Thursday, February 12, 2009

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


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 hot
lamproite 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.


mileage fromRock 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.

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.



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.

Several gem-quality peridots (raw) found in the Leucite Hills of Wyoming.

Faceted peridot surrounded by raw peridot from the Leucite Hills of Wyoming. Olivine was known to occur in the Leucite
Hills for more than a century, but no one bothered to look to see if any of these material was gem-quality. Some well
known geologists and PhDs had looked at this, but missed the fact that nearly all of the olivine is gem-quality peridot.
All one has to do is open their eyes!
And if you would like to find some gemstones, I tell you
how I found dozens of gemstone deposits and even tell
you exactly where you can go (using GPS coordinates)
to find some gemstones and likely found some new gemstone
occurrences. All of this is in my recent book available at Amazon