Gemstone Deposits of Eastern Kenya and Tanzania Controlled by Ancient Meteorite Impacts and Continental Collision – an Exploration Model

By John M. Saul

General Introduction

Sometimes features are so obvious as to be completely ignored. The following paper deals with a couple of these. Take the surface of the Moon. We all know it’s riddled with impact craters, obviously resulting from the mother of all bombardments. And yet few of us have thought to ask how our planet could have avoided the same fate as its neighbour in space. In fact, it didn’t.

Similarly, we all know that our planet is built of a vast mass of rock. Rocks are made of minerals. Most gems are minerals. So why are transparent gems so rare and restricted in occurrence when opaque minerals are so utterly common and found literally everywhere? Modern geology doesn’t have an answer, which is why exploring for gem occurrences is such a devilishly difficult affair.

Few of us have given this much thought.

The key feature of an artist is an ability to show us the ordinary in an extraordinary way. The best scientists do the same. They provide a novel perspective, a vantage point that forces us to see with fresh eyes.

When I was studying geology back in high school in the mid-1970s, I did a paper on what was then considered a fringe theory. The idea was that earth was made up of masses of solid rock, floating on a fluid core.

Now imagine this. If you gave the average child a crude map of the globe, many could probably connect the shapes of the continents with one another. And yet it took many “adult” scientists centuries after the earth was accurately mapped to accept the obvious. Witness the following from the journal Science:

“The theory of plate tectonics has, in the space of the last 25 years, made the transition from lunatic fringe to accepted dogma…”
(Kerr, R. A., 1978. Skepticism persists as plate tectonics answers come harder. Science, 199 (4326), p. 283; DOI: 10.1126/science.199.4326.283)

In the paper that follows, John Saul asks an obvious question: Why are gem-quality minerals so rare? His solution is certainly novel and one I think will change the perspective of many.

While a university student, one of Saul’s professors at MIT taught him an important truth: “All scientific papers are nothing more than progress reports.”

So, as you read through this paper, consider it for what it is: a progress report, a ray of light illuminating a hitherto dark gemmological corner.

Richard W. Hughes
Lotus Gemology, Bangkok

Abstract

Transparent coloured gemstones in eastern Kenya and Tanzania were formed during a continent-to-continent collision approximately 600 million years (Ma) ago. The collision reactivated deep fractures formed by meteorite impacts during the Late Heavy Bombardment of Moon and Earth, c.4100-3800 Ma, causing circular fracture-patterns to be generated upward into younger rocks that had never themselves been impacted.

In the richest gem area of East Africa, the collision was impeded by a circular plug of resistant rock trapped between the colliding continents. Frictional heat, circulating through new and newly regenerated fractures, produced short-lived ultra-hot conditions high in the Earth’s crust. The relatively low constraining pressure at these shallow depths allowed certain minerals to crystallize in a transparent manner.

The recognition of circular scars in eastern East Africa should lead to new exploration targets for transparent coloured gemstones.

Introduction

Certain hard minerals, usually encountered as opaque stony materials, may very occasionally occur as well-crystallized transparent gemstones. This is inherently puzzling because gem-quality crystallization requires a reduction in the external constraining pressure at the depth at which the gems crystallize. Yet many of these gem-forming minerals normally crystallize at great depths under metamorphic conditions (Turner, 1948) in which the pressure is uniformly high.

Minerals may, however, be ‘tricked’ into forming gem-quality crystals in times and places when and where continents collide. The enormous frictional heat generated during such collisions (Veevers, 2003) is dispersed upward into overlying rocks. This produces high temperatures at unusually shallow depths, thereby locally and temporarily reversing the Earth’s normal temperature gradient, in which temperature increases with depth. Minerals – all minerals – then crystallize higher in the Earth, hence with lower constraining pressure, and thus as better quality crystals. For some minerals, both rare and common, this leads to the genesis of transparent gems. In parts of East Africa, surface finds of well-crystallized feldspar and mica are useful as pathfinders in prospecting for gem occurrences.

Yet whereas regions of past continent-to-continent collisions are geographically extensive, the gem-producing areas within them are small and scarce. In these areas, the temperature reversal was substantially greater than elsewhere in the collision zone, as in eastern East Africa and elsewhere where circular plugs of resistant (generally unfractured) rock were trapped in the collisions (Saul, 2018). The excess heat produced in such situations is akin to the heat produced when sandpapering over a knot in hardwood.

The Formation of Hard Transparent Coloured Gemstones

Hard minerals have crystallized as transparent gems at depth in exceptional areas – in East Africa, the Hindu Kush, the Pamirs, and the Central Urals, for example – but not more generally. In each of these areas, the gems crystallized within a very short geological interval, but not at other times.

Numerous varieties of transparent coloured gemstones are found within each of these geographical areas. (See Breakout Box 1.)
Over thirty different types of gems, including sub-varieties, are reported from the Umba River area of northeast Tanzania, for example. Pyrite, quartz and mica found in association with gems in such areas may also be exceptionally well crystallized.

As already recognized by Robert Boyle (1627–1691), gem-quality crystallization requires space (Boyle, 1672). This constitutes a problem in understanding the crystallization at depth of tanzanite, tsavorite, ruby, and other transparent gemstones of metamorphic origin because it appears to require places deep in the Earth where the pressure that constrains crystal growth is somehow greatly relaxed, a seeming impossibility.

Certain hard transparent gemstones – beryl, topaz, spodumene and many others – are also famously found at lesser depths in pegmatites, a matter not discussed here except to mention the observation that any factor that decreases local pressure within pegmatites increases the chances of formation of gem pockets (Kievlenko, 2003; Simmons, 2007).

Earth’s Very Early History

The formation of metamorphic gems has a ‘pre-history’ going all the way back to the times when the Earth was battered by the bombardment that also scarred the Moon. The Late Heavy Bombardment (LHB), as it is called, has been dated from approximately 4100 to approximately 3800 million years (Ma) ago and arguing that the Earth has somehow escaped its effects was likened by a senior NASA scientist as early as 1976 as “tantamount to invoking magic or divine intervention” (Lowman, 1976).

During the formation of a large impact-crater, materials inside the crater were totally melted. With time, they solidified (froze) into a coherent three-dimensional ‘craterform’. Rocks just outside the craterform were maximally fractured, but not melted. The resultant form would be a solid, unfractured plug of rock, surrounded by severely fractured rim-rocks that would remain locally weak long after. (This is a simple case; the pattern of scars on the Moon indicates that LHB impacts generally overlap and ‘interfere’ with one another).

Many earth scientists believe that fractures produced during the LHB must have been annealed or fully eroded, and that they can no longer exist (as is indeed the case for smaller LHB craters). Instead, as I have argued (Saul, 1978, 2014, 2018), this would not be the case if the initial fractures had been sufficiently deep to have reached the depth of the brittle-to-ductile transition. Such fractures would be sporadically or perennially reactivated by movements in the ductile zone and regenerated upward into new generations of rocks.

That said, the deepest, bottommost, portions of the craterforms cannot remain intact. Below the brittle-ductile boundary, rocks flow and original features are obliterated by shear: ductile materials have no memory. Consequently, LHB scars are truncated at the brittle-ductile boundary, resulting in geometric forms awkwardly describable as ‘truncated craterforms’.

Most LHB scars are covered over by more recent rocks into which the underlying circular patterns have not been repropagated all the way to the surface. Yet under suitable conditions involving the thickness of the local crust, diameter of the craterform, rock types, local history, erosion, re-covering and regeneration, numerous LHB fracture-scars do reach the surface and become visible on one type of imagery or another (Saul, 1978).

On the Moon and on all other solid bodies in the Solar System, arcs with circular curvature are with little hesitation identified by planetary scientists as evidence for an impact origin. On the Earth, however, the very many circular arcs claimed since the 1950s to have been formed by impacts (Kelly and Dachille, 1953; and Gallant, 1964, for early examples) led the founders of the modern science of meteoritics to insist on additional evidence: structures would not be confirmed as due to extra-terrestrial impact in the absence of certain minerals, microstructures, and rock types that provide direct evidence for the high shock-pressures that characterize such impacts. But terrestrial LHB scars are not shock features. The rocks that now host LHB scars were never themselves shocked. LHB scars are inherited scars, regenerated from below. The fractures are older than the rocks that host them.

East African minerals with Mohs hardness over ~6 that form transparent coloured gemstones*

andalusite, axinite-(Mg), beryl, chrysoberyl, clinohumite, clinozoisite, cordierite, corundum, cummingtonite, danburite, epidote, feldspar group gems, garnet group gems (pyrope, almandine, spessartite, grossular, tsavorite, hessonite, rhodolite, umbalite, malaya, colourless, colour change), hypersthene, kornerupine, kyanite, manganotantalite, peridot, phenakite, scapolite, sillimanite, spinel, topaz, tremolite, tourmaline group gems, vesuvianite, zircon, zoisite.

*Quartz is excluded because some quartz-family gems form at extremely low temperatures. That said, the large glass-clear quartz found at Merelani would qualify for inclusion on this list. Diamond is excluded because it forms at pressures far higher than any of the other gem-forming minerals listed here. The cut-off at Mohs hardness ~6 is arbitrary and is intended to exclude the many soft transparent minerals rarely encountered as faceted stones outside of collections.

LHB scars differ from the 190 ‘confirmed impact structures’ in The Earth Impact Database (PASSC, 2022) and there is no overlap between the two phenomena. Confounding the two is a scientific error that has migrated from the field of meteoritics into the field of geology and is now, incidentally, leaking into the field of gemmology, preventing us from understanding how metamorphic gem deposits were formed. (See Breakout Box 2.)

Earth scientists also commonly assume (without evidence) that scar-patterns dating to the LHB must have all been ‘subducted’ deep into the Earth, a matter taken up below.

The Earth Impact Database (PASSC, 2022) is a list of geological structures that have been confirmed as having been caused by a hypervelocity, i.e., extra-terrestrial, impact. In order for a structure to be listed, at least one ‘diagnostic shock metamorphic feature’ must be present. These features include shatter cones, planar deformation features (PDFs), toasted quartz, ballen quartz, shocked zircons, and the mineral coesite. Absent from the list of diagnostic features is circularity itself. Yet circularity was, and remains, the prime characteristic by which lunar and other extra-terrestrial impact-sites are recognized. As of early February 2022, the Database had 190 entries.

Scars formed by impacts during the Late Heavy Bombardment are qualitatively different from those listed in the Database. This might have been expected because:

• all features listed in the Database are younger than the LHB. The oldest site in the Database was formed approximately one and a half billion years after the end of the LHB.
• no site in the Database has a diameter greater than approximately 160km, far smaller than many lunar scars.
• the LHB occurred on a terrestrial crust that was both hotter and thinner than in later times.
• extra-terrestrial impacts listed in the Database were produced from above, but fractures attributed to the LHB were inherited from below.
• all 190 recognized post-LHB impact sites include rocks that had actually been subjected to ultra-high pressures and elevated temperatures. But the far more numerous circular fracture-patterns attributed to the LHB by Saul (1978, 2014, 2018) and Byler (1983, 1992) were produced ‘cold’ in younger rocks by regeneration from below in circumstances unsuitable for the formation of shock features.
• The criteria for shock metamorphism used by the compilers of the Database only form in solid rocks. They do not form in fluids. But large impacts produce quantities of melt-fluids that are disproportionate to the size of the impact structure (French, 1998; Melosh, 2000; Ryder et al., 2000; Manske et al., 2018). Consequently, diagnostic shock metamorphic features may have never been produced by large impacts on the early Earth. Or, if produced, they may have been immediately swamped or eventually annealed by the heat retained in the molten mass, thermal metamorphism overwhelming shock metamorphism.

The Kenya-Tanzania Border Region: Two Circular Scars and Three Major Gemstone Occurrences

Early in the course of the LHB, one impact produced a scar with a diameter of approximately 1800km. Its perimeter is now marked by Mt Kenya, Mt Kilimanjaro, and by the arc of the Pangani Rift on the western edge of the Pare-Usambara Mountains of northern Tanzania (Saul, 2014; Figure 1).

Later, as the heavy bombardment was tailing off toward 3850 Ma, a smaller impact produced a 255km diameter scar that overlapped part of the 1800km diameter scar. This smaller scar is geometrically defined by three sites familiar to gemmologists: the John Saul Ruby Mine, the Umba River sapphire and garnet occurrences, and the Merelani tanzanite deposit (Figure 2). The association of this scar-circle with gemstone deposits is puzzling. For from comparison with the Moon, we know there must have been many terrestrial LHB scars with diameters in the range of a few hundred kilometres. But we do not know how the rim-zone of this particular 255km circular scar came to host deposits that would produce gems of great commercial value.

Snowball Earth

Erosion accompanied and followed the LHB, which ended approximately 3800 Ma, and rocks of various types were deposited on top of the LHB impact sites during the workings of ‘geology as usual’. These sediments and lavas were subsequently eroded and redeposited in the course of multiple cycles of sedimentation and erosion, with the fracture-patterns eroded, regenerated, covered over, eroded, and again regenerated upward several times over.

Then, during the interval from approximately 720 to 640 Ma – approximately 3000 million years after the bombardment – our planet underwent three so-called ‘Snowball Earth’ episodes during which the Earth was covered with ice up to two kilometres thick. During these episodes, glacial scouring removed an estimated average of three to five vertical kilometres of rock (!) worldwide (Keller et al., 2019).

After the final Snowball episode, new generations of rocks were deposited on top of the impact scars, by then ‘decapitated’ by the glacial scouring. The subsequent erosional fate of these later-generation ‘post-scouring’ sediments and lavas was not, however, a simple repeat performance of the erosion of the older generations of rocks that had been deposited on top of the LHB craters.

The situation following Snowball Earth was fundamentally different: this time, the walls of the LHB impact scars no longer entered the Earth at near-vertical angles as they had done when they were originally formed. Instead, as a consequence of the glacial scouring, the crater walls intersected the surface of the Earth at angles that were far more gentle (Figure 3).

The newly exposed rim-zones were both (i) gently dipping and (ii) weakened from below by the regeneration of the ancient fractures, in particular, by the rise (‘outgassing’) of deep fluids, liberated by the removal of overburden during Snowball Earth. This caused slabs of crustal rock that had been cooling and densifying through the preceding 3000 Ma to tip and slide downward into the Earth, to subduct, along the gentle, downward dips.

Subduction occurs along the rims of LHB scars. In consequence, sizable LHB scars are not, in general, removed by subduction.

Collisions, Frictional Heating and Geological Trickery

Continent-to-continent collisional settings of particular interest to gemmologists involve sheets of continental rocks that were insufficiently dense to subduct (sink). Instead of sinking, they become interleaved as ‘thrust sheets’, somewhat in the style of shuffled playing cards, with the ongoing collision of Peninsula India with Mainland Asia as a familiar instance. Another was the oblique continent-to-continent collision that involved eastern Africa that took place after the glacial scouring. Sheets of rock, some of which were kilometres thick, were thrust over one another at depth. In places the thickness of the Earth’s crust was doubled (Fritz et al., 2013), building a ‘Supermountain’ (Squire et al., 2006), parts of whose eroded roots now provide the gems of East Africa. 

The juxtaposition of deep (hence hotter) rocks above rocks from closer to the surface caused a temperature inversion. In East Africa, this was greatly augmented by the frictional heat (Veevers, 2003) generated by the thrusting of kilometres-thick sheets of rock, one above another. Excess heat was dispersed upward into overlying rocks, temporarily producing elevated temperatures at unusually shallow depths. Minerals then crystallized higher in the Earth, hence with lower constraining pressure. Figure 4, which shows a thermal profile in central Nepal along part of the Himalayan chain, illustrates how overlying rocks became more than 200°C hotter than those below.

Excess heat is the key factor necessary for understanding occurrences of hard transparent crystalline gemstones of metamorphic origin. Yet the heat associated with thrusting of this nature is not fully explanatory on its own. For whereas thrust faults are not excessively unusual, gems are rare.

Contributing Factors

Within the gem-producing area of eastern Kenya and Tanzania, there were spots and locations where rock temperatures had been particularly high, specifically characterized as “ultra-hot” (Fritz et al., 2013). These included places where the movement of thick thrust-sheets (Fritz et al., 2009; Fritz et al., 2013) had been impeded by the resistant 255km diameter plug where it was caught between colliding plates. Gems then formed, in particular, in zones within the uppermost regions of the higher thrust sheets.

Better quality gems were commonly associated with additional contributing factors. In eastern East Africa, two such contributing factors have been recognized. One was a set of deep parallel linear fractures produced within the 255km scar by the collision. The schematic in Figure 5 shows one of these fractures (“Fault <640 Ma”) and Figure 6, a map, shows several more as parallel red lines. These fractures facilitated the penetration of heat and permitted the circulation of fluids, including those bearing chromophores and fluxes. The fluxes, which constitute the second contributing factor, came from the metamorphism of salts from impure limestones from near the edges of the colliding continents (Garnier, 2003; Garnier et al., 2008).

To be clear, gems do not crystallize exactly where the frictional heat had been greatest, but only in the nearby vicinity. The places of greatest frictional heat were also places where the pressure was particularly high. But whereas heat flows along irregular paths of lesser resistance, the same is not true of pressure, which decreases in time and space in the manner of a rapidly fading halo (Saul, 2014).

Folding during collisions also allowed deep dense rocks to be squeezed upward, producing ‘rootless masses’, some of which host fine gems (Mercier et al., 1999).

Despite differences in local geological circumstances, such scenarios applied at the John Saul Ruby Mine, Penny Lane, Umba, Kalalani, the tsavorite deposits on Mgama Ridge (Bridges Mines and others) and elsewhere, in particular with those gemstone occurrences associated with the eastern, coastal, portions of the 255km scar (Figure 6).

Farther Inland to Merelani and the Green Line

The 255km diameter scar overlaps part of the rim-zone of the 1800km diameter scar (Figure 6). This resulted in a doubly weakened area within which the Pare and Usambara Mountains rose at least 1000 vertical meters (as measured in North Pare; Le Gall et al., 2004). To the west of the Pare Mountains is a featureless flat area, a rift valley (marked RV in Figure 6) that has been in-filled with perhaps 6 to 8 vertical kilometres of sedimentary material (as judged from a possible analogue in southern Tanzania; Le Gall et al., 2004). The steep east-dipping side of this valley is provided by the Pare Mountains, which is to say, by the 1800km diameter scar (Figures 6 and 7).

As the valley filled with sediment and its bottom sank, it also expanded laterally but only to the west (in the direction of the thick horizontal arrow in Figure 7) because it was blocked in the east by the Pare Mountains.

Westward expansion of the rift valley pushed the Lelatema structure up against the inside of the rim of the 255km LHB scar and caused severe folding, producing the present day -shape of the Lelatema Fold Belt (Figure 8). 

Folding was stopped at the point of contact of the Lelatema Fold Belt with the 255km scar, i.e., at Merelani, where it was blocked by resistant beds of dolomitic marble. The contact caused these brittle rocks to fracture – presumably by 606 to 585 Ma, which are the ages of crystallization of tsavorite (Malisa, 1987) and tanzanite (Naeser and Saul, 1974) at Merelani – producing a linear fracture (green line in Figure 9) that is tangent to the 255km circle at its point of contact with the Fold Belt, which is to say, at Merelani.

Heat and fluids then penetrated the fractures, permitting the formation of gems at Merelani in a zone orientated along the tangent at approximately 41° East of North (Figure 8). A critical contributing factor for the formation of gem-quality tanzanite was the formation of ‘sausage-string’ fold structures (“boudins”) that provided centimetre-sized zones of additionally reduced pressure between ‘sausages’ (Olivier, 2008). Fluxing materials were also present, produced by the metamorphism of saline sediments possibly derived from an ancient ocean between Tanzania and the Congo (Feneyrol, 2012).

Gem mineralization at Lemshuku (tsavorite), Nandonjuk (tourmaline), and Namalulu (tsavorite) adhere to the structure of the Lelatema fold belt, not the 255km scar. I speculate that the Lelatema structure itself had been originally circular.

The folding of the Lelatema rocks against the 255km craterform at Merelani produced a major fault, shown as a green line in Figure 9. Extended to the northeast, the Green Line fault intersects the 1800km diameter scar (the Pare-Usambara arc, extended) at Mawenzi, Mt Kilimanjaro’s easternmost and main peak. Extended still farther, it is manifested by the oriented kink in the Yatta Plateau, a geographical curiosity whose existence has been a long-standing puzzle.

In passing, I note from personal experience in Tanzania in 1967 that local people and visiting geologists presumed that there must be a relationship between the tanzanite occurrence and Mt Kilimanjaro nearby, but the idea was abandoned for lack of evidence.

Concluding Remarks

The presence of circular scars in the gem producing area of eastern East Africa should allow new targets for gemstone exploration to be identified. Areas with little potential can also be eliminated, which is a major goal in all mineral exploration projects. Ideas set out here should be useful in the search for transparent gemstones in East Africa; the Hindu Kush and the Pamirs (both of which are circular and uplifted; Saul, 2018); the Central Urals (circular and uplifted; Burba, 1991, 2003a, 2003b; Saul, 2016, 2018); and minerals other than gems (Saul, 1978, 2016; Burba, 2003b).

Acknowledgements

I thank Dr Robert Coenraads for his detailed and insightful review and for many useful suggestions. Janice A. Glaholm and Lauriane Pinsault helped with the mapwork and suggested improvements to earlier versions of the text. I also thank Antonio de Quadros and John B. Southard for discussions and encouragement at the outset of this project.

This article is dedicated to the memory of Dr Alain Mercier (1956-2020).

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Editor’s Note

This article was reviewed by five independent geologists regarding its interest, relevance and suitability for publication by The Australian Gemmologist. Four out of five reviewers supported the publication of the article, and the author responded positively to suggestions to provide additional explanations of unfamiliar concepts and terminology. The Editor, on behalf of the Editorial Committee, has published the article in good faith but with caveats, as noted by one reviewer, which are reproduced below with permission.

“The article is an endeavour to answer how the gem deposits occur in their current geological setting. The author’s hypothesis of upward migration of very deep meteor impact crater fractures of LHB [late heavy bombardment] origin, driven upward toward the cooler earth’s surface by collision forces, fractures which then provided a channel for mineral-rich fluids (hence a favourable environment for gem mineral crystallisation), is contentious. There is no …[direct] evidence of this set of circumstances having happened. However, ideas do need to be advanced and they spring from unlikely sources. I’m of the view in that regard, that The Australian Gemmologist can provide a spark; a platform from time to time to test ideas and to ignite new directions in scientific thinking – ie ‘thinking outside the box’.”

Michelle Clark BSc (Hons 1) PhD FGAA
Editor / Chair Editorial Committee