OPAL & AGATE

I couldn't believe my eyes - opals everywhere!  Some weighed more than 100,000 carats sitting adjacent to the highway & along road cuts in an oil field north of Sweetwater Junction to Riverton.

Opals everywhere! all of the white cobbles in the photo are actually opals! 

I found beautiful agate, some jasper, tons and tons of common opal, tons of fire opal and a few specimens of precious opal all within a large area covering a few square miles. What lies beneath the surface? I found out in one location where a company had trenched to put in a oil pipeline and the rock chips from the trench were filled with opal and agate including the source rock for the famous Sweetwater agates. Dr. Dave Love of the USGS reported in his classic professional paper on the Granite Mountains that the source of the Sweetwater agates was unknown. But here is was - sitting right in front of me. In this field likely hundreds of millions in potential gemstones and it was all just sitting there - and after I reported it to the press, the BLM had a hissy fit - but it didn't matter. No one had the ambition to mine or market this material - so it may sit for decades before someone gets the ambition to market this material.

I began reconnaissance at Cedar Rim (also known as Cedar Ridge) in the Sand Draw oil field of central Wyoming. Based on a clue from a local rock hound, it was apparent that geologists & prospectors again overlooked another major mineral deposit. A later literature search showed that the US Geological Survey had identified opal in the area in the 1930s and 1940s, but paid no attention to the gem other than to mention it in passing.

 Location map of the Cedar Rim opal field, central Wyoming.

 

It became apparent that one of the larger opal fields in North America lay along State Highway 135 between Sweetwater Station & Riverton. Graded roads constructed to the field exposed thousands of cobbles & boulders of opal in road cuts. The opal had remained overlooked for a century even though millions of carats lay exposed! I kid you not, I found boulders of opal in these road cuts that were giganic and some weighed more than a few hundred pounds - all untouched and unexplored!

All boulders & cobbles in this photo are opal in the White River Formation. Note the large boulder next to the hammer. These are all exposed in a road cut.

The Cedar Rim deposit, based on size & extent, is one of the largest in North America - and possibly is the largest. As with any colored gem, success will require serious exploration, marketing, an understanding of geology, & cooperation from government regulators. However, as soon as the discovery was announced, regulators marked their territory.

The BLM claimed protection of an endangered flower that may not even be indigenous to the area & reportedly only found where cattle drop their waste. It was a time that would make any bureaucrat proud (& there were many standing knee deep in cow waste). I even had a BLM manager call me demanding to know the location of the deposit so they could withdraw the area before it was announced. I refused to cooperate. I knew that the deposit would have ended up in the hands of some manager's cousin, nephew, or uncle.  Even with the bureaucrats working their magic, many mining claims were staked on this giant deposit.

The geology of Colorado-Montana-Wyoming is very favorable for discovery of additional opal deposits. During the geological past, silica-rich volcanic ash erupted from the Yellowstone caldera and blanketed this region several times providing an excellent source for soluble silica for opal.

Precious opal from Cedar Mountain. Note that it occurs as secondary vein deposits mixed in common opal.

Opal (SiO2.nH2O) is a precious to semi-precious stone classified as an amorphous mineraloid containing 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 sign of crystallinity. Opal has a hardness of 5.5 to 6.5 & is fragile to relatively durable semi-precious gemstone. In general, the higher the water content, the less stable the opal.

Photo above - a large (>29,000 carat) common opal from Wyoming.

Three general categories of opal include: (1) common, (2) fire, & (3) precious. Precious opal is the most valuable. Precious black opal is considered to be the most valuable by gemologists because of an internal color play that is enhanced against the dark matrix. Precious white opal is considered to be less valuable as the internal color play is less distinctive against the white opaline matrix. Even so, this preference is based on taste.

 

Fire opal, which may or may not have color play, can be translucent to transparent red, orange-red, orange and/or yellow. If transparent to translucent, fire opal may be faceted: Opaque to translucent fire opal is usually cut into cabochons. Translucent to opaque milky white common opal may contain streaks of blue, red, brown, or yellow, & is also cut into cabochons. Hyalite, a colorless to transparent opal found as globules that resemble drops of water, is most often cut into cabochons. All varieties have been observed at Cedar Rim either as massive material or in trace amounts. Cedar Rim is extensive, covering hundreds of acres of land, thus the potential for significant discoveries of high-quality precious opal is high, especially since little surface and no subsurface exploration has occurred.

 

Most primary opal deposits are found in sedimentary or volcanic (rhyolitic) rocks. It is less common in basalt and metamorphic rocks. The majority of the world’s precious opal is mined in Australia where it occurs in Cretaceous marine sedimentary rock of the Great Artesian Basin of New South Wales, Queensland, and South Australia. There are many opal fields in eastern Australia- probably the most famous is Coober Pedy and Lightning Ridge with their underground homes and businesses that are described in my upcoming book on Gemstone Prospecting scheduled for publication in late 2014.

 

Opal has a low specific gravity, conchoidal fracture, and tends to craze (lose water and fracture). As a result, placer opal is unheard of. Opal 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.

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.   

The Cedar Rim deposit consists of vast amounts of white to light-blue opaque common opal, with lesser amounts of translucent to opaque yellow, yellow-orange to orange fire opal & some clear, transparent hayalite. Only trace precious opal has been identified in samples collected by the author. In that all varieties of opal have been identified in the deposit & much of the field remains unexplored, the potential for discovery of valuable seams of opal must be considered. During my study, I identified opal within 12 different sections (~1 mi2/section) covering hundreds of acres. Large amounts of agate were also identified including the source beds of the popular Sweetwater dendritic agate. In places opal beds are a few feet to >50 feet thick. Since there has been no subsurface exploration, the true thickness remains unknown.

 

Location. Cedar Rim occurs along Cedar Rim Draw near the NW margin of Beaver Rim. Beaver Rim is a topographic ridge that marks the northwestern edge of the Granite Mountains uplift. The nearest towns are Riverton (25 miles to the northwest), Lander further west, and Jeffrey City to the southeast. Much of the deposit is located on the US Geological Survey Lander 1:100,000 topographic sheet and on the Rattlesnake Hills 1:100,000 sheet.

 

Opal & chalcedony were observed replacing tuffaceous limestone at the top of the Oligocene sediments that cap Beaver Rim as well as in several buttes to the south. In places, the limestone has layers of massive white chalcedony and nodules of opal enclosed in calcareous crusts. The presence of cylindrical pipes of silica cutting through some limy layers was noted.

 

The source of the silica for the opal and chalcedony was interpreted to have come from underlying (silica-rich) volcanic ash beds. The silica was mobilized by percolating water which surfaced in springs. Opal with chert and chalcedony occurs in the Wagon Bed Formation (now known as the Wiggins Formation), the White River Formation, and the Split Rock Formation suggesting that the opal is widespread and potentially very thick.

 

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 are found in masses up to 3 feet in diameter in mudstone of the Wagon Bed Formation in the vicinity of Wagon Bed Spring and northeastward to the Rogers Mountain Anticline. Irregular opal and chert masses up to 15 feet long are found in the Kirby Draw syncline (which runs northwest from NE section 31, T33N, R94W to section 14, T33N, R95W).

 

At Green Cove (section 35, T31N, R96W) it was 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. These were accompanied by 6- to 12-inch thick chert beds and rock formed of quartz, dolomite and opal.

 

 Within this area, sandy limestone lenses up to 5 feet thick have been partly replaced by irregular fibrous chalcedonic chert and massive gray opal with irregular tubes and pores: many of which are filled with calcareous montmorillonite clay.

Irregular domal structures occur that are several feet in diameter and formed of sand adhering to an opaline skeletal structure that resembles tuffa or algal mats in the Split Rock Formation. 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 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.

 

Based on reconnaissance investigations, the Cedar Rim deposit is one of the largest in North America. If such a large deposit can remain hidden right in front of everyone’s eyes for such a long time, it makes one wonder what else is out there to be found. The geology of Wyoming, in particular, is favorable for opal due to the abundance of silica-rich volcanic ash deposited from past Yellowstone and Absaroka eruptions. In addition to sedimentary-hosted opal at Cedar Rim, precious opal was described in volcanic rocks in the Absaroka Mountains in northwestern Wyoming (J.D. Love, personal communication, 1989) and reported in the Yellowstone caldera.

 

Opal samples collected by me from the Cedar Rim field were from White River (Oligocene) and Split Rock Formation (Miocene) marlstone, limestone, 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. These included giant opals of 25,850 carats (11.4 lbs), 57,100 carats (25.18 lbs) and 77,100 carats (34 lbs). For the prospector, I recommend getting maps of this area, following up on this work as only a small part of this deposit was investigated. Thus the potential for discoveries all around this area and at shallow depth is considered to be very high.

 

Sketch map of Cedar Rim opal field showing location of opal. Areas between the outcrops and reported occurrences of opal are likely underlain by hidden opal beds under shallow sediments and soil. Each square on the map is a section representing 1 square mile.

 

This vast field at Cedar Rim potentially includes extensive deposits hidden at shallow depth. The known opals range from small cobble size nodules to large boulders encased in caliche. Several varieties of opal and agate were identified:

 

(1) Opaque milky white, tawny to translucent common opal with localized fracture fillings of transparent clear opal. Some have light blue opal with black dendritic inclusions. Some are perfectly transparent. Many are fractured but include large consolidated unfractured pieces of several carats.

 

(2) Translucent light-blue opal enclosed by milky opal which in turn is enclosed by narrow perfectly transparent and banded opal crusts that exhibit a pleasant color play (bands of blue-yellow-violet red) in natural light. Some are enclosed in a thin rim of tan to pink quartz. With further exploration, it is likely that more precious opal will be found.

 

(3) Opal with milky quartz breccia & light gray to light blue translucent to transparent opal clasts & veins set in black opal to black chalcedony matrix. The black opal rarely exhibits color play.

 

(4) Gray black to black translucent opal and agate. Some agates are similar to the Sweetwater agates & represent the source beds for the agates. Some material was collected in place in section 7, T31N, R94W.

Some of the many sweetwater agates found in the Cedar Rim field.

(5) At one location in section 25, T32N, R94W, a hill was discovered capped by fractured, varicolored fire opal. This opal occurs as replacements and fracture fillings in silicified arkose (sandstone). The material is translucent to opaque yellow, orange and red similar to Mexican fire opal. What this means for the prospector is simply that there is a lot of possible discoveries to be made at Cedar Rim. Even though only minor precious opal was found in this deposit I suspect that with some serious exploraation and trenchinig, excellent veins of the precious opal will be found. Precious opal usually is found at shallow depth in narrow bands, seams & veins. If you would like to find a deposit of your own - search the old geological literature on stratigraphy of Tertiary rocks in Montana, Wyoming & Nebraska. Many of the esoteric geologists of the past didn't have much of a grasp of gem deposits. Often such deposits are described in passing.

For example, Cedar Ridge lies along a highway & is cut by numerous roads & no one had recognized that this place had one of the largest opal deposits on earth! A few old geological reports from 50 years ago briefly mentioned opal, so one would have to be shocked to find opal scattered over 14 square miles, opal masses >79,000 cts along the edge of the road & common, fire & precious opal with scattered Sweetwater agate & some of the nicest decorative stone on earth.

This gave me a clue - nearly all of Wyoming was blanketed by volcanic ash (as was Nebraska and South Dakota). Guess what? There are other giant opal fields awaiting discovery. I found evidence for other deposits. 

At Cedar Ridge, I found millions of carats of common & fire opal with veinlets of precious opal (including black opal). It is likely that significant precious opal veins will be found at depth!  So how did everyone miss this deposit? No one looked even though hundreds of people had been driving through the opal fields each year, oil wells were drilled, roads were cut through the opals and a pipeline was run right through the middle of the field. 

 

During reconnaissance I found fire (above) and precious opal  - neither had been verified in Wyoming prior to this discovery. I found the source bed for the famous Sweetwater agates at Cedar Ridge along with excellent decorative stone.

 

 

 

 Sweetwater agates (above) and sample of sweetwater agate found in situ - note the classical gray matrix with dendrites. This lower sample also contains coating from the country rock.

 

 

 

 

 

 Fire opal breccia - the first fire opals found in Wyoming. During reconnaissance of the Cedar Rim field, the author found an entire hillside of red and orange fire opal.

 

 

 

 

Left - some fire opal breccia containing blue-precious opal. Right - two boulders of common opal (with some fire opal) weighing several thosands of carats with a few cabochons cut from the common opal.

If you don't want to search in Wyoming - fly to Australia (but take me along as a translator). If this doesn't work out because of the long drive, try the opal fields of Nevada, such as the Royal Peacock, Rainbow Ridge, Bonanza, Dominion and Virgin Valley deposits in northern Nevada. Or you can drive to Spencer Idaho and dig for opal and even southern California. Be sure to tell them I sent you and they will respond, "who?".