Geological Background

Arizona, New Mexico, Sonora and parts of Chihuahua share common geology, cultural, economic and technological history through most of the period under consideration, so the geographical scope of this blog will include them in what we’ll term Southwestern North America. This region has been an important copper producer since the late-19th C, when copper demand took off with what one might call “the electrical phase” of the Industrial Revolution. Prior to that, it was a producer of copper and copper minerals used for artistic, decorative and religious purposes by the native inhabitants and for coinage, by the Spanish. My principle references for this series of blogs are Leveille and Stegen (2012) and the three volume series of books on the history of mining in Arizona, published by the Mining Foundatiion of the Southwest, from 1987-1999 (see reference list for more complete information).

The vast majority of the region’s (and the world’s) copper production comes from porphyry deposits. They supply over 60% of the world’s annual newly-mined copper (figure 1). They are large, low-grade deposits associated with “granites”, but granites with a difference: they have large feldspar crystals in a fine-grained groundmass. This is the geological definition of a porphyry. In addition to this texture, porphyry copper deposits have disseminated and veinlet copper sulfides (figure 2), or oxides and secondary sulfides derived from them. Porphyry deposits are common in magmatic arcs and are amenable to bulk mining methods, both of which contribute to their economic importance.

Figure 1. Global Cu production by deposit type, millions of tonnes

Figure 2. Porphyry with stockworks quartz-chalcopyrite veining, Christmas, AZ

There’s an additional key factor in Southwestern North America. Favorable tectonic and climatic conditions from 45 million years ago to the present included cycles of uplift, erosion and burial, that resulted in episodic lowering of the water table.  This led to oxidation and leaching of sulfide minerals. Copper was subsequently redeposited in blanket-like deposits of secondary copper sulfides (e.g. chalcocite) at the water table. These were occasionally uplifted and oxidized again (figure 3). Multiple cycles of this led to extremely rich, relatively shallow ore bodies that were amenable to relatively primitive mining and processing technology.

Figure 3. Supergene profile of porphyry copper deposit

So let’s get into some more detail on porphyry deposits and why Southwestern North America has so many of them. The vast majority of the region’s copper deposits range in age from 80-45 million years old (figure 4), a period of time known locally as the Laramide. During this period, a magmatic arc, represented by the red blobs on the map in figure 5, was emplaced in thick continental crust.

Figure 4. Temporal distribution of ore deposits in Southwestern North America

Figure 5. Porphyry copper deposits relative to geology, SW North America. Brown = Precambrian, dark green = Paleozoic, blue = pre-Laramide Mesozoic, red = Laramide, orange = post-Laramide Tertiary rocks, yellow = Quaternary alluvium. Numbered light green dots are porphyry copper deposits (key can be found in Leveille and Stegen (2012).

If we look at a 3D re-creation of a slice through the region from 80-45 million years ago (figure 6), it shows a subduction zone, where water is driven off of the cold down-going oceanic slab, rises into the overlying mantle wedge and fluxes melting to generate magmas. These water-rich magmas rise through the crust differentiating and segregating metals until they “pop”, kind of like a beer can taken to altitude in an unpressurized plane. This, in simple terms, is what produced the numerous large low-grade porphyry copper deposits in the region.

Figure 6. 3D model of subduction zone/arc

So here we have a four dimensional picture of the “strike-zone” for porphyry copper deposits in Southwestern North America: We’ve got the Laramide arc in continental crust. We’ve got post-mineral uplift and weathering. But we can’t have too much erosion, otherwise the porphyry deposits, which form at relatively shallow depths in the crust, will be washed away. That’s why there are almost no green dots west of about Ajo (point 21 on figure 5)….its too deeply eroded to preserve them.

Now that we’ve gotten roughly 80 million years of earth history out of the way, let’s focus in on the last few thousand years of human history, beginning with Native American uses of copper.