Insights on the Origin of the Laramide Porphyry Copper Deposits of Northwestern Mexico

As currently accepted, the porphyry copper deposits are hydrothermal systems centered on relatively small stocks that were popped out from larger and deeper seated cooling intrusive bodies. In northwestern Mexico, this parental relationship is supported by the nearly contemporaneous ages of corresponding pre- and mineralized plutons. Moreover, the Sr and Nd radiogenic isotopic data also suggest a strong consanguinity among them. Based on this, the preferred idea regarding the origin of the ore-related stocks is that they formed by decompressive partial re-melting of the “still hot” cores of the major intrusives due to crustal attenuation. Further cooling of the more buoyant magma may have played a critical role in the exsolution and chemical behavior of hydrothermal fluids, which developed a particular ability to concentrate and transport copper and other metals. The attenuation of the crust may have occurred in response to important adjustments in the conditions of the plate convergence, prior to the subsequent detachment of the Farallon slab. The initial stages of the crustal extension developed important fracturing that facilitated the ascent of the re-melted material to shallower depths, and prepared the rocks for the vigorous hydrothermal fluid circulation. According to the available ages, including new 39Ar/40Ar dates, many of the porphyry copper deposits of northwestern Mexico were emplaced near the transition between the culmination of the Laramide compression and the beginning of the conspicuous episode of Tertiary extension that affected most of southwestern North America. This may explain why the main part of the porphyry copper belt in north and central Sonora and the southwestern United States, largely overlaps with a highly extended region where metamorphic core complexes were exhumed in mid-Tertiary times.