Abstract At least five altered and mineralized porphyry centers related to the cooling of a polyphase Eocene intrusion occur within a 25-km 2 "pampa"-type area in the southwestern sector of the Chuquicamata district in northern Chile. These deposits take place 1 to 2 km apart as discrete porphyry "columns" covered by postmineral, poorly consolidated Miocene sedimentary rocks. Such copper oxide and sulfide deposits were discovered and evaluated by drilling done by Codelco from 1996 through 2007 during a brownfield exploration program, driven by the necessity to replace and increase leacheable ore consumed by the Chuquicamata and Radomiro Tomic operations. During this program a resource of more than 20 million metric tons (Mt) Cu was discovered, including 6 Mt Cu of oxide, mixed and secondary sulfide ore, representing one of the largest supergene copper resources discovered worldwide during the last 10 years. Despite their close location and their genetic relationship to a single, polyphase intrusion mineralization event, the five porphyry centers display contrasting host-rock and structural framework as well as different hypogene alteration and ore mineral assemblages. This picture reaches high levels of complexity because of the different levels of exposure of the mineral systems, resulting from primary emplacement processes and post-mineral faulting. These hypogene features and the effect of landscape and climate evolution controlled supergene alteration, thus generating different profiles in each specific porphyry center. The key controlling factors in the supergene overprint are discussed on the basis of their relationship to ore and gangue mineralogical abundance and occurrence, assemblage distribution, geochemical response, and the broad geologic setting. As exploration for covered porphyry copper deposits in the southwestern sector of the Chuquicamata district progressed, numerous lessons were learned about the origin of supergene profiles and the analysis and use of supergene effects and their products as a guide for exploration. These lessons, which include geological and geochemical criteria among others, are discussed in the context of the appraisal of the mineral potential of copper oxide-mixed-secondary sulfide blankets and underlying sulfide protore.

Most of the coal in Mexico is concentrated in three widely separated regions Coahuila on the northeast, Sonora on the northwest, and Oaxaca on the south-southeast. In the Coahuila region, bituminous coking coals are present in Upper Cretaceous rocks. The Sonora region contains deposits of Upper Triassic anthracitic coal, altered by probable thermal metamorphism. The Oaxaca region contains anthracitic and some bituminous coking coal in Lower(?) to Middle Jurassic sequences. Through 1973, only the coal deposits of the Coahuila region had been developed commercially. The coals of the Sonora and Oaxaca regions are undeveloped because of unfavorable geo-economic conditions and lack of exploration. Through 1973, the Coahuila region had yielded about 110 million metric tons of coal, most of which was used to produce coke. Present production is 4,250,000 metric tons per year. The largest mines include one underground mine (Sabinas no. 2) and one openpit mine (La Florida), which produce 40,000 metric tons per month. Only reconnaissance-geologic investigations, complemented locally by a few limited drilling projects, have been made in the Sonora and Oaxaca regions. Exploration has been more extensive in the Coahuila region. There are many totally unexplored areas with potential coal resources. The estimated resources in the Sabinas area of the Coahuila region total 1,781 million metric tons classed as reserves in situ, and 1,180 million metric tons classed as potential but not commercially important at present. North of the Sabinas area, in the Río Escondido basin, the estimated resources total 120 million metric tons classed as measured. In the Sonora region the estimated resources total 11 million metric tons classed as reserves in situ for the Santa Clara coalfield and 4 million metric tons classed as measured, 9 million metric tons classed as inferred, and 18 million metric tons classed as possible for the San Marcial district. In the Mixtepec and Tezoatlán coal fields of the Oaxaca region, the estimated resources total 24 million metric tons classed as inferred and 26 million metric tons classed as possible.