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. Several thousand of these formations can be found from the Nevada state line east through Parashant and the Grand Canyon, all the way to the Navajo Nation. Called 'breccia pipes,' these formations have been the source of great wealth, untold hardships, heartache, and heartbreak for many. In a region overflowing with incredible scenery and geologic wonder, these mineral-rich 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 then became mineralized with rich deposits of copper, silver, gold, and uranium 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 80% 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. Their mineral deposition story, called a porphyry deposit, is similar to the way that minerals were deposited in breccia pipes but not concentrated in narrow pipes. In the case of open pit mines, and the reason they cover so much more of Earth's surface compared to a narrow breccia pipe, is that the minerals were spread out over a wide area at much lower concentrations, maybe 0.25% to 2%. Modern copper mines in Arizona and Utah use the open pit method to remove copper ore from these porphyry deposits. 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 Great Pipe. Note the faint color change to yellow in the center of the feature and the little erosional canyons that flow into it. It is softer than the surrounding rock so it erodes quickly. This pipe is inside Grand Canyon National Park now so it not open to mining.

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 need soft, easily erodable rock layers to form in. This came to pass as the layer cake of rocks in the Colorado Plateau that formed over hundreds of millions of years.

Here is very brief explanation for the formation of the layers of the Colorado Plateau. Going back 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 subducting Pacific Ocean plates in the west, such as the Farallon Plate. These subducting plates pressed on the continent as they dove under North America. This in turn warped the crust. The warping at times lowered the crust, allowing ocean water to invade the land. These warm shallow seas were perfect for the formation of limestone beds, such as the Kaibab, as well as the most important for this story, the Redwall limestone. At other times, forces from the west allowed the land to rise up a little bit, and the inland seas drained away and the land rose above sea level. However, compared to the rest of the topography of the continent, it was still a lowland and continued to collect sediments. These sediments were delivered by rivers from the ancient Appalachian mountains far to the east when they were much higher than they are today. These rivers deposited great beds of sand or mud on this flat land, forming layers like Kayenta mudstone or Hermit shale. At other times great deserts formed, just like the Sahara, where sand dunes formed next to the ancient ocean, similar to the Skeleton Coast of Africa. Eventually these dunes were covered and compressed into sandstone, such as the Navajo and Coconino formations. It was within this layer cake 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. 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 empty spaces.

Caves are not stable on a geologic timescale. Once caves form they begin to break down. As anyone who has visited a cave knows, giant boulders litter the cave floor. They clearly fell off the ceiling. Don't let this make you concerned about visiting the cave. It is usually thousands of years between boulderfalls. Still, over geologic time, the roof of any cave will at some point collapse for lack of support. 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 can climb upward, creating a vertical 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. What remained was a vertical tube hundreds of yards across filled with collapsed rock and gravel.

We now have our breccia pipes. The third phase of this process was mineralization. Something had to deposit the valuable copper, silver, gold, 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. Most important, it contains lots of dissolved minerals, gases, and salts. This includes everything from quartz to copper to carbon dioxide. Some may wonder how there 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. Some of it is left over from when Earth first formed. It is said that there is several times more water underground 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 plumes of extremely hot solid rock (which is a lot like putty) slowly flow outwards from the core at the speed your fingernail grows. As the heat is transferred to the crust it acts on 'hydrated' minerals that contain water. This heat releases the water from the mineral. The newly free water finds other microscopic water. Together, all this water moves toward the surface because the water is buoyant and it can rise into lower pressures near Earth's surface.

Often, hydrothermal fluid intrudes into the cracks of rocks, leaving behind thick veins of quartz. Miners dig these 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 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 intead concentrated in the skinny breccia pipes, resulting in high mineral concentrations. In these breccia pipes, there is not much deposited quartz, but there is plenty of copper, silver, uranium, and even small amounts of gold.There are thousands of known pipes in northern Arizona, with many more hidden beneath the surface yet to be exposed by erosion. Lastly, only about 4-8% of the known pipes contain enough minerals to be mined profitably.

 

This is a 'black smoker' hydrothermal fluid vent on the ocean floor. The black cloudy water is made of the minerals that had been dissolved in the hydrothermal fluid similar to the copper, silver, gold, and uranium that had been transported by hydrothermal fluid up the breccia pipes.

USGS

So how exactly are the minerals deposited? As the hot hydrothermal fluid rose closer to the surface and up 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 hydrothermal fluid rises into cooler rock layers near the surface. Lastly, the hydrothermal fluid was passing through different rock layers of the Colorado Plateau. Right around the Hermit Shale and Esplanade Sandstone layers, the temperature and pressure changes were just right for the dissolved minerals to come out of solution. 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 as much as 750 degrees Fahrenheit. Some may wonder why the water doesn't boil as it comes out if it is so hot. This is because even though the pressure in the ocean water is less than it was underground, the water pressure at this depth is still too high to allow boiling. Water only boils at sea level at 212 degrees Fahrenheit because of air pressure which is around 14.7psi. At, say, 7,000 feet elevation on the rim of the Grand Canyon, there is less air pressure, so water boils at 200 degrees F. The more pressure there is on the water, the higher the temperature needs to be to make it boil.

Here at these hydrothermal vents, as the water flash cools in the oxygenated ocean water, the minerals come out of solution and oxidize into that black billowing cloud. This is similar to how the minerals precipitated out in the breccia pipes. The hydrothermal fluid cooled enough and dropped in pressure enough to let the dissolved copper, silver, uranium, and gold to come out of solution and stick to the rocks.

Curiously, while both copper and uranium can be present in breccia pipes in northern Arizona, there was a notable shift in concentrations between the two. From the Grand Wash Cliffs near the Nevada border east to Mt. Trumbull, the breccia pipes contain much more copper and not much uranium. East of Mt. Trumbull, uranium became more prominent while copper concentrations dropped. It is not known why this chemistry gradient occurred.

This brings us to the human aspect of this story. How did miners discover these breccia pipes? Erosion is needed to expose the 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 soft 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 Navajo sandstone. This exposed the tops of some of the pipes. Eventually the colorful blue and green copper ore eroded out and 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. This would be expensive to build but would decrease shipping costs. The other method was to transport the heavy ore to a mill far away for processing, which was also expensive. The Grand Canyon region was so remote that no large scale transportation syste, like a train line or ore processing facility existed nearby. For years ore had to be shipped by wagons pulled by mule teams, which was a time intensive process. 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. It would solve so many problems. Eventually, this did happen where a train spur was laid to nearby St. Thomas, Nevada. However, there was still 25 miles between the train and the Grand Gulch mine, which was a lot of hot, harsh desert to cross. Another train line was run to the south rim of the Grand Canyon. The St. Thomas line was dug up decades ago as Lake Mead began to fill. The Grand Canyon line is still in use, primarily transporting tourists. The cost to transport and process the ore was so high back then that only the richest ore could be shipped. Today, with modern transportation networks including semi trucks capable of reaching the most remote rugged places, the cost would be much lower to ship lower quality ore, but it was not to be. Mining in the Grand Canyon region diminished before the new transportation system existed.

 

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. While there is still active uranium mining on the Arizona Strip, many of the mines are currently closed due to the low price of uranium ore on the world market. 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 Nation which has 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. The more radioactive breccia pipes further east, however, can be up to 2% uranium ore where miners must take special health precautions when extracting the ore.

A final question is if these breccia pipes inside areas like Grand Canyon National Park or GC-Parashant National Monument ever be mined again? The answer is mostly no, as laws have been passed to prohibit mining in these protected places. Other types of copper and uranium mining tap into much greater amounts of minerals, even if they are at much lower concentrations. 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 on the Arizona Strip. That doesn't mean that some time in the future there becomes a need to extract minerals still buried in breccia pipes, but the likelihood of that is low. This story of mineral wealth in breccia pipes is mostly historic 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 some of Earth's great wealth.