The Laramie Mountains near Wheatland are becoming increasingly interesting due to the potential wealth of buried treasure contained within its unique geology. At certain locations, an “x” could mark the spot.
There’s one spot in Sybille Canyon, along Wyoming Highway 34, where stripes of orangish rock intersect, forming an “X” clearly visible from the road. It’s a natural, unique geologic formation, although it’s not indicative of anything other than hot rock breaking into cool rock.
Wyoming might have a wealth of mineral resources, but it isn’t Treasure Island. Anyone who attempts to dig at this “X” will find nothing but more rock.
“If you dug into it, you would find more of what is exposed along the surface of the cut,” said Patty Webber, a minerals geologist with the Wyoming State Geological Survey. “It's an exposed 2D ‘slice’ through a complex 3D structure of unusual geology in the Laramie Mountains.”
Jay Hancock lives in Douglas and said he’s seen that giant X since he was a kid since work crews uncovered it along the road.
I have been traveling through Sybille Canyon since I was a child, visiting my aunt and uncle who lived in Centennial,” he told Cowboy State Daily. “My connection to this area continued during my time at the University of Wyoming.
“After the rebuilding of the canyon road, I frequently visited my son and daughter-in-law while they were attending UW, often pointing out the landmark to my wife, saying, ‘There's the X.’"
A Complex Complex
Geology is a complex science, and Wyoming is a geologic wonderland where some of the oldest rocks on the planet can be studied. The “X” in Sybille Canyon is an instance where an event that occurred miles under the surface has been lifted and exposed at the surface.
Sybille Canyon is part of the Laramie Anorthosite Complex, a vast exposure of plutons in the central Laramie Mountains. Plutons are formed when hot igneous rock intrudes into solid country rock and slowly cools and crystallizes far below the surface of the Earth.
“It’s some very unusual geology,” Webber said. “This group of anorthosite, monzonite, and syenite plutons intruded during the same geologic event over a relatively short period of time — 12 million years — over 1.4 billion years ago.”
Anorthosite, monzonite, and syenite are different igneous intrusive rocks with different compositions that form when they intrude into different kinds of rock.
If it’s hard to visualize, the best example of intrusive rock in the world is in northeast Wyoming. Devils Tower is an igneous intrusion that was exposed when the softer rock it intruded into eroded away.
Webber said the Laramie Anorthosite Complex (LAC) has been extensively studied because of its position. The igneous rock intruded into the Cheyenne Belt, a major geologic structure along the southern edge of an ancient continental fragment known as the Wyoming Province.
“This suture zone is well documented in the Sierra Madre and Medicine Bow Mountains to the west but is mostly obscured in the Laramie Mountains due to the LAC,” she said.
Dueling Dikes
The “X” along Wyoming Highway 34 isn’t really an “X.” That’s just how it appears from our perspective along the exposure.
Webber said that the singular “X” is actually a cross-section of a “thin sheet” of intrusive magma called a dike.
“Dikes are typically thin, tabular to planar intrusions of magma that form extensive ‘sheets,’” she said. “As dikes intrude, they often move through fractures or other easy-to-exploit pathways. Rocks tend to fracture at conjugate angles — about 60 degrees between the fracture planes - which is pretty similar to the intersection angle of these dikes.”
Webber said there are several dikes throughout the LAC. If geologists could get a top-down view of the exposed landscapes, they would appear much more expansive, sometimes extended laterally for several miles.
The “X” in Sybille Canyon is definitely a dike. The question is, how many dikes are visible in the formation?
“The X shape may have formed when two separate dikes cut across each other at opposing angles,” Webber said. “Another possibility is that while this dike was intruding, it branched out into many different arms that just so happen to look like an X.”
Webber said other dikes in the LAC have similar orientations, where smaller dikes branch off of larger intrusions. There could be dozens of similar formations throughout Sybille Canyon.
‘X’ Isn’t The Spot
Geological explanations are all well and good, but a natural “X” on the landscape might tempt treasure hunters. However, Webber is confident there’s nothing worth pursuing in the hard rock of Sybille Canyon.
“The dike is likely composed of granite or monzonite,” she said. “There are many granite and monzonite dikes in the LAC.”
Granite is an abundant igneous rock primarily used for construction or monuments. It’s not in short supply in Wyoming. Meanwhile, monzonite can contain several different minerals, but none have value on the commercial market.
Ironically, the rock surrounding the “X” is rarer and potentially more lucrative. It’s an exposure of Poe Mountain Anorthosite, one of the many plutons in the LAC.
“Anorthosite is an unusual and uncommon rock,” Webber said. “They can be high in aluminum and are sometimes viable sources to mine aluminum. They also commonly have large amounts of magnetite and ilmenite, iron oxide minerals that can be high in titanium and vanadium.”
Webber said the Union Pacific Railroad had explored a different section of the Laramie Mountains looking for aluminum-bearing anorthosite. Its high-temperature resistance gives it valuable aerospace and defense applications, and some anorthosites contain large amounts of gem-quality labradorite and ilmenite, a titanium-oxide ore.
Geologists treasure anorthosite for its potential mineral wealth and because the factors of its formation are still largely unknown. The most valuable thing most Wyomingites will get from anorthosite is a factoid of trivia Webber shared that might come up during their next trivia night.
“Fun fact — the moon is primarily made of anorthosite,” she said.
REEs In The LAC
The next frontier of Wyoming’s extractive industry is the ongoing pursuit of rare-earth elements (REE). The plutons of the LAC are a target of opportunity for companies looking for more REEs, but they aren’t following the “X marks the spot” methodology.
“Only one pluton within the LAC is being explored for REEs,” she said. “The Red Mountain pluton, which is further east of Sybille Canyon, is one of the youngest parts of the LAC, which is why it’s actively being as a potential REE deposit.”
The Red Mountain pluton is the main target for the Cowboy State Mine, which is being explored by the Australian-founded exploration company American Rare Earths. They believe it could be one of the richest rare-earth deposits in the world.
Webber said the Red Mountain pluton is a “monzo-syenite body” that has been proven to contain allanite, a kind of mineral whose composition can be up to 20% rare-earth elements like cerium, lanthanum, neodymium, and yttrium.
American Rare Earth is specifically looking for neodymium and praseodymium. According to their estimates, up to 2.34 billion tons of these and many other REEs could be waiting to be extracted from the Wheatland-area pluton.
The Red Mountain pluton could prove to be Wyoming’s wealthiest treasure chest. The “X” in Sybille Canyon contains a wealth of potential knowledge for geologists, but that’s the only treasure worth seeking.
“This is a very common phenomenon,” Webber said. “It's just a lucky spot where the roadcut exposes these dikes at that particular angle.”
Andrew Rossi can be reached at arossi@cowboystatedaily.com.
