Thursday, September 19, 2013

Photo Gallery of the Leucite Hills, Wyoming

Boars Tusk lamproite (wyomingite) forms a distinct and prominent volcanic neck with Table Mountain in background

The Leucite Hills were one of my favorite places to visit while I worked at the Wyoming Geological Survey. After the discovery of diamonds in lamproites in Australia in the 1980s, I was able to wrangle some money out of the legislature to visit the diamond deposits in Australia. It should have sparked much greater diamond interest in Wyoming - particularly since the Aussies opened diamond mines at the Argyle and the Ellendale lamproite fields and produced some of the most valuable diamonds in the world - pink diamonds - some which have sold for as much as $1 million a carat, or many tens of thousands of times more valuable than an equivalent weight in gold.
Black Butte lamproite peaks over horizon as if to say, sample
me, sample me!
 
One of the methods used to predict possibilities for diamonds in a lamproite is to test the chemistry of specific minerals found in the rock and then compare the chemistry of these minerals to the chemistry of diamond inclusion minerals (minerals actually found inside of diamonds). One particular mineral that is often used is chromite. After processing some samples from Endlich Hill in the center of the Leucite Hills, we recovered some chromite. The chromites had favorable chemistry and indicated that those particular lamproites originated at a depth where diamonds were formed. Thus, this suggested to use the Leucite Hills represented a great target for diamonds. Later, some researchers from the University of Wyoming confirmed my results.


Anthill with a couple of gem-quality pyrope garnets from
Butcherknife Draw
So, you would think that the legislature would provide the Survey with a little extra cash to do some serious testing. However, this is where we ran into problems. First one has to convince the State Geologist to ask for the money. State geologists are politicians. At least in Wyoming, they have to declare political affiliation. This is a serious problem when considering science as politicians like to modify science to fit their agenda. We all know how much we can trust politicians whether republican or democrat. 
If anyone compared personality traits of mass murderers, politicians and used car salesmen, there likely are distinct similarities. So, we found this to be a problem. If you can convince the state geologist there is something of value in a project for him or her, you next have to find a subtle way to show a legislator, governor and congressman he or she might be able to turn a project into a way to collect money under the table. So the project died with no funding.

Boars Tusk at sunset
 
Anyway, one can still visit the Leucite Hills and the Butcherknife Draw area to the southwest and search anthills for pyrope garnet, chromian diopside, peridot and even diamond. Its a beautiful place with some nifty wild horses.












Mt Gytha, Noonkabob lamproite field, Western Australia. Note the large layered sandstone xenolith in the side of this lamproite volcano.
 
And if you can dig the olivine out of the lamproite, or take them
from an anthill covered with fierce ants, this is what one can do - produce a nice,
faceted, peridot gemstone. Can you imagine, these were examined
by geologists for more than 100 years, even described in professional papers
and books, yet nobody ever noticed they were mostly gemstones.
A few things I immediately noticed when I visited Australia is their rabbits hop higher and are much larger than Wyoming's - they also spoke a strange dialect of English - but after a few Aussie beers, it didn't matter. Here is the Argyle diamond mine as it appeared in 1986. It is much deeper now.
 



Olivine lamproite from Black Butte, Wyoming. Note the large olivine crystal in the sample - about 0.4 inch across.

 
Lamproite breccia, Wortmans dike, Leucite Hills
Lamproite scoria from Zirkel Mesa in the Leucite Hills.
A diamond in the rough - wild horses in the Leucite Hills. It is tragic, but the BLM has done its best to eradicate these horses. Even still, a few of them survive.
 


Leucite Hills from the south. One can never have enough volcanoes
Emmons Mesa, Leucite Hills

Kind of an ugly, uninteresting rock wouldn't you say? When this olivine lamproite was discovered at Argyle, Australia, it yielded some of the richest diamond ore on earth. I collected this particular sample from a zone that had an average grade of 6.8 carats per ton.
Badger's teeth lamproite breccia, Leucite Hills.



Pegmatitic lamproite from Walgidee Hills, Australia.
 

Diamond-bearing olivine lamproite from the Ellendale field, Australia
 

 

 


Chocolate diamonds from the Argyle lamproite, Australia. Are there similar
diamonds in a hidden lamproite pipe(s) in the Leucite Hills? We may never know.
Ellendale 9 diamondiferous olivine lamproite, Australia


Typical leucite lamproite with considerable mica, Leucite Hills.


 


Diamond Hunter, Gem Hunter, Gold Prospector and Geologist - Hey, that good-looking guy was me mapping in the Leucite Hills.

Thursday, September 15, 2011

The Search for Diamonds In the Leucite Hills

Note that all the members on our field excursion to the outback
found the only two boab trees for hundreds of kilometers around
and at temperatures of 120 degrees F and above, shade was
at a premium.
I ended up looking for diamonds in the Leucite Hills: one indicator mineral for diamonds in lamproites is olivine. There is a correlation between the amount of olivine in lamproite and the possibility of finding diamonds that was recognized by Australian geologists several years ago.

When I visited the Ellendale lamproite field in Western Australia in 1986, some things were notable - first, it was hotter than Phoenix (but not by much). And the second was that there were a lot of leucite lamproites in the district that were similar to those in the Leucite Hills.

But then there were a couple of other interesting rocks in the Ellendale field that were not exposed at the surface. This was because these particular rocks had considerable olivine. The olivine serpentinized over geological time resulting in the breakdown of the rock. These olivine rich rocks were also diamond-bearing rocks that were buried under thin layers of soil in the outback.
The Ellendale 7 pipe hidden under a few inches of dirt, but
has enough diamonds to mine.
These hidden lamproites were found under a few inches to a few feet of dirt because of the serpentinization process (serpentinization produces some magnetite that can be detected by magnetic geophysical surveys). Over the years, when I was at the Wyoming Geological Survey, I periodically had prospectors stop in my office with possible apple- and emerald-green jade specimens. They were curious as to why their jade was so soft. I would get out a magnet and show them that the samples contained pockets of magnetite (the rock was magnetic), and this was due to serpentinization that results in some iron separating from the magnesium silicates and reducing to magnetite. The remaining magnesium silicates are relatively soft.
Black Rock butte lamproite plug seen in the background.
But this magnetite is strong enough to result in a magnetic signature over a hidden lamproite pipe. And soon, an Australia company searched the Ellendale field and found hidden lamproites. The hidden lamproites all contained diamonds, unlike the lamproites exposed on the surface, and this was because of the correlation of the olivine, serpentinzation and diamonds.

So when I returned to the Leucite Hills, I started searching for olivine. This is how I made the discovery of more than 13,000 carats of peridot in just two anthills at Black Rock.

Then I started sampling some of the olivine leucite lamproites in the area as well as volcaniclastics for diamonds and indicator minerals. AND we recovered diamond-stability chromites from lamproites in the northeastern portion of the Leucite Hills. Diamond-stability minerals have the same chemistry as mineral inclusions found in diamonds - thus, it is thought that such minerals formed at depths in the earth where diamonds exist. Finding such indicator minerals suggests these rocks originated within a portion of the earth's mantle that has diamonds - so it is very likely one will find diamonds in the Leucite Hills (if only someone will seriously look).

But let's use Ellendale as an example. The real rich olivine lamproites in this area are also going to be serpentinized and the host rocks hidden. Where? If I were to search this area, I would start with aerial magnetic/conductivity surveys and start mapping out geophysical anomalies in the northeastern portion of the Leucite Hills. I would bet that there are hidden lamproites in this portion of the field and possibly further north under some of the dune fields.
Ellendale field with termite mound in foreground

So why doesn't somebody look? Good question. I tried to get the Director of the WGS to ask the legislature for funding for an airborne survey before I realized this guy was not in this position for the science. So, it never got done. Over the years while I sampled and mapped this region, I did recover one excellent, transparent micro-octohedron with surface trigons. Was it a diamond? Who knows?





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Thursday, February 12, 2009

Self-Guided Field Trip to the Leucite Hills & BUTCHERKNIFE DRAW, SW WYOMING in seach of Gemstones

LEUCITE HILLS LAMPROITES - SOURCE OF PERIDOT and possibly DIAMONDS

The Leucite Hills, north of Rock Springs, Wyoming, are a great place to visit and examine the physical geology related to rare volcanic eruptions. The rare volcanics in this region are known as lamproites - ultrapotassic mafic volcanic rocks that are closely related to kimberlites. These are so rare that some of the volcanics were originally identified as 'Wyomingite', until it was discovered years later that similar rocks had been found in Western Australia and were also given local names in Australia. Thus in the 1980s, it was decided that all such lamproites would be named by their mineralogy - e.g., Wyomingite would be better known as a phlogopite lamproite.

This volcanic field has 22 flows, dikes, necks, plugs, cinder cones and pumice cones along the northern flank of the Rock Springs uplift. The flows are thought to range from 3.1 to 0.9 million years in age.

The volcanic rocks are lamproite. These are some of the rarest rocks on earth and classified as ultrapotassic to ultrabasic with 42.65-56.34% SiO2; 2.52-12.66% K2O; and 5.8-12.75% MgO. Individual flows are limited and typically 50 to 122 ft thick. They include vesicular lavas, scoria, intrusive breccias, lapilli tuffs, tuff breccias and agglomerates of diopside-leucite-phlogopite-lamproite, diopside-sanidine-phlogopite-lamproite, and diopside-madupitic lamproite. Vents are associated with most flows, although vents are not conspicuous at South Table Mountain, North Table Mountain, Black Rock or Hatcher Mesa. Either the vents were removed by erosion, or buried by lavas which is not uncommon for lamproite eruptions.
Microscopic studies show the rocks are unusual. They have diopside, phlogopite, titanian-potassium-richterite, leucite, sanidine, wadeite, priderite, and/or olivine, with minor apatite, perovskite, ilmenite, armalcolite and spinel. Typical 'kimberlitic' indicator minerals are rare although diamond-stability chromite was found in flows in the northeastern part of the field.

Some common xenoliths (foreign rock fragments) are found in lavas. Most are fragments from the underlying Green River Formation. Granitic xenoliths are common and some arkose, tuffaceous sandstone, argillite, siltstone, gabbro, and anorthosite are reported. Cognate xenoliths include lamproitic fragments of earlier flows. Phlogopite-chromite harzburgite, orthopyroxene amphibolite, clinopyroxene-rich pyroxenite, and mica-rich xenoliths are described.

Zirkel Mesa baked zone (red) where the overlying lava
baked the underlying mudstones and shales.
Cognate xenocrysts include olivine with reaction rims of phlogopite, chromite with similar reaction rims and green spinel. Sediment samples collected adjacent to Endlich Hill by the author yielded one pyrope garnet. Considerable amounts of gem peridot was collected around Black Rock by the author. For those hunting gemstones, pay attention to any flow that contains olivine and search the rocks and sieve nearby soils!

The field trip log begins at the BLM parking lot at the north end of Rock Springs along Highway 191.

Cumulative
mileage from
Rock Springs (with Description) 0 miles
-  BLM parking lot on Highway 191.
8 miles - Approximately 8 miles north of the BLM office, note the Tri-territory road #216. Turn east.

9 miles - Cross old railroad grade and turn north. This grade was a captive railroad spur of U.S. Steel Corp which used the spur to ship 5 million tons of taconite pellets/year from the Atlantic City iron mine near South Pass. The Geneva Steel blast furnaces near Provo Utah. The mine and spur were active from 1962 to 1983.

12 miles - Continue left at "Y" in road.

22 miles STOP 1.  Fifteen Mile Knoll is a low-lying hill north the road intersection. The hill is capped by pediment gravel with pebbles of quartzite, schist, sandstone, metagabbro, epidotite, jasper, and agate. North of Fifteen Mile Knoll is a prominent volcanic neck-Boars Tusk. Matthews Hill, another volcanic neck forms a low-lying hill to the right of Boars Tusk. Behind Boars Tusk on the horizon, are the snow caps of the Wind River Mountains. Directly behind Boars Tusk are dunes of the Kilpecker Dune field which lie on Wasatch Formation sediments. The Kilpecker dune field lies on a major active shear zone in sedimentary rock. The Nitch Gulch oil field can be seen NE of the dunes, and a little farther NE is Steamboat Mountain with a visible cinder cone on Green River Formation rock. Approximately due east is North and South Table Mountains: lamproites located on Fort Union Formation shale and sandstone. South of Table Mountain is Endlich and Hague Hills on Almond Formation (Cretaceous) sediments.

Endlich Hill is composed of olivine orendite and is a good target for diamond. The rocks have more than one population of olivine. In addition to xenocrysts and microphenocrysts, the lamproite has anhedral olivine mantled by phlogopite (out of equilibrium with the host magma) and represent upper mantle xenocrysts.

To our west, the promient white, Green River Formation supported cliffs of White Mountain are visible. Pilot Butte, the western-most lamproite in the volcanic field, can be seen rising above White Mountain. Madupitic lavas from Pilot Butte lack in chrome spinel and suggest this lava to be a poor target for diamond. Moving on, take the east fork of the Y prior to turning north at first trail to Boars Tusk.

25 miles - Matthews Hill is a remnant of a volcanic neck that forms a low, rounded knoll rising 40 feet above the surrounding terrane.

28 miles STOP 2. BOARS TUSK. Boars Tusk forms a prominent volcanic neck of wyomingite (phlogopite-leucite-lamproite) rising 300 feet above the valley floor. The neck is an agglomerate with abundant Green River and Wasatch Formation xenoliths & autobreccia fragments of lamproite. In addition some granitic xenoliths are found. Continue east towards Table Mountain.

37.5 miles Continue past the Westpine Canyon oil field turnoff on the right & pass Table Mountain on the south. Table Mountain consists of North Table Mountain, Middle Table Mountain, and South Table Mountain. North Table Mountain is a volcanic mesa capped by a single flow that exhibits well developed flow layering.

On the southern margin of North Table Mountain is a small volcanic plug known as Middle Table Mountain. South Table Mountain is the prominent mesa behind (southeast) North Table Mountain. Olivine phenocrysts in the rock led to this rock being named olivine orendite.
The lamproites at South Table Mountain are MgO-rich due to presence of phenocrystal and xenocrystal olivine.

48 miles Turn left onto County Road 83 to Steamboat Mountain.

51 miles STOP 3. STEAMBOAT MOUNTAIN Overlook of Leucite Hills. Looking south (L to R) are Black Rock, Spring Butte (in the distance is Black Butte), Zirkel Mesa with its five cinder cones, Hatcher Mesa (flat top butte in front of Emmons Mesa) and to the right is Deer Butte on the skyline (Aspen Mountain lies between Emmons & Deer Buttes on the skyline). Emmons mesa is made up of a cone and two flows.

Rocks from Streamboat Mesa are vesicular flows. Only sedimentary xenoliths have been found here. A glassey sample of wyomingite (lamproite) from this mesa yielded 12.66% K2O indicating this lava is one of the most potassic lavas in the world. Backtrack down the mountain and turn east (left) on Tri-territory Road.

56.5 miles - Intersection, turn east.

59 miles Turn on jeep trail to right towards Black Rock.

60 miles STOP 4. BLACK ROCK. Black Rock forms the easternmost extent of the Leucite Hills volcanic rocks. Black Rock to consist of a basal pyroclastic olivine orendite tuff overlain by a lava flow. The top 80 feet of the mesa consists of alternating layers of vesicular and nonvesicular lava. In all probability, the mesa represents an lava flow which obscures hidden vent facies lamproites.

Half of the >13,000 carats of facetable
olivine (peridot) recovered from two ant
hills by the author. Too small to cut?
Look at some of the faceted gems produced
from these stones.
Small nodules of dunite have been found at Black Rock. Olivine occurs in the rock. During a cursory search for evidence for diamonds in this area, the author recovered two anthills with 13,000 carats of olivine. Most was peridot and gem-quality. The ants were able to collect material up to 12 mm in length and soils between the anthills and Black Rock are rich in olivine. Some olivine up to 0.5 inch across was found in the rock at Black Rock. Much of the material is facetable. Backtrack on jeep trail.
61 miles - Turn west (left) on Tri-territory road and continue south past Spring Butte. The butte is a compound volcanic center with six cinder cones, at least six flows, and three dikes. The cones consist of welded clastic flows with ribbon and breadcrust bombs.

71 miles - Deadman Gulch. Turn right towards Hatcher Mesa. Caution, this is a jeep trail!

79 miles - STOP 5. HATCHER MESA. Hatcher mesa is an eroded remnant of a small ponded lava flow. The center of the mesa is depressed by 6 feet relative to the edges indicating the surface sagged and the lava flowed back into the vent. The vent is not conspicuous. The mesa contains the most abundant and diverse assemblage of xenoliths and xenocrysts in the Leucite Hills. Petrographically, these rocks consist of microscopic clinopyroxene, phlogopite, apatite, and iron-titanium oxide with uncommon grains of olivine, in a groundmass of leucite, clinopyroxene, potassium richterite, apatite, priderite, wadeite, and glass.

First faceted peridot produced from
olivine from anthills at Black Rock.
Even though these were known for
>100 years, no one ever bothered to
look at the quality of the stones. The
author recognized in 1997 that the
majority were flawless!

Olivine grains from Hatcher Mesa are comparable to olivines in lherzolite and harzburgite in kimberlite.

84 miles - Return back to the Superior Road. Turn south towards Zirkel Mesa.

94 miles - Turn north on jeep road leading to south flank of Zirkel Mesa.

96 miles - STOP 6. Zirkel MESA QUARRY. Highly vesicular pumaceous leucite lamproite is exposed in the quarry showing flow banding with common country rock xenoliths. Locally, the lava has a ropey appearence.

Zirkel Mesa is the largest of the exposures in the Leucite Hills. Flow thicknesses vary from less than 3 feet to as much as 50 feet.

Six cinder cones which rise more than 240 feet above the mesa surface. These coelesce into a single, volcanic-capped plateau. K-Ar dates from two micas yielded 1.25 million year old ages.

Group of faceted peridot from Black Rock anthills. Note the
color variations and clarity. Although small, much larger
material should be found in the soil as well as outcrop in this
area. The presence of olivine in these lamproites and
recovery of diamond-stability chromite suggests the liklihood
of hidden diamondiferous lamproites in this region.
It was estimated in 1912 that 197 million tons of potash were available in the Leucite Hills. Liberty Potash Company quarried wyomingite from Zirkel Mesa during the war and operated a plant in Green River to produce KCl for fertilizer.






This is the end of the field trip - Have a nice day.


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Recommended Reading
Carmichael, I.S.E., 1967, The mineralogy and petrology of the volcanic rocks from the Leucite Hills, Wyoming: Contributions to Mineralogy and Petrology 15, p. 24-66.
Coopersmith, H.G., Mitchell, R.H., and Hausel, W.D., 2003, Kimberlites and lamproites of Colorado and Wyoming, USA: Field Excursion Guidebook for the 8th International Kimberlite Conference, Geological Survey of Canada, 24 p.
Hausel, W.D., 2006, Geology & Geochemistry of the Leucite Hills Volcanic Field, Wyoming Geological Survey Report of Investigations 56, 71 p.
Hausel, W.D., 2009, Gems, Minerals and Rocks of Wyoming. A Guide for Rock Hounds, Prospectors & Collectors. Booksurge, 175 p.