New Mexico Mineral Symposium — Abstracts

New Mexico's place in geologic history reflects its location on the margins of tectonic plates. Hence, the state has seen abundant geologic activity throughout its 1.8 b.y. history. Mineral deposits often result as a consequence of tectonic activity and record the history of interactions between magmas, sediments, and the movement of fluids in the crust. Much of New Mexico’s long and unique geologic history can be told with respect to copper minerals that are contained in a large number and wide variety of deposit types.

Part of New Mexico’s copper history is a direct result of tectonic activity that resulted in the gradual accumulation of continental material. Repeated accretions of microplates and island arcs eventually resulted in the formation of continental crust during the assembly of the supercontinent of Rodinia. During this stage, massive sulfide deposits were obducted from the sea floor and onto the continent, represented by the deposits at Copper Hill and Pecos. New Mexico remained perched near the margin of the continent through much of the Paleozoic after the breakup of Rodinia.

The next episode of significant copper deposit formation came during the assembly of Pangea. The Ancestral Rocky Mountains rose in the Pennsylvanian, and as they weathered, the copper from the older Proterozoic deposits was transported and deposited as red-bed copper deposits on the arid supercontinent during the Permian–Triassic. One can find these types of deposits in almost any of the “red rocks” of the state, but the largest is the Nacimiento mine near Cuba.

Following the breakup of Pangea, New Mexico rode slightly inboard from the continental subduction zone off the west coast. Andesite stratovolcanos developed in the southwestern portion of the state as part of a continental volcanic arc. The porphyry copper deposits at Chino, Tyrone, Fierro, and Hillsboro formed beneath the volcanos of the Laramide orogeny during Cretaceous to Eocene time.

The complete subduction of the Farallon plate and spreading center led to the great ignimbrite flareup of the Oligiocene. Rhyolitic calderas were popping off throughout New Mexico, and many ore deposits formed on the margins of those calderas. Many became the mining areas that were developed in the late 1800s, predominantly for silver.

It was not until the formation of the Rio Grande rift, approximately 30 m.y. ago, that the modern geologic situation in New Mexico became established. Initial rifting requires the crust to bulge up and eventually fracture, creating the horst and graben structures that dominate the modern landscape. As uplift commenced, the earlier-formed ore deposits were exposed to weathering, and a myriad of secondary copper minerals formed from the pre-existing deposits. Modern tectonic and hydrologic processes continue to create and modify ore deposits along the rift further encoding the history of geologic processes in new ore deposits.