A New Class of Nonvolcanic, Large, Peraluminous Copper-Oxide Greisen Deposits in Arizona, USA

An economically significant new type of large copper deposit is herein named the peraluminous copper-oxide deposit type (Gunnison type). These deposits can be economically significant, as the giant Gunnison system in southeastern Arizona contains three million tons of produced or drill-defined copper resources. The main ore mineral in these deposits is chrysocolla, with associated specular hematite. The paucity of sulfur results in only minor amounts of pyrite or copper sulfides. Other copper oxide minerals (in the context of leach processing) include minor malachite and azurite. Beryl (goshenite), fluorite, powellite, and hübnerite also occur. The latest Laramide emplacement ages of these copper-oxide greisen deposits at 68–43 Ma in Arizona are slightly younger than the more mafic, hornblende-bearing plutons associated with classic porphyry copper deposits emplaced between 78–53 Ma. Classic porphyry copper deposits formed in the upper crust (2–5 km deep) during the emplacement of mafic, metaluminous plutons during moderate-angle subduction. Porphyry-copper-related plutons acquired their copper from gabbroic melts developed by hydrous melting of layered mantle in the hanging wall of the flattening subducting Farallon plate. Peraluminous copper-oxide greisen deposits formed in midcrustal settings (10–15 km deep) and are associated with aluminum-rich, biotite granitoids formed by hydrous “water flooding” of the lower to midcrust during flat subduction in the latest Laramide. No volcanism is associated with these peraluminous copper-oxide greisen deposits. The peraluminous plutons acquired their copper from minimum melt, sialic, plutonic to metasedimentary sources in the lower to middle crust, under middle to upper amphibolite conditions.