The evolution of supergene enrichment in the Morenci porphyry copper deposit, Greenlee County, Arizona

Supergene enrichment in the Morenci porphyry copper deposit was formed as a result of the coupled processes of erosion and chemical weathering that accompanied five stages of landscape evolution in the Cenozoic Era. During Stage 1 (64 to 53 Ma), low-grade primary chalcopyrite and pyrite mineralization was deposited as a result of Laramide magmatic and hydrothermal processes at about 55 Ma. During Stage 2 (53 to 30 Ma), initial unroofing and erosion removed approximately 1.8 km of rocks overlying the deposit and shed detritus to the north in the Eocene and to the south in the early Oligocene. During Stage 3 (30 to 18 Ma), the deposit was preserved under 640 to 950 meters of volcanic rocks as a result of mid-Tertiary extension and volcanism. During Stage 4 (18 to 2 Ma), most of the supergene copper enrichment at Morenci appears to have been formed as a result of Basin and Range deformation between ∼13 and ∼4 Ma. Sixteen new ⁴⁰Ar/³⁹Ar ages from alunite, jarosite, and potassium-bearing manganese oxides in the district recorded three cycles of enrichment and leaching that peaked at about 7.3 Ma. Microbiological and geological studies revealed that acidophilic iron oxidizing bacteria and dissimilatory sulfate reducing bacteria contributed to leaching and enrichment of copper in the supergene environment, at least since the late Mocene. During Stage 5 (2 Ma to present), destruction of the current enriched blanket accompanied base-level drop and stream incision as a result of progressive drainage integration in southern Arizona in the late Pliocene and Pleistocene.