Summary:
Deposit Type
The Johnson Camp Mine copper deposit is a type of copper skarn. The copper skarn at Johnson Camp and collectively in the Cochise mining district is presumably related to the Texas Canyon Quartz Monzonite. Copper skarns generally form in calcareous shales, dolomites, and limestones peripheral or adjacent to the margins of diorite to granite intrusions that range from dikes and sills to large stocks or phases of batholithic intrusions, and frequently are associated with mineralized intrusions. Copper mineralization forms along structurally complex and fractured rocks and convert the calcareous shales and limestones to andradite-rich garnet assemblages near the intrusive body, and to pyroxene and wollastonite rich assemblages at areas more distal to the intrusive that are subject to retrograde alteration with mineral hydrated silicate assemblages that overprint earlier garnet and pyroxene.
Mineralization at Johnson Camp occurs approximately 500 ft northeast of known occurrences of the Texas Canyon Quartz Monzonite intrusion as proximal skarn related to a porphyry copper system. This assumption is supported by the high abundance of garnet-epidote alteration in the mineralized zones, and the characterization of the deposits in numerous historical publications.
Geological Setting and Mineralization
The Johnson Camp Mine is located within the Mexican Highland region of the Basin and Range province, which is characterized by fault-bounded mountain ranges, with large intrusions forming the cores of the ranges. The project lies on the eastern edge of the Little Dragoon Mountains within the Cochise mining district. The Little Dragoon Mountains are an isolated, fault bounded horst block comprised of rocks spanning from 1.4 billion years ago (Ga) Pinal Group schists to Holocene sediments. The southern portion of the Little Dragoon Mountains consists predominately of the Texas Canyon Quartz Monzonite of Tertiary age, whereas the Pinal Group schists and a sequence of Paleozoic sedimentary units dominate the northern half of the range. At Johnson Camp, the important Paleozoic host is the Cambrian Abrigo Formation. The Texas Canyon Quartz Monzonite is porphyritic intrusion that crops out to the southwest of the Burro Pit at the Johnson Camp Mine.
Several deformations have occurred in the area with the most recent being the latest Cretaceous-Paleocene Laramide Orogeny compression, followed by Miocene and younger Basin and Range extension that has modified the topography to its current appearance.
The stratigraphy of the Burro pit and Copper Chief pit includes, from lowest to highest, Pioneer shale, diabase sill, Bolsa quartzite, three members of the Abrigo formation, and the Martin dolomite. Most mineralization is hosted in the lower and middle members of the Abrigo formation.
Moderate to intense calc-silicate alteration including garnet, epidote, and diopside are common in various assemblages, most intense calc-silicate alteration in the Lower and Middle Abrigo formations. Pervasive quartz veining occurs in both the Abrigo Formation and underlying Bolsa Quartzite throughout the Johnson Camp Mine area. Quartz vein orientations are typically sub-parallel to the stratigraphic units.
Primary copper mineralization at Johnson Camp is dominantly found along bedding planes or in veins and replacements as chalcopyrite along with quartz and pyrite, closely associated with skarn and calc-silicate alteration in the rock. The host formations are generally within the Bolsa Quartzite, Diabase Units, Lower and Middle Abrigo Formations. The mineralization has been oxidized, consisting of chrysocolla, malachite, copper limonite, and manganiferous wad, decreases with depth but penetrates along faults and stratigraphic contacts. Supergene chalcocite and occasional native copper occur generally below the oxidized zone. Below the supergene zone, the mineralization transitions to primary sulfides despite local zones of supergene mineralization.