Diatremes (and Maars)

[1dave]

A continuation of Geology For Rockhounds.

Ours is a differentiated planet. Heaviest minerals in the center, lightest on top - but . . .

Our continents are floating around and crashing into each other, AND over-riding the ocean bottoms.
This forces lightweight elements like carbon down deep among heavy minerals where they don't belong.



Ever force a beach ball under water, then let it go?
Well, that is what happens to this material, kind of like a lava lamp, but much more forceful.
Unlike water, the earth it passes through leaves a deep carrot shaped hole known as a diatreme.
The hot mass nearing the surface usually encounters groundwater that turns to steam, increasing the explosive force.

Small surface explosions of this nature are called "Maars."




They are important because of the unusual minerals they may contain.
For example, the State Line Diatremes on the Colorado/Wyoming border has:

*** Most of the area is private property or under claim. ***

General Info
More than 100 kimberlite pipes, also known as diatremes, and dikes occur in an area called the State Line Diatreme which is on the Colorado-Wyoming border. There are two main groups of kimberlite pipes that are being mined currently. Please see the Kelsey Lake Mine and the Sloan Kimberlites locations for more information.


Minerals
Chrome Diopside - Emerald-green in color.

Diamond - Usually occur as colorless octahedrons or dodecahedrons that are less than 2mm in diameter. Colors can be yellow, light orange, bluish-white and black.

Pyrope - Up to 6mm crystals have been reported.

Xonotlite - Murphy and Modreski (2002) state that massive, pale pink xonotlite was found by Ray Storey in a Sload kimberlite dump. It has a silky-fibrous, radiating texture that closely resembles pectolite except for the color.

The Big Hole, Kimberly South Africa.

Missouri has more than it's share:

Diatremes often form along fractures. A long line of them may join to form a dike as "The Mule Ear" and Moses Rock in Utah

Moses Rock, San Juan, Utah

The Mule Ear is part of Comb Ridge.

[1dave]

PROSPECTING for PLACER DIAMONDS - Better than the Lottery!

Many gold prospectors likely recover diamonds with black sand concentrates in stream placers but fail to recognize them. I also receive several inquiries each year from prospectors who believe they have diamonds, but can’t find anyone to verify them. Diamond is not all that difficult to identify with a microscope or hand-lens, but it is difficult to find someone with experience in raw diamonds who can confidently identify the gem.



During my first year as a research geologist for the Geological Survey of Wyoming, as it was known in 1977 (the agency changed its name periodically due to politics and is now referred to as Wyoming State Geological Survey or something similar), I was contacted by a prospector who asked me to identify suspected diamonds in his concentrates from a long-tom built on the side of Cortez Creek in the northwestern Medicine Bow Mountains of southeastern Wyoming (Figure 1). He found them during the previous winter while looking through his concentrates for gold, palladium and platinum, but couldn’t find anyone at the university who could identify them. So, right after I was hired by the Survey, he contacted me and I was able to verify them. To be 100% positive, I followed up with an XRD (x-ray diffraction) test. The diamonds were genuine gemstones with few flaws.



Figure 1. Cortez Creek showing location of diamond discovery (section 2, T 16N, R81W); and the Boden diamonds from Cortez Creek (below).



The original source for these diamonds has never been found, and most likely lies buried somewhere nearby. This is because Cortez Creek has a limited drainage basin and the only glaciers in this region during the last ice age were Alpine glaciers restricted to nearby high peaks, so it is unlikely that the diamonds were carried into the region by glaciers.



This part of Wyoming has many diamond-related anomalies. On one of my public field trips 8 years ago, some prospectors panned dozens of diamond indicator minerals (pyrope garnets) from a drainage located 15 miles east of Cortez Creek in the Middle Fork of the Little Laramie River of the Centennial Ridge district (Figure 2). Several years prior to this trip, some of our diamond research at the Survey led to identification diamond-indicator mineral anomalies in this area (Hausel and others, 1988).



Follow-up exploration by Superior Minerals Company in the 1980s to find the source of the Cortez Creek diamonds, identified a diamond-indicator mineral trail in the Iron Creek – South French Creek region to the south. This area is known for many gold and platinum mining scams because of the prominent massive pyrite in graphitic schist gossan, but the source for the diamond-indicator mineral trail was never found. More recent examination in the Medicine Bow Mountains using aerial-photos resulted in nearly 100 anomalies classified as cryptovolcanic structures with characteristics similar to kimberlite (one of the principal host rocks for diamonds) (see Hausel, 2008a). However, these all remain unexplored even though any one could be a significant diamond deposit (see Sutherland and Hausel, 2005).


Figure 2. Prospectors at Centennial Ridge search for gold, but only find diamond indicator minerals in the Middle Fork of the Little Laramie River. So many indicators were found in 2000 that it even surprised the author. Earlier research at Centennial Ridge showed visible gold in black sand concentrates (see Au on map below) at some sample sites, as well as a group of kimberlitic (diamond) indicator mineral anomalies. These anomalies suggest the presence of undiscovered kimberlite pipes in the region.



Cortez Creek lies between the State Line district, the Iron Mountain kimberlites, the Green River diamond indicator mineral anomaly and the Leucite Hills lamproites, areas that have received diamond exploration activity (Figure 3). Most anomalies identified, to date, are marked on maps available to the public and provide an excellent starting point for diamond prospecting (Hausel and others, 1988, 2003).



It is hard to imagine that nearly all of these anomalies remain unexplored. And with so many anomalies, one can almost guarantee that a major diamond discovery(s) will be made in this province in the future, when the right company invests in exploration, or when that one persistent prospector spends the necessary time to find the source of many of the anomalies.



Since diamonds are the hardest naturally occurring mineral on the surface of the earth, they are resistant to damage during stream transport. This means that diamonds (unless they have internal flaws, such as many industrial stones) can be carried in streams over great distances.



The best model for diamond placers is southern Africa. The Orange River basin in southern Africa drains a region with more than 3000 kimberlite pipes including some of the richest diamond pipes in the world. Erosion of these pipes over the past 140 million years resulted in liberation of millions of diamonds. Many were captured by the Orange River and its tributaries and carried 700 to 1000 miles downstream to the Atlantic Ocean. River sediments from Kimberley to the Atlantic Ocean, and beach sands extending from Port Nolloth, Namaqualand at the mouth of the Orange River northward to Luderitz, Namibia, and southward to Mitchell Bay are enriched in diamonds eroded from the kimberlites.



Figure 3. Map of Wyoming showing principal gemstone, precious and base metal mineralized areas and mining districts showing the two Cortez Creek diamonds recovered west of Gold Hill in the Medicine Bow Mountains. Areas underlined in red are mentioned in text. Diamonds and indicator minerals have also been found near Pinedale, in the Granite Mountains, north of Thermopolis, near Gillette, in the Haystack Range, in the Iron Mountain district, and throughout much of the Laramie and Medicine Bow Mountains. Several hundred anomalies have been identified and >130,000 diamonds recovered in this region with little exploration effort. This suggests to the author that this region probably encloses a major diamond province.



Off-shore marine and beach placers in this region provide a seemingly endless supply of small gem-quality diamonds. The diamond placers extend along the shore to the north and south over a distance of 300 miles and are mined along the beach and in the ocean. Diamonds have been traced all the way to the edge of the continental shelf many miles from shore. The process of stream transportation and stream abrasion served to eliminate many flawed industrial diamonds, such that the placers are formed primarily of relatively small gem-quality diamonds.



Diamonds have moderate specific gravity (3.5) that is favorable for concentration with black sands. When found in streams, diamonds may have been liberated from nearby kimberlites, lamproites, or related lamprophyric pipes or dikes, or may have come from diamond pipes many tens of miles (even hundreds of miles) away. However, if the diamonds are found with the classical kimberlitic indicator minerals (pyrope, chromian diopside, picroilmenite, chromite – see Hausel, 2008b), then the source of the diamonds must be much closer as these minerals do not survive large transportation distances. For example, pyrope may only be transported 1 to 3 miles before it is completely disaggregated in most streams. Chromian diopside typically does not survive transportation distances much greater than 0.25 to 0.5 mile. There may be many opportunities for prospectors to mine placer diamonds in the US, most notable is the Pacific Coast and the Colorado-Wyoming region.


Pacific Coast

Most geologists would not consider the Pacific Coast as a target for diamonds because the geology is not consistent with exploration models that indicate only very old crustal regions of the world are favorable for diamonds. However, this thought needs to be reappraised as there have been many discoveries made in younger geological terrains considered unconventional, such as California (Hausel, 1996). The simple fact that many diamonds have been found along the Pacific Coast supports that some primary diamond pipes are likely to be found in this region.

California. California has been the third most productive region in the US for diamonds following the Colorado-Wyoming region and Murfreesburo, Arkansas (Hausel, 1998). Placer diamonds were discovered at Placerville in the vicinity of Sacramento in 1849. Three years later, diamonds were found in the Cherokee placer gold mine near Oroville.



Between 1853 and 1918, about 400 diamonds and 600,000 ounces of gold were recovered from various placer mines along the Feather River near Oroville. It makes one wonder how many diamonds were not recognized by the gold miners. Drainages and tailings in the area likely have many hundreds (if not thousands) of diamonds.

Figure 4. Diamond localities along the Hayfork Creek and Trinity River, northern California (after Kopf and others, 1990).

This area is dominated by Round Mountain which forms a prominent flat-topped mountain capped by basalt. The basalt overlies gold-bearing conglomerate. Diamonds were initially washed from conglomerate along the north end of Round Mountain at Cherokee placer and also found in gravel along the west bank of the Feather River at Thompson Flat, Yankee Hill, and Morris Gulch (Hill, 1972).

The largest diamond found in the area, known as the Moore diamond, weighed 2.25 carats. However, local folklore claims that a diamond of 6 carats was found outside the entrance of the Spring Valley mine at Thompson Flat (Rosenhouse, 1975). A larger diamond of 7.25 carats was found to the southeast at French Corral (Heylum, 1985).

Streams south of Placerville also yielded diamonds. More than 60 (the largest was 1.5 carats) were recovered at Jackass Gulch. At Fiddletown, diamonds were recovered from gravel under volcanic ash at Loafer Hill (the largest was 1.33 carats). At least 50 diamonds were recovered from the Placerville area from Live Oak Mine Reservoir Hill, Smiths Flat, Spanish Ravine, White Rock Ravine, Webber Hill, Wisconsin Flat, Texas Hill, Newtown, Fairplay and Prospect Flat. The diamonds were described as white, canary yellow, greenish, and blue, weighing from 0.1 carat to 1.82 carats. At least 13 stones weighed >0.95 carat (Hill, 1972). The primary source for the diamonds remains unknown, but the author believes that the most likely source is serpentinized breccia pipes to the east in the Sierra Nevada, which may be related to subduction of the Pacific plate riding under the continental plate.

Northern California. Kunz (1885) reported that diamonds were found in the Trinity River region of northern California; in the vicinity of Coos Bay, Oregon; and on the banks of the Smith River. Five large diamonds were recovered from a tributary of the South Fork of the Trinity River (Hayfork Creek). One was the 4th largest in the US and weighed 32.99 carats: it was a yellowish-brown opaque aggregate that would be unlikely to survive great transportation distances in a stream or river (Figure 4) (Kopf and others, 1990).

At least four other large diamonds were recovered from Hayfork Creek: 17.83 carats, 14.33 carats, a poorly documented diamond estimated to weigh between 10 to 15 carats, and a 3.9 carat diamond (Kopf and others, 1990). These, all being industrial diamonds suggest minimal transport distance from the relatively nearby source rock.

Oregon. Diamonds were also reported in black sands near Coos Bay, Oregon (Kunz, 1885). The number of recovered diamonds was estimated to be about 100 - most yellowish in color and included some good quality diamonds up to 3 carats in weight (Blank, 1934).

Washington. Microscopic diamonds have also been described in black sands along the coast in the northwestern part of Washington. In 1932, a diamond of a little more than 4 carats was found in a Skamania County gold placer. The stone was a pale yellow octahedron (Blank, 1934).

Colorado –Wyoming Region

In the State Line district of Colorado-Wyoming, more than 40 diamondiferous kimberlite pipes and dikes were partially eroded during the past 300 to 600 million years (Figure 5). In addition to the known kimberlites, the author found evidence for another 200+ probable pipes in this region, thus there are considerable primary source rocks to supply some potentially rich diamond placers.



Figure 5. Map of the Colorado-Wyoming region showing location of the State Line district. The geology provides evidence that essentially all streams draining the State Line area to the south into Colorado, should contain many hundreds of thousands to potentially millions of diamonds, yet practically no placer mining has occurred in the district.



Kimberlites in the State Line are 600 million and 350 million years old. Elsewhere, diamondiferous intrusives are found to the west, which are about 50 million years old. Diamondiferous kimberlite in eastern Montana is also 50 to 60 million years old. Thus, there has been ample time for considerable erosion.



When kimberlite magma erupts, it erupts from unusual volcanoes known as pipes that are circular in morphology. However, at about 1000 to 2000 feet deep, the kimberlite pipe will take on the characteristic shape of the feeder fractures that it migrated along at depth. The fractures being linear results in the kimberlite having a more elliptical morphology at depth that progressively become more and more linear until a feeder dike is intersected at depths of 2500 to 4000 feet.





The Schaffer 15 kimberlite in Wyoming (left) forms an open park in the forest. The same is true for the diamond-rich Sloan 1 kimberlite (below).



The shape of these kimberlites shows a distinctly elongated morphology suggestive of deep erosion supporting that a considerable amount of the diamond-bearing kimberlite column has been removed by past erosion.





Interpretations based on morphology and kimberlite type (facies) suggest that 2000 to 3000 feet of upper diamond pipe column was eroded in the past. The many hundreds of thousands of diamonds from the upper parts of the kimberlite pipes thus lie somewhere downstream within a few feet to many miles.



Hundreds of thousands (if not millions) of diamonds must have been carried downstream from kimberlites in the State Line district and should still be in all of the drainages in the area - such as Fish Creek, George Creek, Rabbit Creek all the way from the State Line district to Ft. Collins and further - but nobody bothers to look. Would you like to win the lottery? You have a much better chance (actually a very good chance) of finding a diamond or diamonds worth $hundreds of thousands, even possibly a $million in the drainages that lie downslope from the known kimberlites. Not only that, diamonds are extremely hard and will transport dozens and even hundreds of miles, so all you have to do is plot the kimberlites and the nearby drainages and see where they go and where you can get access to pan for the gemstones. So why doesn't somebody do this? Good question!

According to some early work by McCallum and Mabarak (1976), the state line diamond pipes may have lost 2,000 feet of vertical column of diamond-bearing rock and the Iron Mountain district to the north could have lost as much as 4000 to 5000 feet due to erosion. Why no diamonds were found in this region in the 1800s as in California is simply due to the fact that the Californian diamonds were found as a by-product of considerable gold mining. In the State Line district, where the country rock is unmineralized granite, there was little to no placer gold mining in past years. Even so, a few diamonds were accidentally found by a prospector at the Yaussai placer on the North Fork of Rabbit Creek adjacent to the Sloan kimberlite in the past, but few people believed him at the time (Frank Yaussai, personal communication).



During some of the early testing of the Kelsey Lake kimberlites along the Colorado-Wyoming border, a 6.2-carat diamond (and other smaller diamonds) was found in Fish Creek (Howard Coopersmith, personal communication (1998) (Figure 7). Using only a gold pan, another prospector panned a diamond in a very short time from the Poudre River (Vic Norris, personal communication, 2002). Superior Minerals Company also recovered placer diamonds downstream from the George Creek kimberlite dike. Diamonds from the district have ranged from microdiamonds to gemstones up to 28.3 carats. One diamond fragment came from an estimated >80 to 90 carat diamond.

Figure 7. Generalized map of the Colorado-Wyoming state line district showing known kimberlites, nearby streams and roads (Modified from Hausel, 1998). This area also includes more than 200 other targets within and surrounding this district that are likely kimberlites, so there is a very good source terrain for placer diamonds.



Summary

Essentially, every kimberlite in the State Line district is diamondiferous. The George Creek kimberlites yielded the greatest number of diamonds: during testing, these produced more than 89,000 diamonds in the 1980s. These diamond-rich dikes undoubtedly supplied tens of thousands of diamonds in George Creek and Poudre River to the east. Another good source for placer diamonds is the Sloan kimberlites adjacent to Rabbit Creek. These yielded about 40,000 diamonds during bulk sampling tests in the early 1980s, including stones as large as 5.51 carats. But like all of the diamond mining operations in this district, the recovery mills were poorly designed which resulted in many of the diamonds not being recovered from samples during testing, and the rejection of essentially all large diamonds. As unthinkable as it may seem, the North Fork of Rabbit Creek was not tested for diamonds – yet this some diamonds were found in this drainage at the Yaussi placer. Fish Creek, George Creek, Rabbit Creek, the Poudre River will all contain diamonds. Since about 30 to 50% of the diamonds have been gem quality from the kimberlites, a fortune in diamonds awaits the right, persistent prospector.



As a final note, when searching the BLM records of mining claims in this area, it is apparent that some of my past research is getting noticed. I was surprised to see all of the claim blocks that are now tied up in the State Line district. However, there appears to be a considerable area that is open to leasing or claim staking – to search the claim records - access

(http://www.geocommunicator.gov/NILS-PARCEL2/map.jsp). In an upcoming issue, I will show the reader a method that can be used to find a diamond pipes from your home computer.



References Cited

Blank, E.W., 1934, Diamond finds in the United States: Rocks and Minerals, v. 9, p. 147-182.

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 & Mantle Source Rocks in the Wyoming Craton with Discussions of Other US Occurrences. WSGS Report of Investigations 53, 93 p.

Hausel, W. D., 2008a, Prospecting for Diamonds: ICMJ's Prospecting & Mining Journal, v. 77, no. 9, p. 19-50.

Hausel, W. D., 2008b, How to locate diamonds: ICMJs Prospecting & Mining Journal, v. 77, no. 11, p. 18-40.

Hausel, W.D., Sutherland, W.M., and Gregory, E.B., 1988, Stream-sediment sample results in search of kimberlite intrusives in southeastern Wyoming: Geological Survey of Wyoming Open-File Report 88-11, 11 p. (5 plates).

Hausel, W.D., Gregory, R.W., Motten, R.H., and Sutherland, W.M., 2003, Geology of the Iron Mountain Kimberlite District & Nearby Kimberlitic Indicator Mineral Anomalies in Southeastern Wyoming: Wyoming State Geological Survey Report of Investigations 54, 42 p.

Heylum, E.B., 1985, Diamonds near Oroville, California: California Mining Journal, Dec., p. 63.

Hill, M., 1972, Hunting diamonds in California: Naturegraph Publishers, Healdsburg, California, 80 p.

Kopf, R.W., Hurlbut, C.S., and Koivula, J.I., 1990, Recent discoveries of large diamonds in Trinity County, California: Gems and Gemology, v. 25, no. 3, p. 212-219.

Kunz, G.F., 1885, Precious stones, in Mineral resources of the United States: U.S. Geological Survey, p. 723-782.

McCallum, M.E., and Mabarak, C.D., 1976, Diamond in State Line kimberlite diatremes, Albany County, Wyoming, and Larimer County, Colorado: Wyoming State Geological Survey Report of Investigations 12, 36 p.

Rosenhouse, L., 1975, Diamonds along the Feather River: Gems and Minerals, October, 1975, p. 36-55.

Sutherland, W.M., and Hausel, W.D., 2005, Preliminary geological map of the Saratoga 1:100,000 Quadrangle, Wyoming, WSGS Map Series.

[rockpickerforever]

Thanks, 1dave! Great information.

Maybe I should start looking for diamonds, instead of gold...

[1dave]

In Montana:

The Missouri Breaks diatremes are some of the best exposed diatremes in the United States, and display remarkable subsidence features. The diatremes and associated intrusions are important in showing (1) genetic connections of alnoitic, kimberlitic, and carbonatitic magmas, (2) xenolith suites representing the upper mantle and lower, middle, and upper crust of the Eocene lithosphere, and (3) mechanisms of eruption and emplacement of volatile-rich magmas. Some of the diatremes are fairly accessible in good weather. However, the Missouri Breaks is a remote area in which most of the roads are unimproved tracks across clay-rich sedimentary rocks, and even a small amount of rain may immobile four-wheel drive vehicles and make steep grades terminally hazardous.

[1dave]

Finding Diatremes

Most diatremes are millions of years old, and have therefore had their surfaces eroded.



So to find them, search for HEAVY TRACER MINERALS that the melts drug along with them from depth.



Hand pan gullies for:

Pyrope Garnets heavy in Chromium.



Chromium Dioxide



Enstatite (beer bottle ore)



Magnesian Ilmenite



And of course, those pesky Diamonds!

Garnet is probably the best tracer mineral because:
1. It is present in most kimberlites
2. It is usually more abundant than most tracers
3. It is more resistant to attrition than most other tracers.

[1dave]

Lamproite
From Wikipedia, the free encyclopedia

Lamproites are ultrapotassic mantle-derived volcanic and subvolcanic rocks. They have low CaO, Al2O3, Na2O, high K2O/Al2O3, a relatively high MgO content and extreme enrichment in incompatible elements.

Lamproites are geographically widespread yet are volumetrically insignificant. Unlike kimberlites which are found exclusively in Archaean cratons, lamproites are found in terrains of varying age, ranging from Archaean in Western Australia, to Palaeozoic and Mesozoic in southern Spain. They also vary widely in age, from Proterozoic to Pleistocene, the youngest known example being 56,000 ± 5,000 years old.

Lamproite volcanology is varied, with both diatreme styles and cinder cone or cone edifices known.

Petrology

Lamproites form from partially melted mantle at depths exceeding 150 km. The molten material is forced to the surface in volcanic pipes, bringing with it xenoliths and diamonds from the harzburgitic peridotite or eclogite mantle regions where diamond formation is stabilized.

Recent research and lead-lead isotope geochemistry have revealed that the source of lamproites may be transition zone melts of subducted lithosphere which has become trapped at the base of the lithospheric mantle. This observation also reconciles the depth of melting with the peculiar geochemistry, which is most easily explained by melting of already felsic material under deep mantle conditions.
Mineralogy

The mineralogy of lamproites is controlled by their peculiar geochemistry, with a predominance of rare silica-deficient mineral species and rare, mantle-derived minerals predominating.

Minerals typical of lamproites include: forsteritic olivine; high iron leucite; titanium-rich aluminium-poor phlogopite; potassium- and titanium-rich richterite; low aluminium diopside; and iron-rich sanidine. A variety of rare trace minerals occur. The rocks are high in potassium with 6 to 8% potassium oxide. High chromium and nickel content is typical. The rocks commonly are altered to talc with carbonate or serpentine, chlorite, and magnetite. Zeolites and quartz may also occur.

Lamproites are characterized by the presence of widely varying amounts (5-90 vol.%) of the following primary phases (Mitchell & Bergman, 1991):

titanian (2-10 wt% TiO2), aluminium-poor (5-12 wt% Al2O3) phenocrystic phlogopite
titanian (5-10 wt% TiO2) groundmass poikilitic "tetraferriphlogopite"
titanian (3-5 wt% TiO2) potassium (4-6 wt% K2O) richterite
forsteritic olivine
aluminium-poor (<1 wt% Al2O3), sodium-poor (<1 wt% Na2O) diopside
nonstoichiometric iron-rich (1-4 wt% Fe2O3) leucite, and
iron-rich sanidine (typically 1-5 wt% Fe2O3)).

The presence of all the above phases is not required in order to classify a rock as a lamproite. Any one mineral may be dominant, and this, together with the two or three other major minerals present, suffices to determine the petrographic name.

The presence of the following minerals precludes a rock from being classified as a lamproite: primary plagioclase, melilite, monticellite, kalsilite, nepheline, Na-rich alkali feldspar, sodalite, nosean, hauyne, melanite, schorlomite or kimzeyite.
Geochemistry

Lamproites conform to the following chemical characteristics:

molar K2O/Na2O > 3, i.e., ultrapotassic
molar K2O/Al2O3> 0.8 and commonly > 1
molar K2O + Na2O/ Al2O3 typically > 1 i.e., peralkaline
typically <10 wt% each of FeO and CaO, TiO2 1-7 wt%, >2000 and commonly >5000 ppm Ba, >500 ppm Zr, >1000 ppm Sr and >200 ppm La.

Economic importance
The economic significance of lamproite became known with the 1979 discovery of the Argyle diamond pipe in Western Australia. This discovery led to the intense study and re-evaluation of other known lamproite occurrences worldwide; previously only kimberlite pipes were considered economically viable sources of diamonds.

The Argyle diamond mine remains the only economically viable source of lamproite diamonds. This deposit differs markedly by having a high content of diamonds but low quality of most stones. Research at Argyle diamond have shown that most stones are of E-type; they originate from eclogite source rocks and were formed under high temperature ~1,400 °C (2,600 °F). The Argyle diamond mine is the main source of rare pink diamonds.

The pipe had been discovered as part of a regional exploration program that had sampled streams from all over the Kimberley region. The sampling comprised collecting about 40 kg of –4mm gravel from heavy mineral trap sites, i.e. from areas where the heavy minerals like, gold, garnet, ilmenite, chromite and diamond, would accumulate.

The Argyle pipe and alluvial gravels were very rich in diamonds and at least two diamonds were seen on the surface of ant hills (termite mounds) - see photo below taken by Peter Temby in November 1979 (click on the image to see a larger view). This diamond was about 4 mm in size and was washed off the anthill by the next thunderstorm!

View from south across the Argyle pipe, with the assay trenches of the Limestone Creek gravels in the foreground and Smoke Creek in background heading towards Lake Argyle.

Olivine lamproite pyroclastic rocks and dikes are sometimes hosts for diamonds. The diamonds occur as xenocrysts that have been carried to the surface or to shallow depths by the lamproite diapiric intrusions.

The diamonds of Crater of Diamonds State Park near Murfreesboro, Arkansas are found in a lamproite host.
Nomenclature

Lamproites, as a group, were known by a variety of localised names due to the fact their mineralogy is quite variable and because of their rarity often few examples of the following lamproite variants were known. Modern terminology classes all as lamproites but modifies this term with the mineral abundances as per the standard IUGS rules.
Historic Modern
Wyomingite diopside-leucite-phlogopite lamproite
Orendite diopside-sanidine-phiogopite lamproite
Madupite diopside madupitic lamproite
Cedricite diopside-leucite lamproite
Mamilite leucite-richterite lamproite
Wolgidite diopside-leucite-richterite madupitic lamproite
Fitzroyite leucite-phlogopite lamproite
Verite hyalo-olivine-diopside-phlogopite lamproit
Jumillite olivine-diopside-richterite madupitic lamproite
Fortunite hyalo-enstatite-phlogopite lamproite
Cancalite enstatite-sanidine-phlogopite lamproite
Related rock types

Kimberlite
Lamprophyre
Ultrapotassic igneous rocks

References

"Rock Library". Mars Exploration Rover Mission. NASA Jet Propulsion Laboratory. Retrieved 2 June 2014.

Further reading

Bergman, S. C.; 1987: Lamproites and other potassium-rich igneous rocks: a review of their occurrences, mineralogy and geochemistry. In: Alkaline Igneous rocks, Fitton, J.G. and Upton, B.G.J (Eds.), Geological Society of London special publication No. 30. pp. 103–19.
Mitchell, R. H.; Bergman, S. C. (1991). Petrology of Lamproites. New York: Plenum Press. ISBN 0-306-43556-X.
Murphy, D. T.; Collerson, K. D.; Kamber, B. S. (2002). "Lamproites from Gaussberg, Antarctica: Possible Transition Zone Melts of Archaean Subducted Sediments". Journal of Petrology 43 (6): 981–1001. doi:10.1093/petrology/43.6.981.
Woolley, A.R., Bergman, S.C., Edgar, A.D, Le Bas, M.J., Mitchell, R.H., Rock, N.M.S. & Scott Smith, B.H., 1996. Classification of lamprophyres, lamproites, kimberlites, and the kalsilitic, melilitic, and leucitic rocks. The Canadian Mineralogist, Vol 34, Part 2. pp. 175–186.

External links

LAMPROITE-HOSTED DIAMONDS retrieved June 7, 2005
Microscopic image of lamproite retrieved June 7, 2005
Argyle pipe retrieved June 7, 2005
Igneous rock classification flowchart