Thursday, March 25, 2010

Cedar Rim Opal Deposit, Granite Mountains, Wyoming

In 2003, I began searching for opal in central Wyoming. My search began because of a rockhound from Riverton who had indicated some pieces of opal had been reported in this area. After searching the geological literature, I was surprised that some geologists had noted (in passing) that some rock in the area was opalized, but there was no investigation into these reports or the extent of the deposit. The reports suggested that this was a very isolated occurrence.

When I began field work, it soon became apparent that the Cedar Rim field enclosed one of the larger opal fields in North America. Road cuts exposed Tertiary (the Tertiary was a geological period that began 65 million years ago and ended with the last major global cooling event that sparked the advance of major ice sheets at about 1.2 million years ago) sedimentary rocks that had considerable opal - apparently one of the larger opal fields in the world. Following investigations, I released a report to the public which resulted in one of the largest claim staking rushes in Wyoming during the past few decades. Even so, this legal activity was attempted to be stopped by the BLM, which wanted to protect the area even though the agency had no idea where in the state the deposit was located.

The Cedar Rim field was one of many gemstone discoveries made in Wyoming since 1975. Prior to 1975, the state was considered to be relatively unmineralized in gems other than jade. Following several geological studies by the author, it became apparent that Wyoming has a variety of gemstones including some world-class deposits. Notable are gems associated with mantle-derived kimberlites, lamproites and lamprophyres, some of which contain gem-quality diamond, peridot, ‘Cape Ruby’ (pyrope garnet), ‘Cape Emerald’ (chromian diopside), almandine garnet and spessartine garnet.


Other discoveries were made in amphibolite-grade metapelites including a world-class deposit of iolite. These discoveries suggest the metamorphic conditions and later metasomatic events provided favorable environments for crystallization of a number of gems that include ruby, sapphire, iolite, and kyanite. The geological environments are thought to be favorable for other alumino-silicate gemstones including andalusite and sillimanite. Other gemstones include aquamarine and heliodor in pegmatites – most notable was the Casper Mountain pegmatite as well as additional iolite deposits in the anorthosite complex.

Other gem material discovered in recent decades include varisite. The geological environment in the area of the varisite suggest a potential for discovery of emerald. Other gem materials found in Wyoming include gold, platinum and palladium nuggets.

OVERVIEW
Following field reconnaissance at Cedar Rim in the Sand Draw oil field of central Wyoming, it became apparent that a major opal deposit was in the area. I became interested in this area due to a lead from a local rock hound. A later literature search showed that geologists of the US Geological Survey had identified opal in the area, but paid little attention to size, extent, or economic potential: the opal was mentioned in passing.
Location Map of the Cedar Rim Opal Field, Wyoming
Following preliminary work, it was apparent that one of the largest opal fields in North America was located along the side of State Highway 135 between Sweetwater Station and Riverton in central Wyoming. Even more incredible, graded roads in the field cut through the deposit exposing thousands of cobbles and boulders of opal in road cuts that were left uninvestigated by the road builders and oil field workers. A later pipeline cut through the field exposing considerable opal and agate that also went unnoticed.


The Cedar Rim deposit, based on size and extent, could become a major source for opal, agate, and decorative stone for many decades. However, like any colored gemstone, success will require marketing skills and an understanding of geological concepts and cooperation from government agencies (which have been less than cooperative to date).

Hundreds of nodules in road cuts along graded roads in the Cedar Rim area in the White River Formation contain opal. (Left) Silo sits on tons of opal. Essentially all rounded boulders and cobbles in the photo and in the photo to the right are opal coated by caliche. Close examination of the caliche (below left), reveals opal.



The variety of opal includes common, fire, and trace precious opal. The deposit also contains a variety of agates such as the popular ‘Sweetwater agate’ as well as large massive outcrops of rock with opal, secondary agate and quartz that is relatively fracture free and excellent material for decorative stone.


Much of the deposit remains hidden and there has been no subsurface exploration. Some specimens collected by the author exhibit weak color play, a few with strong color play, along with transparency and translucency, thus the possibility of significant undiscovered veins of precious must be considered.

OPAL
Opal (SiO2.nH2O) is a precious to semi-precious stone that is classified as a amorphous mineraloid with 6 to 10% water. A mineraloid is a mineral-like substance that does not yield an exact chemical formula; and like volcanic glass, shows no crystallinity. It has a hardness of 5.5 to 6.5 and produces fragile to relatively durable semi-precious gemstones. In general, the higher the water content, the less stable the opal.
Decorative stone tile cut from massive silicified material at Cedar Rim. Specimen contains limestone clasts silicified by blue agate, quartz, chalcedony and common opal.




Three categories of gem opal include: (1) common, (2) fire, and (3) precious. Precious opal is the most valuable and includes two varieties - one with black and one with white matrix.

Precious black opal is considered to be the most valuable by gemologists because it yields an internal color play enhanced against a dark matrix. Precious white opal is considered to be less valuable, as the internal color play is less attractive against a white opaline matrix. Even so, precious white opal has extraordinary inherent beauty and may be considered more attractive depending on individual taste.

Fire opal, which may or may not have color play, may be translucent to transparent and is red, orange-red, orange and/or yellow. If transparent to translucent, fire opal is often faceted. Opaque to translucent fire opal is usually cabochon-grade. Common opal, which is translucent to opaque, is milky white and may contain streaks of blue, red, brown, or yellow, and is cut into cabochons. Hyalite, is colorless, to transparent opal and found as globules that resemble drops of water. All varieties of opal have been observed at Cedar Rim either as massive material or in trace amounts.

Since this deposit is extensive, covering hundreds of acres of land, the potential for significant discoveries of high-quality precious opal is considered high, especially since little surface and no subsurface exploration has occurred. Many similar opal deposits could remain undiscovered in Wyoming, based on geology. Discoveries of opal in Australia led to further discoveries at depth of narrow seams (or veinlets) of precious black opal. Similar discoveries are likely at Cedar Rim if exploration continues on the surface and beneath the surface.

GEOLOGY & GENESIS
Most primary opal deposits occur in sedimentary or rhyolitic volcanic rocks. Opal is less common in basalt and other rock types. The majority of the world’s precious opal is mined in Australia where opal is occurs in Cretaceous marine sedimentary rock of the Great Artesian Basin of New South Wales, Queensland, and in South Australia (Keller, 1990). It is found in joints in deeply weathered Proterozoic gneiss of the Musgrave-Mann Metamorphics in the Granite Downs of northwestern part of South Australia (Barnes and others, 1992). The presence of opal in gneiss appears to be a result of similar processes responsible for sedimentary-hosted opal.

Wayne Sutherland and Dan Hausel examine cobble of opal
from the Cedar Rim deposit.
Opal has a low specific gravity (1.9 to 2.25), conchoidal fracture, and tends to craze (lose water and fracture). As a result, placer opal deposits are unheard of. Preservation is unfavorable where surface weathering has been intense over long periods of time. Alluvial opal deposits are rare and restricted in size and extent.

When found, alluvial opal lies immediately on in situ deposits. Exposure to dry environments and heat, causes opal to lose water, which results in opaque, chalky-white fractured crusts and masses of caliche that replace the opal. Opal cannot survive deep burial nor structural adjustments (movement along faults). Because of durability limitations, opal, whether sedimentary- or volcanic-hosted, is geologically young (Darragh and others, 1976).


Barnes and others (1992) cite various Australian studies that indicate much Australian opal is Early Cretaceous due to digenetic changes related to deposition of the Bulldog Shale. Darragh and others (1966) estimate possible opal deposition rates vary from 0.3 inch of thickness in 5 million years, to a maximum of 0.3 inch in 200,000 years. However, Eckert (1997) suggests that deposition may be considerably more rapid citing partial replacement of wooden fence posts and animal bones by opal.

Sedimentary-Hosted Opal. Sedimentary-hosted opal deposits are formed by movement of silica-rich water through sedimentary hosts. The water flow can be downward, upward, or laterally.

Sinkankas (1959) notes that basalt flows in Washington appear to have acted as confining layers for circulation of silica-rich water in underlying sediments. To form opal deposits, a source for readily soluble silica is necessary. Known sources for soluble silica include volcanic ash beds (which are common in Wyoming), siliceous micro-fossils, digenetic changes associated with bentonite formation (another common feature in Wyoming), or in situ kaolinization of detrital feldspar. Deep chemical weathering of rocks such as pyroxenite and serpentinite have also been suggested as a potential source for silica for some opal deposits (Eckert, 1997).


Sedimentary-hosted opal is found in some Australian deposits to depths of 130 feet. Where found, the host rocks include conglomerate, sandstone, claystone and bentonite. The sites of opal deposition include pore spaces, joints, fractures, shrinkage cracks, partings, bedding planes and cavities (Barnes and others, 1992). In many places, the site of deposition coincides with the bottom of deep weathering profiles and is accompanied by intense bleaching of country rock (Kievlenko, 2003).

Darragh and others (1966) demonstrated that neutral to slightly acidic groundwater at temperatures of 20˚ to 25˚C, similar to that currently found in Australia’s opal fields, could dissolve as much as 100 ppm silica. Field evidence suggests the opal formed in rock openings filled with pockets of groundwater under slow evaporation rates. It is thought the water remained undisturbed for long periods of time and allowed for formation of colloidal silica spheres followed by slow settling of silica in regular arrays. As evaporation proceeded, a steady-state balance was maintained by hydrostatic inflow of additional silica-laden water that resulted in continual accumulation of opal. Field evidence shows several generations of silica. Soluble salts which could potentially disrupt silica deposition are interpreted to have been removed by upward diffusion – and is expressed by the presence of discontinuous gypsum veins above many opal deposits. Overall, deeply weathered rocks, combined with arid climate in Australia’s opal fields appear to be essential components for the formation of precious opal, and siliceous cap rocks (Barnes and others, 1992).

Darragh and others (1966) suggested that opal formed under near-surface conditions (depths of 15 to 130 feet) to account for the necessary steady-state conditions. The arid climate in Australia’s opal fields confined pockets of groundwater beneath impermeable bentonite beds at shallow depth for millions of years. Opal is often found filling voids, rock matrix pore spaces, and joints. Large interconnected cavities are generally filled with common opal and the best quality precious opal is found in small isolated cavities. The rarity of steady state conditions necessary to form precious opal explains its rarity compared to common opal.

Volcanic-Hosted Opal. Volcanic-hosted opal appears to be either related to post-volcanic hydrothermal activity, or to silica-rich waters derived from surface weathering processes similar to sedimentary-hosted deposits. The difference between these two processes may be obscured by deep surface weathering. Opal hosted by volcanic rock generally contains more water than opal formed in shallow sedimentary environments. Consequently, volcanic-hosted opal (with exceptions, i.e. Mexican fire opal) is generally less stable and often exhibits a greater propensity for crazing (Barnes and others, 1992).

Opal may be deposited in vesicles in volcanic rocks from siliceous solutions at temperatures higher than groundwater temperatures. Deposition under these conditions results in tiny silica spheres with close-packed arrays that exhibit almost no interstitial voids. These opals tend to be transparent, exhibit no noticeable grain pattern, and show diffuse bands of color play as the stone is rotated in light (Darragh and others, 1976). Opal associated with siliceous sinter or geyserite is often attributed to post-volcanic hydrothermal activity.

CEDAR RIM OPAL
The Cedar Rim deposit in Wyoming consists of vast amounts of white to very light-blue translucent to opaque common opal, with significant amounts of translucent to opaque yellow, yellow-orange to orange fire opal, and significant clear, transparent hayalite opal. Only trace precious opal (both white and black) have been identified in samples collected by the author. Based on the fact that all varieties of opal have been identified and much of the field remains unexplored, the potential for discovery of significant seams of precious opal must be considered. During this study, opal was identified within 12 sections of land over hundreds of acres, and large amounts of agate were also identified, including the source beds of the popular Sweetwater agate. In places opal beds are between a few feet to more than 50 feet thick and primarily found as siliceous caps. Additional material is also exposed along a pipeline that cuts through the field.

Variety of agates and dendritic Sweetwater agates from the Granite Mountains, Wyoming.

Location. Cedar Rim is located 25 miles south of Riverton along Sand Draw Road. The deposit is found along Cedar Rim Draw near the northwestern margin of Beaver Rim. Beaver Rim is located in the western portion of the Granite Mountains uplift. The nearest towns are Riverton to the northwest, Lander to the west, and Jeffrey City to the southeast. Much of the deposit is located on the US Geological Survey Lander 1:100,000 sheet, with some reported opal further east on the Rattlesnake Hills 1:100,000 sheet.

History. The first report of opal in this area was by Sinclair and Granger (1911). In their report on Eocene and Oligocene sediments, they depicted opal in a cross-section near Wagon Bed Spring (SW section 34, T32N, R95W) and noted opal and chalcedony were repeatedly observed as replacements of soft tuffaceous limestone at the top of Oligocene sediments capping Beaver Rim as well as on several buttes to the south. In places, limestone forms a layer containing masses of white chalcedony and opal nodules enclosed in calcareous crusts. The presence of cylindrical pipes of silica, cutting through some limy layers was noted.

The source for both the limestone and silica was interpreted by Sinclair and Granger (1911) to be from underlying ash beds. The silica was thought to have mobilized in percolating water which surfaced in springs. Some chalcedony and opaline cement was also described in silicified arkose lower in the section, possibly in what is now known as the Wagon Bed Formation.

Opal at Beaver Rim was later noted by Van Houton (1964). Van Houton described opal with chert and chalcedony in the Wagon Bed Formation, the volcanic facies of the Beaver Divide conglomerate member of the White River Formation (now the Wiggins Formation), the White River Formation, and the Split Rock Formation. Numerous chert nodules and silicified zones are found in both the White River and Split Rock Formations. Locally opal and yellowish-brown to light olive gray chert, in masses up to 3 feet in diameter, are found in mudstone of the Wagon Bed Formation in the vicinity of Wagon Bed Spring and northeastward as far as the Rogers Mountain Anticline. Irregular chert masses up to 15 feet long are found in the Kirby Draw syncline (which extends northwest from NE section 31, T33N, R94W to section 14, T33N, R95W).

Cabochons of common opal from Cedar Rim field.
At Green Cove (section 35, T31N, R96W) Van Houton (1964) noted that the uppermost 20 feet of the lower part of the Wagon Bed Formation contained altered yellowish- to light-gray, distinctly bedded tuff with abundant siliceous nodules up to few inches in diameter. These were accompanied by 6- to 12-inch thick chert beds and rock formed of quartz, dolomite and opal.

The Wiggins Formation, which forms a wide channel fill in the basal White River Formation in this area, is characterized by debris derived from the Yellowstone-Absaroka volcanic field. This ranges from sand-sized material to boulders 8 feet long. Within this unit, sandy limestone lenses up to 5 feet thick have been partly replaced by irregular fibrous chalcedonic chert and massive gray opaline silica containing irregular tubes and pores: many of which are filled with calcareous montmorillonite clay.

South of the Conant Creek anticline (sections 3 & 4, T31N, R94W) Van Houton (1964) described a prominent 160-foot high south facing escarpment. In the lower 50 feet of the upper part of the White River Formation, local layers of light blue to greenish-gray, limonite-stained, brittle opaline chert were described to contain rounded pellets up to 3 mm in diameter. Farther east in section 14, T32N, R93W, the lower greenish-gray tuffaceous mudstone of the White River Formation contains several 2- to 4-inch thick layers of slightly calcareous opaline chert. It was noted that the mixed chalcedony and opal layers contained 1- to 2-mm diameter ellipsoidal to sub-spherical pellets. Both the opal/chalcedony pellets and the rock matrix contain abundant ooliths and rounded thick-rimmed particles.

Within the Split Rock Formation, Van Houton (1964) found irregular domal structures several feet in diameter formed of sand adhering to an opaline skeletal structure that resembled tuffa or algal mats. These occur in well-sorted calcareous sandstone southeast of Devils Gap in section 5, T30N, R95W. It was noted that thin beds of chert, irregular concretions of opaline silica, and fibrous siliceous aggregates were commonly found along Beaver Rim in the uppermost part of the Split Rock Formation. These are hosted by 2- to 6-inch thick light-gray limestone interbedded with thin calcareous tuffaceous sandstone.

Reconnaissance investigations by the author were designed to examine the extent and the varieties of opal in the Cedar Rim Draw deposit in order to determine if there is any economic potential. Based on these investigations, the Cedar Rim opal deposit is one of the largest opal deposits in North America. If such a giant opal deposit can remain relatively hidden and unknown for such a long time, developing geological and exploration models to identify similar opal deposits should lead to additional discoveries. Reports of opal in the state suggest that similar deposits occur elsewhere (Hausel and Sutherland, 2000). In addition to the sedimentary-hosted opal at Cedar Rim, precious opal was also described in volcanic rocks in the Absaroka Mountains in northwestern Wyoming (J.D. Love, personal communication, 1989). This study resulted in the identification of an enormous opal deposit extending over portions of at least 12 sections. More detailed sampling and mapping would likely lead to additional discoveries.

Cedar Rim Draw Opal. Based on mapping by Van Houton (1964), opal samples collected by the author were from White River (Oligiocene) and Split Rock Formation (Miocene) marlstone, claystone, siltstone, sandstone, conglomerate and boulder facies. Opal was found in sections 25, 26, 35 & 36, T32N, R95W, sections 31 & 32, T32N, R94W, sections 5, 6, 7 & 8, T31N, R94W, and sections 1 & 12, T31N, R 95W. Numerous samples were collected during this study including giant opals of 25,850 carats (11.4 lbs), 57,100 carats (25.18 lbs) and 77,100 carats (34 lbs). I left other boulders of opal that obviously weighed much more than a hundred thousand carats!

Sketch map of the Cedar Rim opal field showing location of opal discoveries.


This vast field of opal at Cedar Rim potentially includes tens of thousands of tons of opal as well as extensive deposits hidden at shallow depth. The opals range from small cobble size nodules to large boulders encased in caliche. The caliche replaces of the opal as it weathers and devitrifies. Several varieties of opal were recovered and a description of the rock samples collected from the deposit includes:

(1) Opaque milky white to translucent common opal with localized fracture fillings of transparent clear opal. Some material includes light blue opal with minor black dendritic inclusions. Some is perfectly transparent and much exhibits a subtle color play with localized zones of stronger color play. Many opals are fractured but include large consolidated unfractured pieces of several carats.


Milky white to tawny common opal from Cedar Rim.

(2) Translucent light-blue opal enclosed by milky opal which in turn is enclosed by a narrow perfectly transparent and banded opal crust that exhibits a pleasant color play (bands of blue-yellow-violet red) when natural light is reflected from the specimen. These are enclosed in a thin rim of tan to pink quartz.

(3) Opal with milky quartz breccia and light gray to light blue translucent to transparent opal clasts and veins set in black opal to black chalcedony matrix. The black opal rarely exhibits color play.
Transparent opal (left), light blue opal (right), and precious opal (bottom) from Cedar Rim.

(4) Gray black to black translucent opal and agate. Some are similar to the Sweetwater agates described by Love (1970), and is likely the source bed for the Sweetwater agates. Some material was collected in place in section 7, T31N, R94W. Very minor play of colors was observed in a couple of the specimens. Much of the color play appears as a surface sheen with uncommon, tiny distinct rainbow bands within the opal along fractures. It is recommended that several of these be cut into cabochons to see if any play of color continues into the opal,

(5) At one location in the field, varicolored opal is common. This opal includes translucent fire opal as replacements and fracture fillings in silicified arkose. This was the first report of fire opal in Wyoming and the author discovered an entire hillside of this gemstone! Other opal in this area is milky white translucent to opaque with considerable opaque to translucent yellow and lesser opaque to translucent orange opal comparable to the Mexican fire opal. This opal is common capping a hill in sec. 25, T32N, R94W.
Fire opal from Cedar Rim.

CONCLUSIONS
The Cedar Rim opal deposit remains relatively unexplored. The extent of the deposit is poorly known and additional field work and trenching is recommended to determine the aerial extent and thickness of the opal deposit, as well as to search for seams of precious and fire opal. The source beds for the opal and agate are silica-rock ash beds. During field investigations of this deposit, the following discoveries were made by the author: (1) Cedar Rim is a giant opal field - one of the largest in the world and is found in portions of 16 square miles and likely extends beyond the area of investigation. (2) The first verified precious opal in Wyoming. (3) The first verified fire opal in Wyoming. (4) The first verified black opal in Wyoming. (5) Discovery of large massive opal-agate outcrops suitable for manufacturing tile and counter tops, and (6) the source beds of the Sweetwater agate.

Following discovery of this deposit along with world-class iolite-ruby-sapphire-kyanite gemstones deposits in eastern Wyoming, my work was halted by people at every level in the Democratic party in Wyoming. It was apparent that these bureaucrats did not want more jobs or resources to be found in the state. Even so, these gemstone deposits potentially host several hundred billion in gemstones and potentially several $US trillion in gemstones.

There are many other opal deposits in Wyoming - essentially all unexplored. As an example, my new book on Gold (in preparation), I describe many gold deposits around the region. One of these in the Bear Lodge Mountains has the following description:

Prospect; A 150-foot shaft was sunk 1.5 miles northeast of the intersection of the Warren Peaks road with the Bear Lodge truck trail. The dump contains malachite, iron oxide, cuprite and chrysocolla (Chenoweth, 1955). These minerals occur as fracture fillings in altered kaolinized porphyritic syenite(?). The copper minerals are associated with light-green and white opal in veinlets.



REFERENCES
  • Barnes, L.C., Townsend, I.J., Robertson, R.S., and Scott, D.C., 1992, Opal - South Australia’s Gemstone: Department of Mines and Energy, Geological Survey of South Australia Handbook No.5, 176 p.
  • Darragh, P.J., Gaskin, A.J., Terrell, B.C., and Sanders, J.V., 1966, Origin of precious opal: Nature, January 1, 1966, v. 209, no.5018, p.13-16.
  • Darragh, P.J., Gaskin, A.J., and Sanders, J.V., 1976, Opals: Scientific American, v.234, no.4, p.84-95.
  • Eckert, Allan W., 1997, The World of opals: John Wiley & Sons, Inc., New York, 448 p.
  • Hausel, W.D., and Sutherland, W.M., 2000, Gemstones and other unique minerals and rocks of Wyoming - a field guide for collectors: Wyoming State Geological Survey Bulletin 71, 268 p.
  • Hausel, W.D., and Sutherland, W.M., 2006, World Gemstones: Geology, Mineralogy, Gemology & Exploration: WSGS Mineral Report MR06-1, 363 p.
  • Keller, Peter C., 1990, Gemstones and their origins: Van Nostrand Reinhold, New York, 144 p.
  • Kievlenko, Eugenii Ya, 2003, Geology of gems: Ocean Pictures Ltd, 432 p.
  • Love, J.D., 1970, Cenozoic geology of the Granite Mountains, central Wyoming: US Geological Survey Professional Paper 495-C, 154 p.
  • Sinclair, W.J., and Granger, W., 1911, Eocene and Oligocene of the Wind River and Bighorn basins: Bulletin of the American Museum of Natural History, v. 30, part 7, p. 83-118.
  • Sinkankas, John, 1959, Gemstones of North America: Van Nostrand Company, Inc., New York, 675 p.
  • Van Houton, F.B., 1954, Geology of the Long Creek - Beaver Divide area, Fremont County, Wyoming: USGS Geological Survey Map OM 140 map scale 1:62,500.
  • Van Houton, F.B., 1964, Tertiary geology of the Beaver Rim area, Fremont and Natrona Counties, Wyoming: USGS Bulletin 1164, 99 p., map scale 1:62,500.

Black Opal from Cedar Rim.




Precious opal found in veins (Cedar Rim)
Rare blue Cedar Rim opal.


Fractured fire opal from Cedar Rim. Much of the opal and agate at Cedar rim is massive with few fractures. Only in the fire opal are fractures common.












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