in an average ore grade of approximately 0.75 percent copper in the chalcocite zone.The rocks at La Caridad have been oxidized and almost completely leached of copper minerals to an average depth of 50 meters. This leached capping contains various combinations of indigenous iron oxide minerals that include hematite, goethite, and jarosite. The distribution of these minerals was used during mapping to interpret the preexisting sulfide mineralogy.