BASE METAL PORPHYRIES AND DIAMOND-ENRICHED KIMBERLITES OF THE LARAMIDE OROGENY: PRODUCTS OF CONVERGENT MARGIN MAGMATISM

Widespread deformation of western North America associated with the Laramide Orogeny has been recognized for well over a century. Shallow and rapid subduction of the Farallon Plate beneath the North American Plate has in recent decades been established as the cause, and convergent margin magmatism associated with the Laramide is responsible for the dispersion of many base metal porphyry deposits near the southern margin of the North American Plate. We propose that this process is also responsible for the occurrence of diamond-enriched kimberlites erupting well over 1000 kilometers inboard of the plate margin. Hydrous minerals can be subducted into the upper mantle, and seismic imaging has revealed that a competent Farallon Plate extended far beneath North America. As with the widespread deformation, fluids similar to those forming base metal porphyry mineralization in Arizona also reached far inboard to the Slave Craton, generating kimberlite magma to carry commercially viable diamonds to the surface. A significant change in the convergence geometry between the plates may be postulated to trigger the formation and alignment of both deposit types, as deep penetrative basement faults become dilatant under the ambient far-field stress field to permit the rise of magma from mantle depths to the near surface environment without significant interaction with the overlying crustal fabric. As a consequence, kimberlites of similar age in the Slave align themselves parallel with the convergence direction of the Farallon Plate, while plutons in the porphyry systems of Arizona exhibit fracture density patterns that parallel the convergence direction modified by local basement fault and fracture geometry. A dramatic decrease in convergence velocity toward the end of the Laramide increased the heating rate of the slab and subsequent fluid release, forming most mineralized porphyry systems and most diamond-enriched kimberlites at about 55 Ma.