Breccia Pipe Mining on the Arizona Strip and in the Grand Canyon

The Copper Mountain mine shaft in Parashant Canyon is a good example of a very productive copper mine.

NPS - J. Axel

 

Generalized breccia pipe illustration. The red area indicates uranium ore. Copper and silver ore would have been found just above the uranium.

An unusual type of geologic formation exists in northern Arizona. It is thought that there may be up to a few thousand of these from around the Nevada state line east through the Grand Canyon region and all the way to the Navajo and Hopi reservations. Called 'breccia pipes,' a few of these formations have been the source of great wealth, untold hardships, and heartbreak for many. In a region overflowing with incredible scenery and geologic wonder, these breccia pipes add yet another incredible facet to the landscape and its impact on human history in the Grand Canyon region.

The term 'breccia' is a common feature familiar to geologists. Breccia is essentially broken rocks underground that have fallen to fill a void (basically an empty pocket with no connection to the surface). What is so unusual in and around the Grand Canyon is that these breccia features come in the form of vertical pipes that are filled with broken rock. How they formed and a few became mineralized with rich deposits of copper, silver, uranium, and a few other valuable metals is quite remarkable and an area of continued geologic research.

Breccia pipes in the Grand Canyon region first attracted miners in the 1870s. Within Grand Canyon-Parashant National Monument are many such mines. Two of the most popular for the public to visit include the Grand Gulch Mine in the Grand Wash Cliffs, and the Copper Mountain Mine, in Parashant Canyon. Near the Grand Gulch Mine are the Savannic and Cunningham mines that require a UTV to visit due to the incredibly rough roads. At its height of productivity in the early part of the 20th century, the Grand Gulch Mine produced up to 75% pure copper ore, which astonished geologists and miners alike. At nearby Grand Canyon National Park, several breccia pipe mines can be seen such as the Orphan Mine near Hopi Point or the Grand View Mine on the Horseshoe mesa.

Watch Parashant's new 12 minute film about the history of the Grand Gulch Mine

Some may ask if breccia pipes are similar to open pit copper mines found in other areas of the southwest. A porphyry deposit is similar to a mineralized breccia pipe in that hot fluids from deep in the crust brought minerals dissolved in the water and were deposited as they cooled. In breccia pipes these were concentrated in narrow pipes up to 75% copper. In a porphyry deposit the copper is much more diffuse, perhaps 0.25% concentration, which requires open pit mines. Examples include the Morenci Mine in eastern Arizona and the Bingham Mine near Salt Lake City.

 

Diagram showing the cross section of the north side of the Grand Canyon. Well known breccia pipes are shown in yellow.

US Geological Survey

 

Looking straight down on the Grand Pipe. Note the faint color change to yellow in the center and the little erosional canyons that flow into it. It is soft so it erodes quickly. This pipe is in Grand Canyon National Park. Copper ore wasn't present, so like most breccia pipes, it wasn't mined.

Google Maps (click the image to go to this location in Google Maps)

So what exactly is a 'breccia pipe' and how did it form? First we need to set the stage. These vertical pipes formed in the layer cake of rocks in the Colorado Plateau that were deposited over hundreds of millions of years.

The region that would come to be known as the Colorado Plateau kept slowly sinking due to interactions between the North American continent and subsiding ocean floor that was close to sea level. Sometimes the region was below sea level. Sometimes it was just above it. Over time though the trend was that the crust was sinking, allowing deposition of layer after layer. These warm shallow seas were perfect for the formation of limestone layers, such as the Kaibab, as well as the most important for this story, the Redwall limestone. At other times, plate tectonics allowed the land to rise up a little bit. The land rose above sea level and mudstones like the Kayenta or Hermit were deposited in ancient river deltas like what the Mississippi River is creating today. At other times sand dunes covered the land right up to the edge of the ocean similar to the Skeleton Coast of Africa today. These sand dunes became formations like the Coconino. Over several hundred million years, because this was a lowland compared to the rest of the continent to the east, it continued to collect sediments. These sediments were delivered by rivers from the ancient Appalachian mountains far to the east. It was within this layer cake of ancient deposits that thousands of breccia pipes could develop. But how and why?

 

Grand Canyon stratigraphic column.

University of Arizona

It is time for the next phase to begin, which is the creation of voids in the Redwall limestone, at about the mid-level of the Grand Canyon's layers. This bed of limestone is about 350 to 330 million years old and up to 800 feet thick. Around 260-200 million years ago, giant caverns, perhaps several thousand in total, began to appear in the Redwall. These caverns were formed by acidic water groundwater dissolution. Unlike caves we know today like Carlsbad Caverns, these were not open to the atmosphere. These voids would have been full of deadly gases like carbon dioxide and hydrogen sulfide. They did not have decorations like stalactites. They were just giant dark water-filled spaces.

Caves are not stable on a geologic timescale. As anyone who has visited a cave knows, giant boulders litter the cave floor. They clearly fell off the ceiling. Over geologic time, the roof of any cave will at some point collapse. But this doesn't fill in the chamber, it just moves the floor of the cave upward since what was on the ceiling is now on the floor. This then creates a new, higher ceiling. Over tens of millions of years, collapse after collapse after collapse, the cave climbs upward, creating a vertical, rubble-filled pipe. Here in the Grand Canyon region, these collapses eventually ascended several thousand feet until each one finally pinched out in the upper Colorado Plateau layers like the Chinle formation. What remained was a vertical tube hundreds of yards across filled with collapsed rock and gravel. A sinkhole is another term that basically describes a breccia pipe.

We now have our breccia pipes. The third phase of this process was mineralization. Something had to deposit the valuable copper, silver, and uranium ore in the jumble of rocks in the breccia pipe. The minerals got there because of a special liquid called hydrothermal fluid. Hydrothermal fluid is basically extremely hot water that has been heated by Earth's mantle. This water can exist at extreme temperatures since since it is trapped by rock, it can't boil or escape to the surface as steam. It is considered supercritical and can be hotter than 1,000 degrees Fahrenheit deep in the crust. Most important, it contains lots of dissolved minerals, gases, and salts. This includes everything from quartz to copper to carbon dioxide.

How can be water deep underground? It gets there in part by the subduction of the ocean plates under continents. Water gets dragged down with the subducting plate. Much of it is left over from when Earth first formed. It is said that there is several times more water in the mantle than in all the oceans of the world. However, it isn't down there in great pools of fluid. It is spread out amongst the minerals. It needs something to set it free from its rocky trap. What frees it? The key here is that Earth's mantle convects. This means that hot solid mantle rock (with a texture like putty) slowly churns. It flow from near Earth's core toward the crust at the speed your fingernail grows. This is called convection. As the heat is transferred into the crust it mobilizes water that starts to flow toward the surface. As the water cools it is ready to leave metals behind when conditions are right.

Often, hydrothermal fluid intrudes into the cracks of rocks, leaving behind thick veins of quartz. Miners dig the quartz out because valuable metals like gold, silver, and copper are often found mixed in with the quartz. In the copper porphyry deposits of open pit mines in the region, vast networks of microscopic cracks allow minerals to be deposited over a huge area. In the breccia pipes of the Arizona Strip, the hydrothermal fluid and its dissolved minerals were instead concentrated in the skinny breccia pipes, resulting in high metal concentrations.

One mystery continues to puzzle geologists. Only about 4-8% of the known breccia pipes contain enough minerals to be mined profitably. The rest don't have hardly any metals at all in them. This is an area of active research. Are there perhaps high concentrations of copper much deeper in the crust, perhaps in the basement rocks that are Precambrian in age?

 

This is a 'black smoker' vent on the ocean floor. The black cloudy water is made of the minerals that had been dissolved in the hydrothermal fluid.

USGS

So how exactly are the minerals deposited? As the hot fluid rose closer to the surface and up into these breccia pipes, it was moving from high pressure to low pressure. The amount of pressure is one factor that determines when something that is dissolved in liquid, like copper, can come out of solution and be deposited. Temperature is another factor where as temperatures drop, it plays a role when minerals precipitate out, which is what happens when hot fluid rises into cooler rock layers near the surface. Lastly, the hydrothermal fluid was passing through different rock layers of the Colorado Plateau. From the the Supai Group up through the Hermit Shale and Coconino Sandstone and up into the Kaibab Limestone, the temperature and pressure changes were just right for the dissolved minerals to come out of solution. Different mineralized pipes have the metals precipitated at different levels. It is thought as well that the chemistry of those rock layers also helped with precipitation of the minerals.

In the photo at left of the hydrothermal vent, the ocean floor is about the only place to see hydrothermal fluid in action on Earth. The water gushing out of these vents can be under thousands of pounds of pressure per square inch and incredibly hot. While not the same as what happened in the breccia pipes, as conditions change for these hot fluids carrying dissolved minerals, you can see how changing temperatures and pressure can allow metals can be deposited.

Curiously, while both copper and uranium can be present in breccia pipes in northern Arizona, there was a notable shift in concentrations across the landscape. From the Grand Wash Cliffs near the Nevada border east to Mt. Trumbull, the breccia pipes that do have mineral deposits of copper seem to only have trace amounts of uranium. East of Mt. Trumbull, uranium became more prominent and copper concentrations drop. This is another mystery being researched by geologists. Why is copper more common in some places and uranium in others?

This brings us to the human aspect of this story. How did miners discover these breccia pipes? Erosion is needed to expose the deeply buried breccia pipes. Forces in the mantle and the crust caused the Colorado Plateau to begin to ascend. It started to erode as streams and rivers cut into the rocks and stripped away layer after layer. Erosion eventually excavated the landscape down to the top of the breccia pipes between the Kaibab limestone and Chinle formation. This exposed the tops of some of the pipes. Eventually the colorful blue and green copper ore was exposed. It lay on the surface or was washed into streambeds for miners to find.

 

Copper ore and breccia material at Copper Mountain Mine. The green is malachite. There is a small blue piece in the far left just below center that is another copper mineral called azurite. These can be chemically processed into copper. These were the colorful rocks that miners looked for in canyon washes to tell them a copper deposit was nearby.

NPS - J. Axel

 

Once erosion had excavated away the rock layers above a pipe, the breccia pipe was finally exposed. Native Americans and new settlers to the area looked for these colorful stones. If green or blue copper ore was seen, the person would 'follow the color' upstream to the source. This location information could then be sold to a mining company. The mining company would explore the site and if the ore tested rich enough, a claim was filed and mining began.

Even if a mine produces high quality ore, there are costs to consider if the venture is to be successful. One cost is whether to process the ore on site in a mill and smelter. This would significantly decrease costs as the metals would be more more concentrated. The other method was to transport the heavy ore to a mill far away for processing, which was expensive. The Arizona Strip was so remote that no large scale transportation system, like a train line or ore processing facility existed nearby. For years ore had to be shipped by wagons pulled by mule teams. Later it was transported by trucks. Miners must have certainly dreamed that one day a railroad company would put in a spur line to their mines on the Arizona Strip.

 

Uraninite, also known as pitchblende, is a uranium ore found in the breccia pipes.

Wikipedia - Geomartin

During the nuclear age in the 1950s, and again in the 1980s, a new type of mining occurred. The focus was now on uranium in the breccia pipes rather than copper. Uranium can look black, goldenrod yellow/orange, or made of greenish-brown crystals. This mineral of course has caused extensive controversy due to reckless mining methods in the past that have permanently contaminated groundwater, especially on the Navajo and Hopi reservations which have seen a huge increase in unusual cancers in people who live near contaminated groundwater. Even at Grand Gulch mine the rocks from the breccia pipes are just slightly radioactive, but not enough to be a hazard to the public outside the mines. They are closed to entry for safety due to roof instability but also high concentrations of radon gas. The more radioactive breccia pipes further east, however, can be up to 2% uranium ore.

A final question is if these breccia pipes inside areas like Grand Canyon National Park or GC-Parashant National Monument will ever be mined again? The answer is no. Laws have been passed to prohibit mining in these protected places. From concerns about uranium contaminating ground water that could make its way into the Colorado River to prioritizing tourism over industrial mining in the Grand Canyon region, mining has been reduced to a fraction of what it once was. This story of mineral wealth in breccia pipes around the Grand Canyon is mostly part of the nation's history now. Still, visitors to the Grand Canyon-Parashant region can ponder those days long ago when small but rich deposits of minerals tantilized the explorer dreaming of adventure, the spectre of hard work, and the often elusive allure of cashing in on some of Earth's great wealth!