TECTONIC CONTROLS, TIMING AND GEOCHEMISTRY OF SUPERGENE ENRICHMENT OF THE TYRONE PORPHYRY COPPER DEPOSIT, GRANT COUNTY, NEW MEXICO

The Phelps Dodge Tyrone porphyry copper deposit is located 10 miles southwest of Silver City, New Mexico, and is part of the Laramide-aged Arizona and New Mexico porphyry copper province. Tyrone has been dated at 53-56 Ma by K-Ar on hypogene sericite. Tyrone lies in the southeastern part of the Basin and Range structural province. Many porphyry copper deposits in the Arizona and New Mexico porphyry copper province, including Tyrone, have been faulted and rotated by Tertiary Basin and Range extensional faulting.

Tyrone underwent extensive supergene sulfide enrichment during late Tertiary time, which enhanced the copper grades making it economically viable. Supergene enrichment was controlled by Basin and Range extensional faulting that affected uplift, erosion, basin filling, and groundwater levels, and also by the Tertiary paleoclimate. Copper-bearing ferricrete derived from erosion of the Tyrone deposit occurs at the base of Miocene-Pleistocene Gila Conglomerate, a syntectonic basin fill unit. The ferricrete occupies northeast-trending paleochannels that extend over two miles from the deposit, and contains as much as 2700 ppm copper. Mapping in Oligocene volcanic rocks to the northeast of Tyrone has also revealed a ferruginous, copper-bearing sandstone and conglomerate unit that represents earlier erosion of the Tyrone deposit. The ferruginous sandstone and conglomerate contains clasts of quartz-veined intrusive rocks derived from the Tyrone deposit, and as much as 467 ppm copper. Copper values decrease with distance from the Tyrone deposit.

Drill hole logging of the leached cap and top of the chalcocite blanket in the mine indicates that the leached cap is tilted 9 to 21° to the northeast due to movements on northwest-striking Basin and Range normal faults. The leached cap is more goethitic in the souwestern part of the district in the area of lower total hypogene sulfides, and more hematitic in the northeastern part of the district in the area of higher total hypogene sulfides. At least two supergene enrichment events are indicated by limonite mineral distributions in the leached cap. ⁴⁰Ar/³⁹Ar dates on supergene alunite from the deposit indicate two supergene enrichment events in the Miocene at 19.4 to 16.2 Ma and again at 8.7 to 4.6 Ma.