Summary:
Deposit Type
World Copper presently considers mineralization at Zonia to be the product of a porphyry copper system, which is the conceptual deposit model on which current plans for future exploration are based.
While mineralogy and alteration observed at Zonia is similar to that of numerous Laramide-aged porphyry copper deposits hosted by Proterozoic greenstone rocks in central Arizona, the age of mineralization at Zonia is as yet undetermined and deformation of mineralized veins coincident with foliation at Zonia suggests that the age of the Zonia deposit is quite likely concurrent with the timing of Precambrian deformation.
Within the resource area, the Zonia deposit is characterized by mostly oxidized, supergene-enriched stringer, vein, fracture, and disseminated oxide mineralization. The original pyrite-chalcopyrite assemblage underwent intense oxidation, with copper remobilized into fluid flow conduits (fractures) and concentrated in more mafic, calcareous reactive units (the greenstone and chlorite schist) and at the water table, ultimately resulting in the development of chalcocite-rich lenses that are known to extend up to 800 feet deep along the steep, west-dipping foliation of host rocks.
This supergene mineralization experienced a second phase of oxidation and partial remobilization due to regional uplift and erosion, as well as the lowering of the water table, which resulted in a large deposit of in-situ and transported copper oxide mineralization that masks much of the primary sulfidic depositional environment. The processes of oxidation followed by supergene enrichment and then secondary oxidation are well-documented by Locke (1926), Blanchard (1968), and Anderson (1982).
The original sulfide minerals were principally pyrite and chalcopyrite, with minor bornite, molybdenite, and sphalerite. Grid sampling of the pit area tentatively indicates an overall mineral zonation of inner copper, molybdenum, and gold, zoning outwards to zinc and manganese. Such zoning is characteristic of porphyry copper deposits, and additional work should be carried out to confirm/define its occurrence at Zonia. Further evidence to support the porphyry copper deposit model includes the cross-cutting relationships exhibited by quartz-chalcopyrite veins and veinlets, the type and style of both supergene and hypogene alteration, and the regional position of the deposit within an island arc setting. Structure, alteration, and oxide copper minerals are considered the principal guides to further exploration.
Mineralization
Copper mineralization occurs primarily within the foliated Qmpf, the protolith of which is presumed to be argillically altered Qmp, but mineralization is also concentrated along the contacts of various felsic units, as well as between mafic and felsic units. The latter occurrence is considered a late-stage effect of supergene, mobilized copper reacting with the more calcic mafic units.
As described by Schmidt (2021), the pit geology is dominated by a leached cap of pervasive supergene alteration and limonite that typically forms over primary pyrite-chalcopyrite mineralization. Sericite alteration is extensive and obscures original rock textures in places. Clays representative of argillic alteration is readily observed in the South pit. Hematite, jarosite and goethite occur as fracture coatings and boxwork veinlets ranging from 0.1- to 0.5-inches in width, and hematite veins up to 2 inches thick suggest that the chalcopyrite-quartz veins and secondary chalcocite favor the sericite schist.
Known mineralization extends approximately 8000 feet along strike parallel to the regional (northeast) trend of foliation, with a dip of 80 to 85 degrees to the northwest. Width of the mineralized zone is quite variable, ranging from 250 to about 1000 ft. Ore minerals primarily consist of chrysocolla, black copper oxides (tenorite, melaconite, pitch), cuprite, native copper, malachite and azurite, though occasional shipments of chalcocite were reportedly made from the underground Cuprite shaft and from the North pit.
Current interpretation proposes that regional deformation related to the Yavapai Orogeny sheared the originally disseminated and blebby pyrite-chalcopyrite mineralized horizons into folia-form mineralization, parallel to schistosity, and ranging from vertical to a dip of ~45°. Subsequent oxidationremobilization of the copper from chalcopyrite (~35% Cu) followed the foliation down-dip to the groundwater table, where copper then reprecipitated as enriched sulfide minerals, primarily secondary chalcocite (~78% Cu). This chalcocite blanket was then oxidized during a second lowering of the water table and copper further mobilized into reactive units below. The early underground mining at Zonia exploited the high-grade chalcocite horizons preserved at depth.