A legal alien searches for diamonds among the stars (pencil sketch by the GemHunter ). |
Me, I'm the GemHunter. My posts are on Facebook, books, papers, and blogspots. And when I'm not writing, you might find me out in the desert kicking rattlesnakes. I've found many mineral deposits - some worth $billions, I'm just extremely lousy at making money and finding company CEO's who are trustworthy. Yes, others have made money fortunes from my discoveries, but, other than consulting fees (sometime they've even cheated me on those), I only have old field boots, a rusty rock hammer, and an old Honda Fit to show for my discoveries. Several people have suggested I should find an attorney and sue these buggers - such as that scum CEO in Colorado who still owes me more than $1 million in wages and stocks. But then again, what do I do with the attorney when he scams me? I guess there are just professions in the world that attract untrustworthy people - crooks, politicians, CEOs - guess I'm repeating myself.
So, carbon, the basic building block of diamonds, graphite and people is created during nuclear fusion in stars and later disseminated in nebula following supernovae explosions. Considerable carbon was also created during the Big Bang (13 billion years ago) when the universe erupted from either nothing, or a ball of energy. There, meditate on that thought. What did the universe look like at the very beginning. Was there any mass? Was there any energy? And if there wasn't, where did they come from - and was there a real beginning for the universe?
Some meteorites have diamond and/or a carbon polymorph such as lonsdaleite and/or carbonado. Other diamonds arrived at the earth’s surface from the opposite direction, from the upper mantle (90 to 120 miles deep) via extremely rare volcanic eruptions. Some diamonds may even have originated near the earth’s core-mantle boundary and then followed convection cells to the upper mantle based on the presence of an unusual garnet known as majorite found as a rare mineral inclusion in some diamonds (Erlich and Hausel, 2002).
Dozer cut across Sherman Granite (pink) and a diamond-bearing kimberlite (blue ground) in Wyoming (photo the author). |
So, it is apparent we are not going to dig deep enough to reach these diamond shoots, so they somehow need to come to us. And since diamond is unstable at the earth’s surface where atmospheric pressures are insignificant compared to pressures at 90 to 120 miles or more deep, these gems must make it to the earth's surface at a very fast rate; otherwise, they will end up being a number 2 pencil lead (graphite).
One way to get these gems to the surface and into Tiffany’s is by using a geological elevator. This elevator, or what geologists call a diatreme, it an explosive volcano. The magma is that transport these diamonds is referred to as kimberlite, lamproite or lamprophyre by geologists, is an extremely rare magma that forms at great depth in the mantle where the diamonds occur. Diamond may even possibly be thrust up from the mantle, but as such, it may all revert to graphite before making it to the surface, but we’ll just focus on the magmas for now (Hausel, 1996).
Since diamonds are unstable at the earth’s surface, they tend to resorb (melt) or burn unless under pressure or quickly stabilized at the earth's surface. The only way they are preserved is by the hot magmas rising and cooling very fast before the diamonds can resorb. It is not difficult to burn a diamond on the earth’s surface – I burned one back in 1985 using a Bunsen burner flame , and it only took about a minute of heating and blowing oxygen on the gem - then it made a loud "pop", and what was left was graphite.
The primary host rocks for terrestrial diamond are pieces of the earth’s mantle known as peridotite and eclogite. Some of these can be very rich in diamonds. I saw one eclogite sample from the Sloan kimberlite diamond mine in Colorado that had ~20% diamond. To get these rich primary rocks to the surface, they are carried in magmas that begin their journey in the upper mantle.
When kimberlite magma begins its initial migration, it is under tremendous pressure and tends to pick up diamonds along with pieces of diamond-rich eclogite and peridotite. Some of these rocks will disaggregate on the journey to the earth’s surface and disseminate their diamond load throughout the kimberlite magma. And if they really hurry, many of the diamonds will be preserved by time they reach the earth’s surface.
At the surface, the kimberlite magma will erupt to produce a maar-like volcano (depression) that does not have the typical cone we are used to seeing associated with volcanoes. The gas in the kimberlite being under tremendous pressure will erupt from the volcano at speeds of 1 to 3 times the speed of sound. Now that is a volcano! With such speeds, the magma fragments rocks all the way from the mantle to the surface and produces a breccia pipe (maar).
The erupting kimberlite magma will release considerable gas into the atmosphere at 3 times the speed of sound, while some gas (water vapor and carbon dioxide) reacts with the magma itself to produce mica, calcite, and serpentine. Being under pressure, when kimberlite erupts, the associated gas expands and the gas temperature drops to freezing – so if you are standing next to this volcano, you will need to wear a coat, mittens and warm hat. If the EPA gets wind of this, erupting kimberlite volcanoes will be taxed.
Diamondiferous kimberlite breccia, Canada (photo by the author). |
The North American Craton showing locations of some diamondiferous kimberlite and lamproite districts (after Hausel, 1998). |
Diamondiferous kimberlites and lamproites are essentially restricted to cratons and cratonized terrains. The easiest way to think of a craton is to think of them as the very old and ancient continental cores – or rock outcrops that are >1.5 billion years old. Some of the oldest parts of these continental cores are referred to as Archons that consist of rocks >2.5 billion years old. So if you were to start prospecting for diamond-rich kimberlite volcanoes, your chance of finding one will be a heck of a lot better in the very old Archons, since essentially every commercial diamond-rich kimberlite that has been mined in history, has been located in Archons. But there are exceptions, such as the diamond-bearing lamproites at Argyle Australia, Murfreesboro, Arkansas, Ellendale, Australia, and Golconda, India. These are located in the younger Proterozoic age continental cores (1.5 to 2.5 billion years old) (Erlich and Hausel, 2002).
So, let’s take all of the soil, dirt and younger rocks in North America and just strip them off, so we can see where all of these very old continental core rocks are found and then if you want to prospect for diamonds, go to those areas and hopefully you will find a couple of billion dollars in diamonds.
Snap Lake diamondiferous kimberlite from Canada (photo by the author, specimen donated by Chuck Mabarak). |
Good prospects have also been identified in kimberlite, lamproite and lamprophyre in Proterozoic terrains referred to as ‘Protons’ by diamond exploration geologists. The Prairie Creek (Murfreesboro, Arkansas) lamproite and diamond-bearing kimberlites in Colorado, Wyoming and Michigan are in Protonic terrains. All of these have lower diamond ore grades and smaller diamonds on average. However, the diamond-bearing lamproite at Argyle, Australia, is rich in diamonds (fly to ‘Argyle airport, Australia’ with GoogleEarth to visit this mine).
A view of one of the Schaffer diamondiferous kimberlites in Wyoming. It lies under the open park that does not support any tree growth (photo by the author). |
There are also diamondiferous host rocks that are considered to be unconventional by geologists. Some of these have been identified in cratons as well as outside of the continental cores within tectonically active regions along the margins of cratons (Hausel, 1996). Because very high grade diamond ores have been detected in some of these, unconventional commercial host rocks are anticipated to be found in the future (Erlich and Hausel, 2002). Presently, diamond exploration programs are designed to search for conventional host rocks (i.e., kimberlite and lamproite) or for placers presumably derived from these.
Kimberlite
The majority of diamond mines are developed in kimberlite such as the Wesselton, DeBeers, Kimberley, Bufontein and Dutoitspan and South Africa (fly to ‘Kimberley, South Africa’ on GoogleEarth and look within the city for circular open pit mines – these include some of the richest diamond deposits ever found). Diamonds eroded from these kimberlites produced the richest diamond placers on earth: the diamonds were carried down the Orange River for 500 to 700 miles to the beaches along the west coast of Africa (See 29o39’43”S; 17o03’15E on GoogleEarth and look for all of the piles and piles of tailings along the west coast of Africa that go on for miles – these are all placer diamond deposits).
Kimberlites are carbonated alkali peridotites that exsolve CO2 during ascent to the surface from the earth’s upper mantle. The release of the CO2 gas under pressure (the same gas that produces carbonation in soda pop) results in incredible eruptions producing what are known as diatremes of intensely brecciated kimberlite with rounded xenoliths and cognate peridotite and eclogite nodules. The diatremes form sub-vertical to vertical pipes that taper down at depth to form steeply inclined cylindrical bodies.
In cross-section these have carat-shape. The average angle of inclination of the walls of various diatreme pipes in the Kimberley region of South Africa (Wesselton, DeBeers, Kimberley and Dutoitspan) is 82° to 85°. Ideally, the pipes have rounded to ellipsoidal horizontal cross sections filled with kimberlitic tuff or tuff-breccia. Many taper from the surface to depths of 1 to 1.5 miles where they pinch down to narrow root zones emanating from a feeder dike. The kimberlite rock itself will have so much calcium carbonate, that it will fizz like a soda pop if you place a drop of dilute 10% hydrochloric acid on the rock.
Lake Ellen diamondiferous kimberlite, Michigan (photo by the author). |
Kimberlitic magmas are interpreted to originate from depths as great as 120 miles and travel to the Earth’s surface in a matter of hours (O’Hara and others, 1971). The magma rises rapidly, possibly 5 to 20 miles/hour in order to transport high-density ultramafic xenoliths and cognate nodules (peridotite and eclogite). Within the last few miles of the surface, emplacement rates increase dramatically to several hundred miles per hour. Such velocities could bring diamonds from the mantle to the surface in less than a day. McGretchin (1968) estimated that the speed of the fluidized kimberlite near the surface increased to as much as 870 miles/hour, or about the speed of sound (Mach 1). Some estimates have even suggested kimberlite emplacement at the Earth’s surface may have achieved velocities exceeding Mach 3 (Hughes 1982)!
The temperature of the magma at the point of eruption is relatively cool. Watson (1967) indicated that the magma temperature was less than 600°C based of the coking effects on coal intruded by kimberlite. A low temperature of emplacement is also supported by the absence of any visible thermal effects on country rock adjacent to most kimberlite contacts. Davidson (1967) suggested the temperature of emplacement may have been as low as 200°C based on the retention of argon. Hughes (1982) pointed out that the near-surface temperatures of the gas-charged kimberlite melt may be more on the order of 0°C (32oF) owing to the adiabatic expansion of CO2 gas as kimberlite erupts at the surface. So this is most likely the only volcano you can stand next to and catch a cold when it erupts.
Diamonds in the rough |
Summary
Now that you know where diamonds come from, we’ll discuss some diamond prospecting methods in our next update in the future. And if you have any diamonds you would like to give to me, make sure they are very large.
References
- Davidson, C.F. 1967. The so-called “cognate xenoliths” of kimberlites. In Ultramafic and Related Rocks. New York: John Wiley.
- Erlich, E.I., and Hausel, W.D., 2002, Diamond Deposits – Origin, Exploration, and History of Discoveries: Society of Mining Engineers, 374 p.
- Hausel, W.D., 1996, Pacific Coast diamonds-an unconventional source terrane in Coyner, A.R., and Fahey, P.L., eds., Geology and ore deposits of the American Cordillera, Geological Society of Nevada Symposium Proceedings, Reno/Sparks, Nevada, p. 925-934.
- Hausel, W.D., 1998, Diamonds and mantle source rocks in the Wyoming Craton, with a discussion of other US occurrences: Wyoming State Geological Survey Report of Investigations 53, 93 p.
- Hausel, W.D. 2006, Diamonds in Industrial Minerals & Rocks (7th Edition). Society for Mining Metallurgy and Exploration, p. 415-433.
- Hausel, W.D., 2008, Diamond Deposits of the North American Craton in Woods, A., and Lawlor, J., eds, Topics in Wyoming Geology, Wyoming Geological Association Guidebook. p. 103-138.
- Hughes, C.J. 1982. Igneous Petrology. New York: Elsevier. 551 p.
- Kennedy, G.C., and B.E. Nordlie. 1968. The genesis of diamond deposits. Economic Geology. 63:495–503.
- Lampietti., E.M.J., and D. Sutherland. 1978. Prospecting for diamonds-some current aspects. Mining Magazine, August: 117-123.
- Levinson, A.A., J.J. Gurney, and M.B. Kirkley. 1992. Diamond sources and production, past, present and future. Gems and Gemology 28:4. 234–254.
- McGretchin, T.R. 1968. The Moses Rock dike: Geology, petrology, and mode of emplacement of kimberlite-bearing breccia dike, San Juan County, Utah. Ph.D Diss. California Institute of Technology, Pasadena. 188 p.
- O’Hara, M.J., S.W. Richardson, and G. Wilson. 1971. Garnet-peridotite stability and occurrence in crust and mantle. Contributions to Mineralogy and Petrology. 32:48–68.
- Watson, K.D. 1967. Kimberlites of Eastern North America. Ultramafic and Related Rocks. New York: John Wiley.
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