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3D Model of the Amphitheater Mountains

Three-dimensional model of an ultramafic feeder system to the Nikolai Greenstone Mafic Large Igneous Province answers some important academic questions while revealing mining potential

By Leonard Chan

A helicopter hovers in the cool Alaskan air, a mere speck against the rocky expanse of the Amphitheater Mountains. Attached by a cable to the underside of the helicopter is a magnetometer which measures the magnetic field of the earth.

A team of geoscientists stand on a rocky outcropping that overlooks Tangle Lake, a body of brilliant blue water nestled in the valley below. Using sensitive equipment, they take accurate measurements of the earth's gravitational field.

The sharp crack of a striking hammer echoes through the mountain pass. A small piece of rock, a tiny piece of a larger story, is set aside for further analysis. It will be taken to the lab and measured for density and magnetic susceptibility.

For decades, the Amphitheater Mountains have been the subject of geological studies such as these because they contain a near-continuous section of exposed rock related to the Nikolai Greenstone Formation, a Large Igneous Province (LIP) that makes up a major component of the Wrangellia terrane. The term "Large Igneous Province" refers to continental or oceanic flood basalt eruptions that cover huge areal extents (>100,000 km2).

"These are really big and rare events in the earth's history," explains Jonathan Glen, a geoscientist with the United States Geological Survey (USGS) who has been studying the Nikolai and other flood basalts for some time. "You have lava, kilometers thick, localized spatially, the bulk of which is spewed out in a matter of a million years."

The Nikolai volcanic province is one of the largest LIPs in the world, estimated to be a million cubic kilometres in volume. Its remains are preserved along more than 2,500 km of the western North American margin from south central Alaska to Vancouver, British Columbia.

The Amphitheater Mountains represent a major vent for the Nikolai LIP and provide a unique opportunity to further our understanding of these geological formations. In this case, not only is the basalt exposed, but the feeders that supplied it with magma, the two most prominent ones being the Tangle Lake and Fish Lake complexes, are also easily accessible, which allows for a rare glimpse into the plumbing system of a major oceanic LIP.
"It’s extremely rare to find an exposure of a feeder like this to a flood basalt and walk through it from the sills and dikes right up into the lavas," says Glen.

But despite the exceptional exposure of the Amphitheater structure the vast majority of the formation remains underground. What they needed was a model that could accurately describe what was happening below the surface.

To create this model, Glen and his colleagues spent a great deal of time in the mountains gathering new data, including aeromagnetic surveys, gravity measurements, and rock sampling. Additionally, they compiled reams of historical data going back several decades.

"Gravity and magnetic methods are really effective, especially in the early stages of exploration, as they are relatively inexpensive and easy to undertake, " says Glen. "But these two methods were particularly well suited for studying the Nikolai-related rocks that are characterized by prominent gravity and magnetic anomalies due to their high densities and magnetic susceptibilities that contrast sharply with those of the surrounding rocks."

These anomalies were what Glen and his colleagues Jeanine Schmidt and Gerry Connard needed in order to do their modeling, as it made it possible to distinguish between the rocks that were originally there and those that were emplaced during the formation of the Nikolai.

Feeding this data into forward modeling software, Geosoft's GM-SYS, they developed sophisticated two dimensional  profile models of the Nikolai synform. However, with only two dimensions, the usefulness of these models was limited.

"As long as the structure is essentially the same on either side of the profile, then you can treat it as a 2D structure and model it in two dimensions," describes Glen.  "If, however, just off to one side of the profile you have a whole other rock body, then you have a three dimensional structure, and modeling this in two dimensions is problematic."
Hoping to develop a more complete picture, Glen and Connard, one of the original developers of the GM-SYS software, and Schmidt worked together to create a three dimensional model of the Nikolai LIP in an effort to shed some light on its secrets.

Using the 3D modeling capabilities in GM-SYS, Glen and colleagues wrestled the 2D models together into a model with three whole dimensions.

"The 2D models that we had were our starting point," says Glen. "We had a whole set of intersecting 2D models. We took the layers from that and exported them and then gridded them up individually which then formed the basis for our initial 3D model."

"While this sounds relatively simple, it is challenging to extrapolate 2D models into a complex and useful 3D model," Connard says.

Connard continues to contribute to Geosoft’s development of the GM-SYS software, and he feels the project provided an opportunity to assess the limitations of the current version of the 3D modelling software which will lead to future improvements. “We are using the lessons learned in this project to make it easier for users to work with this type of problem,” says Connard.

In the meantime, having stitched a working 3D model together with the current software, Glen, Connard, and Schmidt started using it to address some academic questions.

One  debate has been whether or not the Nikolai formation was a syncline (a depression resulting from the compression of a horizontal strata of rock) or a sag basin (while resembling a syncline, its shape would be the result of direct magmatic emplacement, possibly accentuated by a later compression event).

Answering this question would help further understanding of the evolution of the Nikolai LIP and other structures like it.

Looking at the results, Glen offers his interpretation, "Original thought was that this was something that had formed simply as a result of a compressional event. But given the results, we believe this was a sag basin that had formed, in part, while the lavas were being erupted. If it was a simple syncline, it would not have been a major center for the eruption of the Nikolai and its subsurface extent would be much smaller, lacking the big root. The geometry of the sills that fed the magma that later erupted to form the lavas, how they extend down and thicken into the root beneath the structure, suggests that it developed during emplacement of that system."

In other words, the Nikolai was born this way.

According to the model, the Amphitheater structure is a relatively simple synform that gradually thickens and then plunges downwards to depths five kilometres below the surface. This suggests that the magmatic feeder system to the Nikolai LIP is much larger than previously thought (1,000 - 2,000 km3).

While interesting from a purely scientific point of view, this has major economic implications as well.
Mafic LIPs such as the Nikolai are major sources of platinum group elements (PGE), nickel, and copper.  The volume of the flood basalt and abundance of layered intrusions suggests that the Nikolai LIP has the potential to host world-class deposits of these valuable minerals.

The geometry of the magmatic plumbing system is critical to controlling the distribution of these minerals, so understanding the subsurface structure of the Nikolai will help to identify areas that warrant further exploration.
"The great thing about the model is that it provides constraints on the actual subsurface geometry," says Glen. "If you make assumptions about the concentrations of minerals within these units, then you can put clear bounds on the amount of possible material in the subsurface that is mineable.  And particularly for Alaska, there's extremely good access from the mountains to major highways, which is a big factor in terms of exploitability."

When asked about the accuracy of the model, Glen responds, "That's a good question. It's hard to say exactly. The only way to know for sure is to start drilling. But if the geologic assumptions are correct then the shallow extent – the upper several kilometers - of the model should be accurate to within perhaps 20%."

So while the 3D model that Glen and colleagues created has answered some questions about the Nikolai LIP,  questions still remain: How does this model relate to other flood basalts? How does the subsurface geometry of these LIPs relate to potential vent locations?

The debates continue. But for now, Glen has moved on from the Amphitheater Mountains and the Nikolai. For his next project, he’s investigating whether this model for feeders is something that is more broadly applicable to feeders of other flood basalts he has been studying.

As for the Nikolai, "It's up to others now to go and explore," he says."They need to get into the mountains and start punching holes in the ground so we can really test the model. There's still a lot to be done, but what this model allows us to do, we could not do before."