Discovery of a Jurassic Porphyry Copper Belt, Pangui Area, Southern Ecuador

Abstract

ABSTRACT Grassroots exploration has led to discovery of 10 porphyry copper prospects in the previously unexplored Jurassic arc of southeastern Ecuador. The prospects are located in steep, wet, jungle-covered terrain in the Pangui area, part of the Cordillera del Cóndor. The exploration program, initially mounted in search of gold in the Oriente foreland basin, employed panned-concentrate drainage sampling. Follow-up of the resulting anomalies utilized soil sampling combined with rock-chip sampling and geologic mapping of the restricted creek outcrops. Scout and infill drilling of two of the prospects, San Carlos and Panantza, has shown hypogene mineralization averaging 0.5 to 0.7 percent Cu overlain by thin (averaging <30 m) zones of chalcocite enrichment or oxidized copper mineralization. The prospects are centered on small, composite granocliorite to monzogranite porphyry stocks that cut the Zamora batholith or, in one case, a satellite pluton. The batholith is emplaced into Jurassic volcanosedimentary formations, which concealed Triassic extensional half-grabens before being incorporated into the Subandean fold-thrust belt along the western margin of the Oriente basin. North- and northwest-striking normal faults in the hanging wall of a major north-striking fault zone controlled the locations of most of the porphyry centers. K silicate and variably overprinted intermediate argillic alteration, containing chalcopyrite as the principal sulfide mineral, characterize the central parts of most of the porphyry prospects and grade outward to pyrite-dominated propylitic halos. Overprinted sericitic alteration is generally less widely developed, although apparently shallower erosion at the Warintza and Wawame prospects resulted in preservation of extensive pyrite-rich sericitic zones. All the prospects contain appreciable (60–250 ppm) molybdenum, but gold tenors are low except at Panantza and Wawame (~0.15 and 0.2 g/t, respectively). Supergene oxidation and chalcocite enrichment zones are immature because of inhibition by the rapid erosion prevalent in the Pangui area. Supergene profiles attain their maximum development on ridge crests but are essentially absent along major creeks. Discovery of the Pangui belt, along with other recently defined porphyry copper systems in northern Perú, Indonesia, and the Philippines, underscores yet again the efficacy of drainage geochemistry as an exploration technique in tropical and subtropical arc terranes as well as the outstanding potential for additional exposed deposits in poorly explored parts of the circum-Pacific region.