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1 Center for Mineral Resources, Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA
The present metallic distribution in northwestern Mexico is the culmination of superposed magmatism, tectonism, erosion, and burial over more than 150 m.y. Detailed palinspastic reconstructions of preextensional configurations—the first study of its kind for this region—clarify the interplay among these features on the present distribution and character of mineralized geologic systems. This new synthesis goes beyond previous metallogenic investigations of northwestern Mexico by separating events into specific timing and structural relationships, and by restoring the geology to its preextensional configuration. Metallogenic factors such as enrichment, preservation, and erosion play major roles in the present distributions and for the first time are related to the overall metallogenic framework of northwestern Mexico. The analysis concludes that modern metallogenic patterns are the result of the complex superposition and subsequent redistribution of geologic systems in a way that is related directly to the regional history, rather than simply metallic belts or interpreted angle of a subducting slab.
Three main extensional events in the Oligocene–Holocene have been restored, and the palinspastic distributions have been analyzed. Reconstructions reveal the following: (1) Mineralization events, igneous centers, and sedimentary sequences are continuous across the Gulf of California and other areas with large amounts of extension. (2) Middle Tertiary gold-silver mineralization in Baja California may be the western part of the Sierra Madre Occidental metallogenic province, thus expanding the previously recognized extent of this province's mineralization. (3) Late Cretaceous–early Tertiary porphyry copper deposits and intrusive centers form a narrower belt than previously noted and are traceable for over 400 km, with parts of the belt buried beneath the younger Sierra Madre Occidental volcanic fields. (4) Interpreted alignments of older geologic features, including lineaments of ore deposits, are displaced in the reconstructions. (5) Sedimentary-rock–hosted gold deposits and low-angle-detachment gold systems are closely related and occur around core complexes.
By using structurally restored time slices, it becomes clear that older deposit types tend to be those formed at greater depths and more proximal to intrusions, whereas younger deposits formed at shallower depths are less eroded and are more commonly volcanic-rock hosted. These characteristics express themselves in the regional distribution of deposit types. Second, mineralization of widely differing ages is spatially superposed, commonly associated with coeval magmatic and tectonic events. The structural and magmatic events together with paleodistribution of ore deposits define a new framework to interpret the metallogenic history of northwestern Mexico.
Key Words: metallogeny • Mexico • ore deposits • reconstruction • Sonora • tectonics
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