Structural reinterpretation of the Ajo mining district, Pima County, Arizona, based on paleomagnetic and geochronologic studies

Abstract

The Ajo mining district of southern Arizona is divided into two main structural blocks by the Gibson Arroyo fault. The eastern Camelback Mountain block contains the Late Cretaceous-early Tertiary porphyry copper deposit which has been previously thought to be associated with the displaced apex of a large intrusion exposed by deeper erosion in the western Cardigan Peak block. However, unpublished U-Pb data support a mid-Tertiary age for the western intrusion. Paleomagnetic and geologic evidence indicates that the ore deposit has been tilted to the south a total of approximately 120 degrees : 68 degrees before and another 55 degrees after emplacement of the overlying Locomotive Fanglomerate and Ajo Volcanics. The Ajo Volcanics have K-Ar ages (24-25 m.y.) slightly older than those from the large intrusion of the Cardigan Peak block (20-23 m.y.). Paleomagnetic directions are consistent with the southward tilting of the volcanic rocks, but they suggest that the remanent magnetization and perhaps the K-Ar dates of the western intrusion were reset by the emplacement of dikes younger than the Locomotive Fanglomerate and Ajo Volcanics. These and other geologic relations indicate the following sequence of mid-Tertiary events in the district: (1) emplacement of the western intrusion, (2) movement along the Gibson Arroyo fault, (3) unroofing and perhaps tilting of the pluton [asymp] 70 degrees to the south along with the Camelback Mountain block, (4) syntectonic depositions of the Locomotive Fanglomerate and the Ajo Volcanics, (5) continued uplift and tilting to the south totaling 40 degrees to 60 degrees , (6) intrusion of the youngest dikes with attendant alteration and remagnetization of the host rocks, and (7) minor (?) oblique movement along the Gibson Arroyo fault.