Green Rocks and Minerals

Nephrite jade from Wyoming

This past fall, I was working on a gold project near Cheyenne, Wyoming and met two old friends for dinner. One had an affection for green minerals and rocks, so I thought I would tell you a little about some green rocks in my back yard.  One specimen has a lot of lumps and kind of reminds me of brains. So I call it jaded brains. It has a pleasing light-green color and was picked up in the jade fields of Wyoming.

Light green jade

The geological and geochemical processes that produced jade are intriguing. The jade fluids produced under great pressure (and temperature) tended to replace much of the original rocks and minerals to produce jade. Each atom of the original mineral or rock was mobilized and replaced by the atoms that make up jade. We don't know if this was a rapid process or a process that took place over millions of years.  But we can see the results and sometimes, much of the original rock matrix was replaced by jade leaving feldspar phenocrysts (larger crystals in a fine-grain groundmass or matrix) somewhat in tact.  In other specimens, we find quartz crystals that are preserved and untouched, but in others, even the quartz is not safe, as we find evidence of quartz (hexagonal) being replaced by jade.

Jade porphyry. Much of the original rock matrix was replaced by jade leaving some greenish
white feldspar in place that are also partially replaced.

Jade specimen with quartz crystal preserved.

GOLD & OTHER PRECIOUS METALS

Mineralized terrains of Wyoming (after Hausel and Hausel, 2011).

Little was known about gold in Wyoming when I began research on the geology and mineral deposits of the Cowboy State; thus, I set out to map, evaluate, & find more gold, and it almost seems like I discovered gold nearly everywhere I looked. I was amazed at how much had been overlooked - me and my small team even found anomalous gold in a paleo-drainage in the Laramie City landfill! I published a few compendiums for prospectors & geologists, & mapped several mining districts that were previously unmapped or only partially mapped. I even found a previously unrecognized ultramafic massif with significant palladium, nickel gold & copper mineralization & a whole new district.

34-ounce nugget recovered from the Rock Creek placer, South
Pass district, Wyoming.

There are many great stories & memories about discoveries & prospectors I met. Hopefully, I will be able to find time to write a book about these, as such stories should be preserved. There are stories about hundreds of nuggets in ball jars in Shorty Haddenham's trailer at Atlantic City, dozens of nuggets found by a Ft. Collins prospector, a prospector who spent one entire winter jumping a claim & panning out barrels of mica thinking he had found the Mother Lode & my research along the UP corridor- we found gold everywhere including the Laramie City dump.

I mapped the South Pass greenstone belt at the southern tip of the Wind River Mountains which included several historic gold districts: Lewiston, South Pass, Atlantic City, Miners Delight & others. I identified several hundred gold anomalies & found gold was structurally controlled in reef ore shoots that are very rich down plunge. One deposit I mapped (Carissa) is a major deposit 1000 ft wide, 1200 ft long & probably continuous a few thousand feet downdip. This deposit was withdrawn by the legislature without any scientific review - such abuse of political power literally took away a mountain of gold & many good jobs.

Geology led me to many discoveries. The Rattlesnake Hills were an obvious target, so in 1981, armed with the concept that the RH were part of a fractured greenstone belt intruded by Tertiary alkalic volcanic rocks, I knew there had to be gold. And I found gold in the Rattlesnake Hills in veins, shears, Tertiary breccia, stockworks, pyrite. I also found significant gold elsewhere in Wyoming - Seminoe Mountains, Mineral Hill, Purgatory Gulch, more.

OTHER GEMS

Blue barite from the Mine Hills near Shirley Basin, Wyoming

While conducting field research over 3 decades while at the Wyoming Geological Survey at the University of Wyoming, we were able to document hundreds of mineral deposits, occurrences and anomalies. We were able to do so by using scientific methods by keeping in mind the geology, tectonics, and geological history. Sure, we found a lot of stuff, but I guarantee you, if you apply similar scientific methods, you will also find a many more mineral deposits in Wyoming - the geology along with mineral anomalies we identified, prove that there are many other deposits including several ruby and sapphire deposits, more iolite deposits, many diamond deposits, dozens of gold deposits, many massive sulfide deposits, etc. 

While conducting reconnaissance, jasper cobbles and boulders were found in several old prospects at Tin Cup, in an outcrop near the south edge of the Rattlesnake Hills, and (jasperoid) at Quaking Asp Mountain. Some of the Tin Cup jasper is extraordinary and found in masses weighing several hundred pounds. The jasper in the Rattlesnake Hills contained some fossil leaf imprints.

Nearly everywhere we explored, we followed trends and examined geology which lead to other mineral deposits. I was curious enough to find out what some of the unusual minerals were that I picked up, and as a result, identified more than a dozen minerals that had never been reported in Wyoming.

OPAL

Resistant, opal, boulder adjacent to rock hammer in Tertiary
volcaniclastics, with a seam of resistant opal lying beneath the
 boulder and hammer, Cedar Rim, Wyoming.

A rockhound from Riverton, Wyoming, mentioned opal was reported south of town near Cedar Rim at one of my rock hound club talks. When I returned to Laramie, I began searching old US Geological Survey reports on the stratigraphy of the area and found, in passing, mention of opalized rock in some of the stratigraphic columns - so, I headed to the area not knowing what I might find, but the reports left me with the impression that these were just trace amounts. So, when I arrived at Cedar Rim, I was absolutely amazed at the extent of the opal. The great majority of the material I classified as common opal, but also found an entire hillside with colorful red-orange, orange to yellow Mexican opal, and Mexican opal breccia, and traces of precious opal, along with dendritic agate (Sweetwater agate) and a host of colored agates. Based on the presence of the precious opal, I suspect some seams might be found at depth. After spending a few days in the opal field, I later found additional US Geological Survey reports describing some opalized tuffaceous sediments located to the west and east of Highway 135 suggesting that there is likely more opal to the west of the Cedar Rim deposit.

Specimen of 'Candy' opal from Cedar Rim - a mix of white
common opal with colored 'Mexican' opal.

The Cedar Rim deposit lies east of highway 135 and is cut by oil field roads. A few old geological reports from 50 t0 80 years ago mention opal in passing, so I was surprised to find opal scattered over 14 mi2, opal masses containing 80,000-carats along the edge of the oil field roads, and common, fire & precious opal and scattered Sweetwater agate, along with some of the nicest decorative stone in Wyoming. All occurring in Tertiary-age volcaniclastic sedimentary rocks that had a notable

contribution of volcanic ash erupted from Yellowstone in the past.

Cedar Rim boulder opal. All of the boulders and
cobbles that stand out as resistant rock, is typically
filled with opal.

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 opal fields waiting to be discovered. So I found millions of carats of common & fire opal & traces of precious opal (including black opal) that suggest as soon as someone digs, valuable veins of precious opal will be found as depth!

The waxy material in the rock matrix is common opal. 

PERIDOT

In 1997, I decided to search the Leucite Hills in western Wyoming for diamonds (see photo at left). I didn't find diamonds, but mark my words - "DIAMONDS WILL BE FOUND HERE". This is an easy prediction to make based on geology.

The Leucite Hills consist of several lamproite volcanoes & flows that erupted 900,000 to 3.1 million years ago. They started their journey under one of the thickest parts of the Wyoming Craton. Cratons are very old, cool, continental cores necessary to have melting deep within the earth's upper mantle (where diamonds are formed). While exploring this region, I collected grab samples from lamproite in the northeastern part of the field & two yielded chromites with similar geochemistry to chromites found as mineral inclusions in diamond - thus this tells me some of the volcanoes began their journey at 90 to 120 miles beneath the surface where diamonds are common.

Diamondiferous lamproites are found at Murfreesburo, Arkansas; Argyle & Ellendale, Australia; Majhgwan-Chelima, India; Kapamba, Zambia; Aldan, Russia & Bobi, Ivory Coast. The richest deposits are typically found in olivine lamproites which often alter to serpentinite (a very soft material) that erodes quickly, thus such deposits are usually hidden within a field of leucite lamproites (which are much harder rock). Thus, this all suggests a few diamonds are likely to occur in the lamproites with diamond-stability chromites, but the locations for rich diamond deposits are hidden. Along the northern edge of this field, a large sand dune field that marks the location of a major continental shear - a favorable structure for hidden lamproites. One can almost guarantee there are diamondiferous olivine lamproites in this area hidden under a few feet of sand. Lamproites are also well known for colored stones - brown, yellow & the beautiful, extremely rare Argyle 'Pinks'. Some have sold for >$US1 million/carat!

While searching for diamonds, I started looking for olivine. In 1997, I came across two green anthills at Black Rock - the ants had collected all of the available olivine in the immediate area & decorated their hills. So I collected the hills & processed them for diamonds - but all we found were 13,000 carats of flawless peridot (gem-quality olivine). Some were 12 millimeters in length. I also found peridot in place that were nearly 0.5 inch across.
Even though olivine had been recognized in this area >100 years ago, no one had ever looked at the quality of the olivine. Later, I mapped the Leucite Hills and identified all of the olivine bearing volcanoes.