LARGE SCALE CENOZOIC EXTENSION IN SW NEW MEXICO AND THE TECTONIC SETTING OF A LARAMIDE PORPHYRY CU-AU SYSTEM IN THE MALONE AND TYRONE MINING DISTRICTS: A REGIONAL EXPLORATION SYNTHESIS

Regions of extreme Cenozoic extension in the Cordillera of western North America are associated with multigenerational normal faults. These faults have significantly tilted and dismembered many Laramide and older porphyry Cu deposits, particularly in the Basin and Range of Nevada and Arizona.

Southwest New Mexico from the San Vicente valley to the Arizona border is part of an unrecognized region of extreme extension superimposed upon broad Laramide uplift. Oligocene and older rocks are tilted generally NE at up to 75°; and Laramide porphyry deposits are tilted 20° to 35° NE in the Lordsburg district, up to 45° at Tyrone, and 50° to 75° in the Malone district.

Porphyry-skarn systems NE of the San Vicente graben; including Chino, Pinos Altos, and the Hanover-Fierro district, are all relatively upright and undisturbed by extension. Paleozoic rocks at the Lone Mountain deposit, just NE of the graben,are tilted NE - reflecting Laramide uplift - but Oligocene volcanic rocks are flat.

At Malone, a basal Cenozoic conglomerate with veined and altered clasts dips up to 65° NE, and overlying volcanic and sedimentary rocks are unaltered. Below the unconformity, several km² of Precambrian crystalline rocks have vein controlled quartz-sericite-pyrite cutting propyllitic alteration. Sizable local zones have sheeted and stockwork quartz-pyrite veins with Au-Pb-Zn-Cu mineralization and pervasive, weak to moderate qtz-ser-py alteration. This is characteristic of the lateral margin of a porphyry Cu system, and is roughly similar to what is found within 0.5 km of open pits at nearby Tyrone. Malone outcrops may be the margin of a NE tilted, faulted porphyry system concealed at depth.

Multiple generations of SW down normal faults separate Malone from Tyrone, and include at least one major low angle fault. Cross-section reconstruction from Lordsburg NE through Malone, Tyrone, Lone Mountain, and Chino indicates that the Malone and Tyrone mineralization were once contiguous, and both appear related to the Tyrone stock, which is now tilted significantly to the NE.

The Malone-Tyrone system was enormous, comparable in size to Morenci or even Chuquicamata. Recognizing Cenozoic extension and understanding the effects on this system in SW New Mexico is critical for future exploration. This principle holds true throughout the Cordillera.