Abstract

K–Ar dating demonstrates that all but eight of 41 dated porphyry copper and related ore deposits of Mexico were emplaced during the Laramide episode of maximum plate convergence. One older deposit is related to the Jurassic volcanic arc of western North America, one is pre-Laramide Cretaceous, four are Oligocene in age, and two late Cenozoic deposits are within the modern trans-Mexican–Chiapenecan volcanic arc. Thirty-three of the deposits lie within a long narrow belt that continues into Arizona and New Mexico, and widens from 100 km to over 300 km in the region of maximum extension in the southern Basin and Range Province. Eighty-five percent of the deposits were emplaced during the eastward transgression of the Cordilleran volcanic arc in middle Cretaceous through Eocene time.The occurrence of the porphyry copper deposits of Mexico appears to be independent of the terrane intruded and the copper content of the wall rocks where the wall rocks predate the volcanic arc, which is syngenetic with the porphyry stock. However, strontium is significantly more radiogenic where the host porphyry has intruded terrane having a Precambrian crystalline basement. Most frequently, the porphyry pluton can be observed to have intruded penecontemporaneous volcanic rocks or the batholith itself. The porphyries appear to be apophyses of the batholiths. The relationships suggest that the ore components are contained within the calc-alkaline batholiths and concentrated in the subvolcanic porphyries and wall rocks during transport of hydrothermal fluids to the volcanic orifice.The shape of the Cordilleran copper belt is controlled by magma composition, existence of a protective capping of dominantly volcanic rock, uplift, time, and erosion. As the continental volcanic arc that produced the porphyry copper deposits progressed eastward, the associated magma became more alkalic and copper poor. Thus, enrichment to ore grade became increasingly improbable to the east. Uplift and ample time for erosion prior to the return of the continental volcanic arc reduced the probability of ore preservation to the west. Optimum conditions for preservation were present within the belt where burial of calc-alkalic porphyry plutons under a thick volcanic cover occurred before removal of the ore zone by erosion. The broader width of the porphyry belt to the north is probably the result of both more extensive basin-and-range extension and basin-and-range taphrogeny that exposed some of the porphyries to relatively recent denudation and consequently made them available for economic exploitation.

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