Despite entire lifetimes of research, there are still countless mysteries in the realms of economic geology. Given that mineral resources are highly important to the continued survival of the world economy, it is beyond necessary to understand the processes behind the various ore deposits that dot the globe. One such intrinsically valuable mystery is that of the Bushveld complex and its surrounding deposits in South Africa. For years, geoscientists have feuded over the natures of these bountiful resource sources. “You ask [the deposit] if it is hydrothermal or magmatic,” joked this past week’s distinguished Van Tuyl lecturer, Dr. Steve Kesler from the University of Michigan, “it doesn’t answer.” Kesler has spent many years working passionately on the puzzle of the nature of the deposits surrounding the Bushveld Complex.
The Bushveld Complex is extremely important, not only for the materials it offers, but also for the surrounding mining districts. “The stuff we’ve been interested in,” said Kesler, “is what’s going on outside the Bushveld.” Many of these mines, such as those that dot the gold-rich Witwatersrand, are crucial for the global supply of gold and other elements such as zinc. The main question posed by geologists is if they are all related, or if the area is just by chance filled with some of the world’s richest deposits. The problem of determining the type of ore body is the complex nature of alteration and the vast similarities between certain non-related ore types. The early work around the subject readily put the final nail in the coffin for the idea that the alteration was magmatic. Now it was necessary for Kesler and his team to define what type of hydrothermal deposit it was. “The first thing that [came] to mind is the Mississippi Valley Type (MVT) deposits,” said Kesler in reference to the thermal anomaly that was present.
MVT deposits are a lead-zinc rich deposit that are hosted within carbonate rocks, such as a marine carbonaceous shale. For the most part, these deposits will have a similar chemical signature to the seawater that they were deposited in. The chemical readings for the deposits were correct for what was imagined, but, of course, there was a problem. “There wasn’t supposed to be oxygen in the atmosphere [at the time of deposition] and there wasn’t supposed to be any sulfate in the world’s ocean,” remarked Kesler. For this type of deposit, is is necessary to have oxygen to oxidize the sulfur material and of course there needs to be a source of sulfur. Luckily, this can be partially resolved depending on the model that is used. Once the MVT idea caught on though, they began popping up all over the place, according to the results from Kesler’s team. They pinned down that the sulfate came from the atmosphere and that the atmosphere was capable of oxidizing, which solved the problem of the environment and cemented the idea that these were MVT type deposits.
The next problem came in the form of the source of the water needed for the hydrothermal alteration. “What was driving the water, where was it coming from? These weren’t the worlds largest basins,” questioned Kesler, “there weren’t many orogenic (mountainous) areas, either the bushveld had to be pushing them out or had to be coming from a nearby orogeny.” While the nearby ancient Kheis orogeny looked promising as a source of water influx, due to a discrepancy in the dating, it turned out to be wrong, which turned the spotlight onto the Bushveld itself as the main source of water. With the source determined, other interesting features began appearing in the data. For one, several nearby deposits began to make sense in the whole scheme of this major inter-deposit interactions. “This is a house of cards, little by little things may fit together in reference to other similar deposits,” said Kesler in reaction to the idea that this may have occurred at other locations around the globe. Nonetheless, the data fits the model and hopefully even more hidden gems in the Bushveld area can be revealed in the near future.