- Economic Geology
A new porphyry copper deposit has been discovered in the Superior (Pioneer) district of east-central Arizona, lying under thick Tertiary cover approximately 2 km south of the historic Magma vein. The porphyry center was encountered in the course of an exploration drilling campaign conducted from 1992 through 1997, comprising a series of subvertical surface core holes and subhorizontal underground core holes ranging in length from 1,000 to 1,955 m. The system is large, structurally intact, and shows indications of a substantial hypogene mineralized zone that averages between 1.0 and 2.0 percent Cu.
The deposit is hosted by a thick sequence of Cretaceous (?) volcaniclastic and siliciclastic sedimentary rocks. The Mesozoic strata rest disconformably on calcareous units of the uppermost Paleozoic Naco Group and are intruded by an east-northeast–striking swarm of quartz porphyry dikes. The dike swarm is truncated at the basal Tertiary unconformity, under which the Mesozoic section is buried by 600 to 1,200 m of unmineralized overburden composed of the Oligocene-Miocene Whitetail Conglomerate and the Miocene (18.6 Ma) Apache Leap dacite tuff. The attitude of the basal Tertiary beds shows that the porphyry center has been tilted 30° to 35° to the east-northeast.
A broad zone of hydrolytic alteration characterizes the high structural levels in Mesozoic wall rocks. Peripheral propylitic alteration in the volcaniclastic rocks is marked by epidote and chlorite, giving way inwardly to a narrow zone of chlorite-dominant alteration in which epidote is destabilized in favor of smectite clays and carbonate. This chlorite zone corresponds closely to the "intermediate argillic" alteration type of Meyer and Hemley (1967). The propylitic fringe of the system is erratically mineralized by vein-controlled pyrite, sphalerite, and specularite with minor chalcopyrite and galena. Chloritic rocks in the intermediate argillic halo contain pyrite and chalcopyrite. The intermediate argillic alteration mantles an inner zone of pervasive quartz-sericite-pyrite. The sericitic zone is strongly pyritic, with bornite and lesser chalcopyrite. Within sericitically altered rock, pyrite-chalcocite-digenite-bornite veins exhibit centimeters- to meters-wide halos of advanced argillic alteration in which sericite is replaced by kaolin group clays.
The quartz-sericite-pyrite zone is superimposed on earlier potassic alteration that remains preserved at deeper levels. The base of the Mesozoic sequence and shaly beds in the upper Naco Group section are pervaded with secondary biotite. Calcareous rocks in the Naco Group are altered to (1) pyroxene-epidote hornfelses and minor garnet-magnetite skarn, overprinted by (2) actinolite and (3) chlorite-carbonate-clay alteration. Biotitically altered rocks are mineralized by quartz-pyrite-chalcopyrite ± anhydrite veins. Molybdenite is present in an early generation of quartz-rich pyrite ± chalcopyrite veins. Hornfelses and skarn in the Naco Group are strongly mineralized by quartz-poor, chalcopyrite-pyrite-anhydrite veins with chloritic envelopes.
Surface and underground drilling into the deposit has outlined a zone of >1.0 percent hypogene Cu that is at least 750 m long by 250 m wide by 300 m high, elongated to the east-northeast. The same volume of rock also averages 0.02 percent Mo and 2 ppm Ag. This zone apexes approximately 300 m below sea level, slightly more than 1.5 km below the ground surface, and remains open in all directions laterally and at depth. Four of the last five drill holes penetrating this zone ended in rock assaying >2.0 percent hypogene Cu.