Geology of the Grand Gulch Mine: Why Was There So Much Copper Here?

The geologic story of the Grand Gulch mine and the breccia pipe mines around the Grand Canyon are the result of a series of geologic events that lined up just right.

Breccia is a term geologists use to describe fallen rocks in a cavity in the ground. Breccia formations themselves aren’t uncommon. What makes the breccia here unusual is how it formed in vertical pipes. The pipes are roughly 300 to 2,500 feet in diameter and several thousand feet from top to bottom.

Breccia pipes are found in only a few places in the world. From the Nevada state line east to the Navajo and Hopi Reservations a few thousand of these pipes are known. Many more will remain hidden until their tops are exposed by erosion. It is likely that hundreds more were destroyed by the Colorado River as it cut the Grand Canyon. Once exposed, the breccia pipes fell piece by piece into the Colorado River. Untold amounts of copper, silver, and uranium were washed out to sea. Remnants of some of these pipes are still visible to Colorado River rafters as they look high up on cliff walls.

These breccia pipes are old. They started to form in the Redwall limestone layer far below the Grand Gulch Mine around the same time as the Redwall was being deposited during the Mississippian period 358-323 million years ago. Acidic water dissolved massive voids in the limestone that had no openings to the outside world. They were filled with water and were structurally unstable. Basically, they were the beginnings of sinkholes. Each time rocks in the ceiling of a void fell, the floor was raised. This could also be considered 'stoping.' The void climbed with every rockfall. These voids in the Redwall limestone eventually rose several thousand feet through layer after layer of the Colorado Plateau while those layers were being deposited. After perhaps a hundred million years of rock collapses, the vertical pipes were choked with fallen boulders, gravel, and clay. Each pipe finally pinched out between the Kaibab limestone and the Chinle formation.

Copper and Silver Ore Deposits

After the breccia pipes formed, the second geologic step would deposit millions of pounds of copper and thousands of ounces of silver in those pipes. The literature describes the depositional environment of the layers from below to above the copper deposits as Mississippi Valley-type where the metals were in a briny fluid below ground. Heat from Earth’s mantle warmed the brine. Since this hot fluid is more buoyant than the rocks around it, it rose upward through cracks and faults. It took dissolved silica, salts, carbon dioxide, as well as metals like copper, silver, and lead with it. Gold is also deposited by hydrothermal fluids. However, for some reason there are no gold deposits at the Grand Gulch Mine so most likely it wasn't in the fluid.

As the fluid neared the surface, it migrated up into the breccia pipes. It was easier for the water to flow through the loose fallen rock than through the surrounding solid rock layers. As the fluid rose pressure and temperature dropped. By the time the fluid reached the depositional zone in the breccia pipe the water was in the range of 176 to 343 degrees Fahrenheit (80-173 degrees Celsius). The dissolved metals were also chemically attracted to the rocks at certain depths and came out of solution. Copper, silver, and lead bonded to the jumble of rocks in the breccia pipe forming ore. Over tens of millions of years, the fluid brought up more and more of these metals, resulting in ore that was as much as 75% copper.

Is There Uranium Here?

The other major economically valuable mineral in some breccia pipes in the region is uranium ore. Uranium is only present at the Grand Gulch mine in trace amounts that are not dangerous to visitors on the surface and far below the concentration needed to be profitably mined. Because the surface of the Grand Gulch Mine is in the Esplanade sandstone, it is also possible that there were uranium deposits long ago in the Grand Gulch breccia pipe. However, uranium is primarily found in the Coconino and Hermit Shale formations. Those two formations were eroded away above the Grand Gulch Mine and all across the middle bench of the Grand Wash Cliffs millions of years ago. To give you some idea of concentration that may have been here, east of the monument where those formations haven't been eroded yet, uranium deposits are between 0.5% and 2%. That is quite rich.

One danger at Grand Gulch Mine is radon gas from the decay of uranium in the rocks. Radon was measured in the mine shafts at levels that would be unhealthy without forced air ventilation. They did not have this during the mining period. It is not known if radon caused cancer in miners, but they would have first developed other ailments like silicosis of the lungs from inhaling silica dust from rock drills. That probably took the greatest toll on their health.

So how did uranium get here? The most accepted reason, albeit still very controversial amongst geologists, is that the breccia pipes were acting as underground drains long after the copper and silver had been deposited. Over millions of years oxygenated rainwater soaked into the ground and became alkaline. This chemistry allowed it to pick up trace amounts of uranium in the Chinle formation from roll-front sandstone deposits. As the uranium-enriched water percolated into the ground, the water entered a more acidic environment and lost its oxygen. This precipitated uranium as uraninite in the Coconino sandstone and Hermit shale. However, others think that the uranium came up after the copper and silver was deposited, but a deep source. More work needs to be done to determine if the uranium came from meteoric water (rain) or from lower in the crust.

Exposing the Breccia Pipes

Until about 6 million years ago, these breccia pipes were hidden. Using seismic tomography (the study of earthquake waves) geologists have shown that part of the Colorado Plateau is delaminating and falling away into the mantle. Just imagine the sole of a shoe that has come unglued and the flap is hanging down. Geologically, this is known as a ‘lithospheric drip.’ Hotter asthenospheric rock from deeper in the mantle is rising and taking the place of the piece that is falling. This has put very hot rock right under the Colorado Plateau. Hot rock is buoyant and continues to lift the plateau today.

While not part of the copper story, the hot rock near the surface has resulted in volcanism all along the southern boundary of the Colorado Plateau in Utah, New Mexico, Colorado, and Arizona. Some of the most recent eruptions in the region (dates are approximate) include the Panguitch Lake flow 1,000 to 2,000 years ago, Little Springs by Mt. Trumbull 960 years ago, Sunset Crater 920 years ago, and Ice Spring by Fillmore, UT 660 years ago. As you travel the region around the edge of the plateau you will see black basalt rock from lava flows everywhere. Many form the caprock of the southwest's peautiful mesas.

To expose these breccia pipes erosion is needed. When a landscape rises, as it is doing with the rise of the Colorado Plateau, it becomes more susceptible to erosion. The Colorado River came together while the plateau rose. It cut through the rock layers of the Colorado Plateau and created the Grand Canyon. The higher the landscape is above sea level, the greater the rate of erosion as water rushes downhill. This is known as stream gradient. Water didn't do all the work though. Wind storms remove layers of loose sediment as well. These forces eventually revealed the breccia pipes.

 

Before the invention of the airplane, finding a copper deposit was often a matter of luck. As people walked cross-country, they would stumble on the top of a breccia pipe. If it was a pipe rich in copper, blue and green rocks could be found all over the ground. Copper-rich breccia pipes hidden in canyons were found by clever prospectors. They saved their energy and let flash floods and gravity do the hard work. They would walk up dry canyons, knowing that floods would wash colorful ore downstream. They just needed to look below their feet for green and blue stones and then ‘follow the color’ up to the ore body. Rocks in breccia pipes have also been chemically altered and softened by hot chemical-rich fluids. Because of that, they often erode more easily than the rock layers around the pipe. This forms a surface depression. Today, these depressions are spotted by low flying airplanes or ground scanning lasers.

Geologists don’t know why some pipes are so rich in ore. Most are not. Less than 8% of the breccia pipes in the region contain enough copper, silver, and uranium to be mined. The rest contain only trace amounts that weren’t concentrated enough to make mining economical. This area is part of the North American Cordillera and made up of a wide variety of rocks that were added to the continent over hundreds of millions of years. Perhaps in the ancient rocks deep below the breccia pipe is a copper-rich ore body brought here by some long ago continental collision.

Ongoing research by geologists will help us understand the area’s incredible geology more and more in the years to come.