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Ask a Geologist. A rocky landscape.

Schultze Granite, near Top of the World. Photo by Greg McKelvey.

Our new series in 2019 on the geology of this region helps to answer questions and bring you a lay mans’ understanding of the rock formations you see as you drive through the area. Hosted by Rick Levelli, a geologist and Greg McKelvey a photographer, we hope you find the series intriguing. 

Gila County exhibits some of Arizona’s geological highlights, easily visible and accessible from highways that traverse it. These range from stacks of rocks representing a billion years of earth history, to cryptic gaps in the rock record where just as much time has gone missing, to the frozen “Mother Magma” for massive porphyry copper deposits, to the rock-record of ancient and modern springs, marine fossil localities and unusual cave formations. Over the next year, we’ll take you on a geological tour of the county’s highways, with this first installment being along US 60, starting from Superior, across the Pinal county line, up to the county seat in Globe.

Milepost 228

Just at the northern edge of the town of Superior (milepost 228), road cuts display the 1.1-billion-year-old dark greenish rocks known as diabase, injected into the earth’s crust during an episode of stretching (or extension) of which you can see evidence as far away as the Iron Range country of Minnesota and the Upper Peninsula of Michigan.

From here, you quickly climb through a succession of quartzite, limestone and shale dipping steeply to the east and deposited under shallow seas that covered the western US from 542-250 million years ago, during the era was known as the Paleozoic (“old life”).

This is when the number and diversity of life forms really exploded on earth. Fossilized corals, funny creatures known as crinoids, that were like upside down starfish connected to the ocean floor by a long, segmented stem; and brachiopods, similar in appearance, but unrelated, to clams, can be found in the limestones in road cuts here. By the time you reach the Queen Creek tunnel at mile 228.5, you have crossed a major unconformity, where roughly 230 million years of the rock record (all of the Mesozoic, or “Middle Life” era, which was dinosaur time, and a large part of the Tertiary era, when mammals, including us, appeared) are missing and enter the cliffs and hoodoos of the much younger Apache Leaf Tuff, ever-popular with rock climbers.

The scattered holes, mine dumps and black and red-stained rocks you seer along the highway from Superior to the Queen Creek tunnel are evidence of mineralization in the Magma Mine, famous for high-grade copper ores and developed by Col. William Boyce Thompson, who later donated to the state the land on which the eponymous Arboretum, locate just west of Superior, was founded. Mineralization in the Magma mine is in rocks older than the Apache Leap Tuff.

Head frame of Number 10 Shaft, Resolution Mine, above Superior

Milepost 230

When you top out on the plateau behind Superior, at around milepost 230, you will notice two mine shaft head frames over a white waste dump, visible just below the ridge to the southeast. These are the number 9 and 10 shafts of the Magma Mine. The former accesses the deep eastern part of the Magma vein system and the latter is designed to give access to the huge Resolution copper deposit, discovered by drilling from the bottom of the Magma mine in 1995. This is a deeply buried (it starts at about a mile deep!) porphyry deposit that stretches to the south of the highway, completely invisible under the rolling, oak-studded topography formed by the Apache Leap.

The Apache Leap Tuff was erupted from calderas in the Superstition Mountains, 30 miles to the east, 18.5 million years ago. It is part of a volcanic field that stretches from the Goldfields Mountains, near Usery Pass, all the way to Top-of-the-World, blanketing older rocks.

Milepost 236

At Top-of-the-World (mile 236), you enter the knobby rounded outcrops of the Schultze Granite and stay in them almost to the town of Miami. This is the 64-million-year-old frozen “Mother Magma” for the copper deposits of Pinto Valley, Miami, Globe and probably Resolution and Superior. Shiny fractures in the road cuts are coated with muscovite, and fantastic igneous textures can be seen on outcrops along the old trace of US 60, away from the traffic and noise of the new version. The copper deposits are related to the borders of the granite, where variants known as “porphyries”, which are associated with most of the great copper deposits in Arizona and the world. Discovery and development of these deposits fomented the opening of the Western US and Central Arizona.

Did you know that porphyry copper (and many other metal) deposits formed from water “boiled-off” from granitic magmas as they rose and cooled in the earth?

That’s right, deep in the crust, where they are generated above subduction zones, such as the one that lay beneath Arizona from about 75-55 million years ago, many granitic magmas contain up to 3% water.

As a blob of magma rises through the crust to shallower and shallower levels, the weight of the overlying column of rocks is less and less, which means that the confining pressure on the magma body drops until, sometimes, the contained water separates violently (“boils off”) from the molten rock. The analogy would be taking a can of your favorite carbonated beverage from sea level to the top of Mount Everest and seeing what happens to it. The dissolved carbon dioxide in your beverage would separate from the liquid (be it water, beer or Coke) collect at the top of the can and, eventually, cause it to explode. When this happens with water in a magma, a couple of things take place almost simultaneously.

Schultze Granite, near Top of the World. Photo by Greg McKelvey.

First, valuable elements, such as sulfur, iron, copper, gold and silver are much happier in hot water than in a molten silicate magma, so that’s where they go. Secondly, that explosive release of metal-charged fluid can fracture the wall-rocks surrounding the magma and, as it cools dramatically as a result of this, fracture the resulting granite itself. Further cooling deposits the dissolved elements in the water as quartz veins, which often contain valuable concentrations of metals as sulfide minerals. Think about that the next time you drive by the colorful rock piles and historical mines around the edges of the Schultze Granite!

The Old Dominion mine, just to the east of US 60, nearly to Globe (milepost 258), is another famous former copper-producer, hosted in the same rocks that you saw driving out of Superior, up to the Queen Creek tunnel. It was a vein, very similar to the Magma vein in Superior, almost its mirror image, on the western end of your tour.

Headframe of Old Dominion Mine in Globe. The mine was closed in 1933. Photo by Greg Kelvey.

With that, we complete our 24-mile Superior to Globe drive which, in the fourth dimension, encompassed over a billion years of earth history (although 230 some million years have gone missing!) and traversed an area with a rich copper, silver, gold and zinc deposits that contributed greatly to the opening and development of this part of Arizona.

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