Saturday, October 28, 2017

Diamonds - Origin and Occurrence

A legal alien searches for diamonds among the stars (pencil sketch by the
GemHunter copyright by Dan Hausel).
Where does diamond come from? A jewelry store? Yes. But before a diamond makes its way to the jewelry store, it comes from nature and is mined from specific kinds of mineral deposits. And finding diamond deposits require considerable geological detective work.

Me, I'm known as the GemHunter. You can find some of my posts on my website, Facebook, books, papers, blogs, and blogspots. I like to prospect and love to write. I am the most successful gold, diamond, gemstone and mineral hunter ever to kick rattlesnakes in the sagebrush of Wyoming, just not good at making money on all of my discoveries. Yes, others have made money from my discoveries, but I'm not good when it comes to finances, which is why I have old field boots, a rusty rock hammer, and an old Honda Fit.

So, some diamonds actually come from stars (nope, not Hollywood) and even from some planets. Carbon is the necessary ingredient for diamond. Naw, not that stuff superman squeezes: coal is a hydrocarbon. But graphite in your pencil, is made from the same exact material as diamond - its just that the atoms that make up both, are carbon - but they have different atomic bonds making one soft and kind of greasy, and the other one very hard and brittle. You can think whatever you want, but to me, it is a miracle - everything in the universe is made up of these atoms that have itsy bits wave-mass particles and by adding a couple of these particles (electrons, neutrons, protons, etc) we can completely change the material into something dramatically different, change a few bonds here and there, and shazam! We have a diamond or a pencil. These atomic materials are just like legos - they are the building blocks of everything. So, who manufactured legos? A bunch of Vikings in Denmark. So who manufactured all those atoms and their building blocks? Think what you want. But to me, you can only explain it as a miracle from God. And then there are people - even we are made from atoms - you can even convert Grandma into a diamond! Yes, it works for most people (politicians excepted - they produce an odiferous form of carbon - and lots of it). 

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 by Dan Hausel).
When subjected to incredible pressures and temperatures, that dirty o' carbon will produce magnificent diamonds. To get necessary pressures to form diamond shouldn't be any problem - just get a 90-mile high column of rock and sit it on pencil lead. If we to take pencil lead, or some unsuspecting insect or plant, and stack a pile of rocks as high as 400 empire state buildings on top of the carbon, and – whoosh – you now have diamond instead of a pencil or insect. This is why diamonds are so rare – it takes rare conditions to form and very rare conditions to get them to the earth’s surface when we mine them. So, if we could dig a mine shaft to a minimum of 475,000 feet, we will be mining diamonds! Pretty simple! But, the deepest mines on earth (and they've been digging for more than 100 years), are gold mines in the Witwatersrand basin of South Africa. These mines are very deep and so, far are only about 12,600 feet deep, so we have a lot more rock to dig! 

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. 

Partially resorbed (odd-shaped) diamond crystals from Argyle, Australia. These diamonds partially resorbed in
 the hot lamproite magma before reaching the earth’s surface and producing many unusual shapes for diamond (photo by Dan Hausel). 


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. 
A piece of the Earth's mantle. A garnet peridotite similar to this sample
was collected from a kimberlite in Wyoming and accidentally discovered
to have diamonds in 1975. Such mantle nodules of peridotite and eclogite
often have diamonds and are the original source of diamonds in kimberlite
pipes (photo by Dan Hausel)

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 kimberlite magma explodes at the surfaced producing a diatreme with many angular rock pieces as 
seen  exposed in the mine highwall at the Kelsey Lake diamond mine in Colorado.  Note the large number of
 white, angular blocks of rock in the bluish-gray kimberlite matrix (photo by the Dan Hausel). Find this mine on
  GoogleEarth by searching for ‘Kelsey Lake, Livermore, CO’.  


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
Carbon dioxide is basically plant food, but the EPA listed this very natural gas and plant food as a pollutant of global warming during the Obama era (At 3 pm on New Year’s Eve, the wind-chill was -32oF in Laramie, Wyoming, a bit colder in Yellowknife, Canada and a lot colder than Nome, Alaska. In good o’ hot Gilbert, Arizona, the wind-chill was 38oF). If you remember high-school biology, plants absorb carbon dioxide and give off oxygen. We in turn, absorb oxygen and give of carbon dioxide – it’s the cycle of life and the Obama government claimed it was pollution that needed to be taxed so Al Gore can get even richer.  
The North American Craton showing locations of some 
diamondiferous kimberlite and lamproite districts (after
Hausel, 1998).
There are literally thousands of known kimberlites and hundreds of lamproites and lamprophyres but only a handful has commercial amounts of diamond. One estimate made years ago suggested that <1% of all kimberlites are commercially mineralized in diamond (Lampietti and Sutherland, 1978). Although, a few thousand kimberlites have been discovered since 1978, that statistic still remains essentially valid. Commercially minable diamond deposits are very rare. They are as rare as Congressmen who pay taxes and tell the truth. 

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 Dan Hausel,
specimen donated by Chuck Mabarak).
Snap Lake, Gahcho Kue (Kennedy Lake), Victor (Hausel, 2006, 2008) and the diamond-rich kimberlites at Ekati and Diavik are in the Slave Province. To visit this area, search ‘Ekati airport, Canada’ (64o43'22"N; 110o36'54"W) on GoogleEarth, and you will fly to the open pit mines at Lac de Gras. While there, look for circular lakes or nearly circular lakes that appear to line up with others: more than 120 diamondiferous kimberlites have been found in this area and lakes often sit on kimberlite in this part of the world! Now search for ‘Diavik airport, Canada’ to visit a nearby diamond mine. 

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 Dan Hausel).
More than 2,000 kimberlites have been found in Canada. One should expect similar discoveries in Michigan, Wisconsin, Wyoming and Montana, but for some reason, politicians and government bureaucracies tend stymie exploration. For instance, both the forest service and bureau of land management often hinder exploration. The forest service in particular has a bad reputation when it comes to exploration and mining: the agency often submits large land withdrawals whenever mineral discoveries are made. 

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). 

Cross-section of a kimberlite pipe from Colorado and Wyoming.
The vertical scale of this cartoon would be about 5,000 to
10,000 feet, thus in the State Line district of Colorado and Wyoming
about 2,000 to 3,000 feet of the upper pipe has been eroded. In other
words, the richest part of these diamond pipes have been removed and the
diamond load is now somewhere downstream from the State Line and
diamonds likely have been scattered down the Poudre River drainage and
tributaries to LaPorte, Fort Collins, Denver and beyond. With all of the
sand and gravel that has been mined along this drainage near
Fort Collins, it is likely that a fortune in diamonds
went unrecognized and thrown away. 
Lampietti and Sutherland (1978) reported only about 10% of known kimberlites are mineralized with diamond. This statistic may no longer be valid in that ~50% of kimberlites found in Canada and Wyoming over the last few decades, and as many as 90% in Colorado have yielded diamond. When economic, kimberlites may contain hundreds of millions to billions of dollars in diamonds; thus kimberlite should be a priority target in any exploration program for mining companies and for prospectors in general. Diamonds are worth much more than gold! 

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 Dan Hausel).
The Kimberley pipe contracted sharply at depth. At the lowest level of mining (3,465 feet), it was no longer pipe-shaped but rather had the appearance of three intersecting dikes (Kennedy and Nordlie 1968). Combined with the estimated 5,100 feet (1,600 m) of erosion since the time of emplacement, the depth to the original point of expansion was probably 1.5 miles (2.4 km). 

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
Kimberlites typically transport xenoliths and xenocrysts to the surface. Many are derived from mantle depths and some form a distinct suite of minerals that are referred to as kimberlitic indicator minerals. The traditional indicator minerals used to explore for kimberlite include pyrope garnet, chromian diopside, chromian enstatite, picroilmenite, chromite, and diamond. 

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 

  1. Davidson, C.F. 1967. The so-called “cognate xenoliths” of kimberlites. In Ultramafic and Related Rocks. New York: John Wiley. 
  2. Erlich, E.I., and Hausel, W.D., 2002, Diamond Deposits – Origin, Exploration, and History of Discoveries: Society of Mining Engineers, 374 p. 
  3. 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. 
  4. 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. 
  5. Hausel, W.D. 2006, Diamonds in Industrial Minerals & Rocks (7th Edition). Society for Mining Metallurgy and Exploration, p. 415-433. 
  6. 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. 
  7. Hughes, C.J. 1982. Igneous Petrology. New York: Elsevier. 551 p. 
  8. Kennedy, G.C., and B.E. Nordlie. 1968. The genesis of diamond deposits. Economic Geology. 63:495–503. 
  9. Lampietti., E.M.J., and D. Sutherland. 1978. Prospecting for diamonds-some current aspects. Mining Magazine, August: 117-123. 
  10. 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. 
  11. 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. 
  12. 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. 
  13. Watson, K.D. 1967. Kimberlites of Eastern North America. Ultramafic and Related Rocks. New York: John Wiley. 

No comments:

Post a Comment

HOW BLUE IS BLUE GROUND?

Dry placer?’ at Cedar Mountain, Wyoming. These rounded boulders and cobbles are part of a lamprophyre breccia pipe that contains abundant...