Botswana Diamonds Subterrane
Andrew Long (left) with MD of Botswana Diamonds James Campbell

Established in early 2018, Subterrane has been pioneering geophysical interpretation to attain new levels of understanding of the Earth’s structure, and transforming conventional understanding of plate tectonics to apply to the exploration of the planet’s natural resources.

The company’s flagship product, Sigmoid Tectonics, has evolved from previous published work employing a variety of geophysical methods from satellite, airborne, marine and ground acquisition of gravity and magnetics.

This article first appeared in Mining Review Africa Issue 2, 2020

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Andrew Long, founder and director of Subterrane says, “The principle key to applying Sigmoid Tectonics lies at the basis of the interpretation of geophysical anomalies.

“Comprehending what geophysical anomalies imply in a profoundly different way can help us unify other theories such as Einstein’s treatment of gravity.”

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Subterrane introduced this concept at a presentation at the Geological Society of London in May 2018 in a discussion: ‘Regional to basin scale influence of strike slip tectonism on the evolution of the western Caribbean Margin implications for petroleum play systems in Patuca and Mosquitia’.

This was part of a petroleum exploration project working on the Caribbean Margin in Central America.

Prospectivity

Subterrane started working with AIM/BSE-listed diamond junior Botswana Diamonds in early 2019 on its Thorny River project in the Limpopo Belt of South Africa.

Over a period of some months all the previous exploration data that had been acquired was reviewed and reprocessed.

This included an array of proprietary ground surveys employing magnetics, gravity and electromagnetic surveys, plus the regional and national surveys available from the Council of Geoscience in South Africa, as well as the important ‘hard’ data (core log).

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Subterrane already had a good idea of the structural evolution of the Kaapvaal craton through its proprietary application of Sigmoid tectonics.

“We can understand the present day structure of the Earth’s crust through different treatment to that conventionally understood by plate tectonics. By interpreting the longer wavelength structure through regional gravity, magnetics, electromagnetics and seismic tomographic models it is possible to understand how the Earth’s crust has been deformed over time, and how it will continue to deform in the future.

The applications of understanding this are far reaching, including the construction and engineering industries, resource extraction, and hazard mitigation in relation to predicting earthquake activity”, says Long.

Subterrane has constructed an evolving global synthesised structural model of the Earth’s structure from the core to the upper crust.

“The idea is to eventually develop software applications for real world practical use in a variety of industry sectors, however we will need to acquire more sophisticated geophysical measurements at the satellite level to fill in areas of sparse measurement coverage.”

In July 2019 Subterrane signed a commercial agreement with Botswana Diamonds to find kimberlite pipes similar to the famous Marsfontein mine.

There are a number of known fissures on the acreage, and attention had been focussed on these with ground geophysical acquisition and drilling.

In October a site visit by Subterrane was conducted to ground truth along fissure-strike targets, carrying out conventional geological field work and take structural measurements from the available exposure.

“The initial prognosis looked good, with dolerite outcropping on strike with their associated indicative geophysical anomalies”.

Following a review of the structural measurements, further understanding of the pipe system was gained. Subterrane conducted final ground work in January 2020 in order to constrain the target locations.

“The pipes we are looking for have little surface expression, and the typical structure of these small kimberlite intrusions have a small areal extent (less than 40 m typical diameter of the indicated sub-crop), and are associated with the older, early Jurassic (180Ma) dolerite magmatism, which was precursory to the later kimberlitic emplacement.

Traditional depth estimation via magnetic anomalies indicates the top of the targets will be intersected in a range between 20 m and up to 40 – 50 m depths in some localities.

“With Subterrane’s understanding of the extensional tectonics around Africa’s cratonic shields’ since the early Jurassic breakup of Gondwana, we can reconstruct the kimberlite trail across the region,” says Long.

The cratonic region represents what Subterrane describes as an extremely low entropy geological setting, that has not undergone significant extension over geological time.

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An example of a high entropy setting would be a coastal setting where sedimentary processes are fast evolving given the influence of the Earth’s tides, aeolian processes and extensional tectonics on the margins of active oceanic spreading ridges.

It appears there were at least three phases of Jurassic kimberlite emplacement, with the curious en echelon configuration of the Thorny River fissures indicating different paths of fissure emplacement, and an evolution of the pipes over time.

The field work revealed that one of these phases indicated a north-south emplacement trend.

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By revisiting the airborne magnetics it is believed there is one very large pipe situated between the two main fissure strikes on the property.

“We will look closely at the outcrop in more detail to further our understanding of the evolution of the kimberlite emplacement and make further detailed measurements of structure so we can target the drill bit more precisely over the geophysical anomalies of interest.”

Subterrane is working with Botswana Diamonds on a direct fee basis and associated royalties based on any diamond production from successful targets.

“This is a common trend in today’s mining industry as junior miners team up with larger players,” says Long.

The company hopes its proprietary processes will find success on the project and will open the way to establishing a renaissance in natural diamond exploration in a timely manner, as the supply of natural diamonds to the market anticpates shortage in the near future.