The geologic history of supergene enrichment in the porphyry copper deposits of southwestern North America.

Abstract

The supergene enrichment present at most of the porphyry copper deposits in southwestern North America is the result of at least three stages of weathering. Field evidence and K-Ar dating of the minerals alunite, jarosite, and illite establish the relationship between these three stages of weathering and the tectonic evolution of the region. K-Ar dates on alunite from Sacaton, illite from Tyrone, and stratigraphic evidence from Safford, Santa Rita, Ray, Ajo, Morenci, and San Manuel prove the first cycle of weathering and supergene activity occurred during Eocene dissection of the Mogollon Highlands. This stage of supergene activity ended when most of the porphyry copper deposits in the region were buried by Middle Tertiary volcanic rocks. There have been two post-volcanic stages of supergene activity. K-Ar dating of alunite from Silver Bell, Tyrone, Santa Rita, and Red Mountain-Patagonia and illite from Pinto Valley provide evidence the first post-volcanic stage of supergene enrichment was initiated by Early Miocene block faulting associated with movement on regional low-angle normal faults. The first stage of post-volcanic supergene alteration and mineralization came to an end when primary sulfides, the limiting reagent in the supergene enrichment process, in the weathering portions of the porphyry deposits were exhausted by oxidation or when supergene deposits were submerged by rising groundwater within the closed drainage basins of the period. K-Ar dating of alunite from Tyrone, Morenci, Ajo, San Xavier North and Silver Bell; jarosite from San Xavier North and Ajo; and stratigraphic evidence from Cactus-Carlotta, Ray, Santa Cruz, and San Manuel provide evidence that the second post-volcanic stage of supergene mineralization in the region was initiated by the Late Miocene Basin and Range event. This third stage of supergene alteration and mineralization came to an end when establishment of semi-arid climatic conditions increased the rate of erosion and decreased the rate of chemical weathering in the region. Therefore, prior to large scale open pit mining most of the supergene deposits in the region were, temporarily at least, inactive.