Dr Armelle Kloppenburg, an independent structural geology consultant, whose clients come from both the petroleum and the mining industry, visited Mines to teach a short course, and she kindly agreed to give a talk to the student chapter of the Society of Exploration Geologists (SEG). She presented on a project she and a team of geologists worked on at the Bingham Canyon Mine in Utah, one of the largest open-pit mines in the world. Bingham Canyon is a porphyry skarn deposit, producing mainly copper, but also gold and molybdenum. The mine has been operating since 1906 (the deposit has been producing copper since the 1860s), so Kloppenburg had no lack of data for her analysis.
She began by explaining, briefly, how geologic structures are important in economic geology. Where a conventional geologist sees unit boundaries and continuities, she said, a structural geologist sees breaks and movement along these breaks. Such discontinuities – faults, fractures, shear zones, and so on – often provide pathways for fluid flow, controlling the interconnectedness of a petroleum reservoir or the movement of mineralizing hydrothermal fluids through the rock. Therefore, by creating a three-dimensional model of the rock and running the clock backwards – undoing the breaks, as it were – a structural geologist can give a picture of the deposit or reservoir at some critical point in the past, which can be useful, for instance, in determining the migration history and maturity of hydrocarbons in a specific spot, or in predicting where a fault might crop up. This four-dimensional model, known as a kinematic reconstruction, is based on movements of the rock. Kloppenburg’s task was to make a kinematic model of the Bingham Canyon deposit.
This task was easier said than done. Bingham Canyon sits in a structurally complex area in the midst of the Wasatch Mountains, a fold-and-thrust belt, at the convergence of several faults running different directions, and at the tip of an anticline running perpendicular to the Wasatch Fault. When a fault “steps” across an area, the space between the two parts of the fault is called a transfer zone, because that is where energy and movement transfer from one fault to the next. These areas behave uniquely depending on the direction of movement along the faults; Bingham Canyon is in a transfer zone, so its structural properties depend on whether the area was in compression or extension when the faults were active.
Though the kilometer-deep deposit is hosted by Pennsylvanian sedimentary rocks (about 300 million years old), the mineralization itself is much younger. At the end of the Sevier mountain-building episode, a little less than 40 million years ago, an intrusion of magma rose in this spot, forming a porphyry rich in copper and other metals and “scorching” the surrounding rock to form a metal-rich skarn. The nature of the deposit means that the miners must continually go deeper to get at the ore. In order to minimize the risks involved in this process, Kloppenburg set out to do a kinematic reconstruction of the area. This involved developing a clear, consistent conceptual model of the geology, validating the model’s geometry to ensure it was balanced, and finally running the model back in time to get a good picture of the deformational history of the rocks.
The main question she had to answer, in order to accomplish these steps, was to determine where the accommodating space for the intrusion had come from in the first place. While a magma intrusion will melt some of the surrounding rock – “eating” its way up – this process only accounts for 5-10% of the gap the magma fills. Therefore, some displacement of the host rock must have occurred to give the intrusion room to occupy. Kloppenburg determined that the means of creating this space was movement along the local fault systems. A transfer zone will become “squished” if the faults creating it are pushing the two sides of the zone together, but when the constituent faults are moving the opposite direction, the sides of the transfer zone will pull away from each other, creating an opening called a fault dilation, or “pull-apart”, basin. In three dimensions, this process can also create vertical spaces, due to movement along normal or reverse faults, known as roof uplift or floor depression. When a series of dikes are intruded into a floor depression or the like, islands of the host rock are left in-situ within the dikes. By modelling the order of dike emplacement, Kloppenburg was able to remove them one by one, giving a picture of the original geology. The final piece of the puzzle – that is, which set of faults was pulling apart – was answered when the reigning stress regime was determined. In other words, the overall area at the time of the porphyry’s intrusion was in a state of relaxation following the Sevier orogeny, meaning that the whole area was in extension. Thus, Kloppenburg knew that the north-south running faults were responsible for accommodating the intrusion, rather than the east-west running faults.
Now that her model was complete, Kloppenburg conducted her kinematic reconstruction and found that the host rock fit back together nicely, with few gaps, all of which could be accounted for by melting, or “assimilation”, of the host rock by the magma. It took many iterations to perfect the model; Kloppenburg warned that the more assumptions one makes, the less constraint there is on a model, and she and her team were forced to make many assumptions about Bingham Canyon. In the end, however, she confirmed that the deposit consists mainly of fault-assisted dykes, and the model allowed for a better geological map, better determination of fault locations where no data was available, and better prediction of where small-scale structures such as fractures are likely to occur. Armed with this knowledge, the company was able to delve ever deeper and to continue production for years to come.