This study investigates temporal and spatial relations between shortening and porphyry copper intrusions in the Late Cretaceous to early Paleocene Laramide arc of Arizona and New Mexico. In many parts of the arc, Laramide reverse faults, associated fault-propagation folds, and intrusions were dismembered and tilted by mid-Cenozoic and younger normal faults or were concealed by younger cover. These challenges were the principal reasons that the locations of Laramide basement-cored uplifts were poorly known in parts of the arc until recently. Here we systematically assess field and geochronologic data bearing on the age of reverse faults and porphyry copper systems in the region to determine whether reverse faults influenced the emplacement of porphyry copper plutons. Reverse faults and related folds offer direct evidence for shortening, but less-direct evidence—such as the distribution of strata at a mid-Cenozoic erosion surface—also indicates periods of shortening and uplift. Whether reverse faults influenced the emplacement of porphyry copper plutons is demonstrated in the few districts where reverse faults are exposed together with mineralized intrusions. Greater inference is required to explain changes in magma composition in time and space and to assess whether deep crustal structures localize magmas or whether local shortening gravitationally triggers magmatism. Here we present new generalized views of several porphyry deposits with Laramide and younger structures in map view, present-day cross sections, and cross sections restored to Laramide geometry, including the best places to assess structural control by reverse faults.
In areas with the best constraints, porphyry copper deposits formed an average of ~5 m.y. after formation of local macro-scale faults and folds. Map patterns show that most mineralizing intrusions were not controlled by reverse faults because intrusions cut across the faults without intruding along them. East-northeast–striking faults, most of which presumably formed during the Laramide orogeny and were oriented parallel to the compressional stress direction at that time, influenced the geometry of porphyry dikes and veins in several deposits and thus are inferred to have controlled their emplacement. The relation between those structures and reverse faults, however, is unknown pending future studies. The possible role of preexisting basement structures, typically inferred from lineaments, in localizing ore systems in this arc is currently poorly constrained, and alignments could have other explanations.
Laramide porphyry copper deposits are commonly located within the footwalls of major reverse fault systems. Given the typical postshortening timing of deposits in this province, the footwall location suggests that topography generated from major uplifts aided preservation of ore deposits in the footwalls of basement-cored uplifts. Paleogeologic maps of the mid-Cenozoic erosion surface thus can be used for regional exploration targeting. Furthermore, comparison of the timing of shortening and the timing and changing compositions of magmatism permits a speculative genetic link between crustal shortening and mineralization. Andesitic volcanism and dioritic stocks predated shortening, whereas magmatism associated with porphyries mostly postdated shortening and was associated with much larger and more felsic intrusions. Following early andesitic volcanism, subsequent shortening may have suppressed volcanism and promoted fractionation of magma in deep crustal storage zones. Upon local lessening of compressional stress, and perhaps after the locus of reverse faulting jumped to another location in the arc, these more evolved magmas ascended, forming upper crustal chambers. Petrologic arguments, supported by eroded exposures of tilted sections of the crust beneath several deposits, suggest that metals probably were scavenged during convection of large felsic magma chambers. A separate metal-bearing, saline aqueous phase accumulated near the top of the chamber and accompanied an upward rise of small volumes of magma in multiple events. Large hydrothermal systems formed porphyry copper deposits as successive pulses of magma intruded and crystallized as porphyry stocks and dikes.
