Abstract

Deposits of copper and zinc at Johnson, Arizona, occur in metamorphosed Paleozoic limestone near a quartz monzonite stock probably of late Cretaceous or early Tertiary age. The metallic mineralization was preceded by a stage of thermal metamorphism during which pure carbonate beds were recrystallized and impure carbonate beds were altered to garnet, diopside, and other contact-metamorphic silicates. Silicate formation, which involved loss of carbon dioxide, was accompanied by shrinkage that reached a maximum of 30 percent. In the following metallic mineralization, the metamorphic rock was replaced by copper and zinc sulfides associated with some chlorite and other relatively low temperature gangue minerals. Nearly all the ore occurs as tabular masses and chimneys in particular beds in the Abrigo formation of Cambrian age.The recently discovered Moore ore body is a lenticular mass in the Abrigo formation about 400 feet below the present surface. Faulted and fractured limestone and dolomite beds of the Escabrosa limestone (Mississippian) crop out above the ore body. Local copper stains, which are abundant in the district, and a greater-than-average amount of faulting are somewhat meager geological evidence for the presence of ore.To determine if there was any geochemical evidence for the proximity of ore, outcrops of the Escabrosa limestone and part of the underlying Martin formation (Devonian), the fault zones, and soils were sampled both over the ore and in the adjoining area, and the samples were analyzed for traces of the ore metals.The ore-metal content varies widely and is determined in part by stratigraphy and structure. Large areas abnormally high in ore metal are indicated by samples from the fault zones. Composite chip samples of the rock between the faults show small high areas within the high areas indicated by the fault-zone samples. One of the chip-sample anomalies is over the Moore ore body but displaced somewhat to one side of the center of the body. Two other anomalies are over unexplored ground some distance from the ore body. Soil samples collected on low ridges, where contamination is unlikely, show the same general anomalies as the rock samples.A genetic relationship between the Moore ore body and the nearby geochemical anomaly is suggested by its proximity and by the presence in the anomaly area of fault zones which carry concentrations of ore metal and project toward the ore. Diamond drilling and further geochemical studies are suggested as possible means of checking the inferred relationship. At present, geochemical studies give promise of becoming a valuable adjunct of geology in prospecting the Johnson district and similar areas elsewhere.

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