AJO MINING DISTRICT, SOUTHWESTERN ARIZONA: LARAMIDE MAGMATISM, PORPHYRY COPPER MINERALIZATION, AND STRUCTURAL EVOLUTION

The Ajo district contains a Laramide porphyry Cu-(Au-Mo) deposit associated with the New Cornelia pluton, Na-Ca alteration spatially associated with the mid-Cenozoic Cardigan Peak pluton, and has been overprinted by Cenozoic extension. Data from maps and drill holes, new mineral composition data, U-Pb zircon age dates, and Al-in-hornblende thermobarometry on the New Cornelia pluton add to the understanding of the geologic history of the Ajo mining district. The New Cornelia pluton, responsible for porphyry-style mineralization in the district, intruded into Cretaceous Concentrator Volcanics. The New Cornelia stock has been rotated ~100-120° S/SE such that the outline of the New Cornelia pluton narrows from northwest to southeast in map view. Alteration progresses from potassic in the northwest to sericitic in the southeast, i.e., from deeper to shallower levels. Bornite-chalcopyrite-molybdenite dominate ore mineralization with K-feldspar-quartz-secondary biotite-anhydrite-magnetite-tourmaline-rutile gangue. Pyritic associations related to sericitic alteration overprint potassic alteration. Two periods of supergene enrichment are recognized in the district; one chalcocite blanket dips ~30ºS, the other blanket parallels the pre-mine topography. West of the New Cornelia mine, and west of the Gibson Arroyo fault, a Proterozoic pluton and its gneissic wall rocks are intruded by the 23-25 Ma Cardigan Peak pluton. Na-Ca alteration overprints the Cardigan Peak pluton and is caused by the circulation of nonmagmatic external brines. Widespread chlorite overprint within the New Cornelia pluton and surrounding Concentrator Volcanics likely formed from circulation of dilute meteoric fluids during the Cenozoic.