Saturday, December 11, 2021

Tonnes and Tonnes of Jasper - A Rockhounds' Treasure Trove!

 

Never seen so much jasper! Tonnes and tonnes of jasper in the Tin Cup district, yet much 
of the area remains to be prospected. Some jasper was exposed in old prospect pits, while
considerable jasper trends along strike lying under inches of soil. All of this on
 public land 
administered by the BLM (Bureau of Land Management).
 

You can follow  these dike-like occurrences of massive jasper for hundreds of feet. In the
same area, are 
low-quality nephrite jade, some rare jade pseudomorphs after quartz,
rubies, sapphires,
agate, and pyrite. These deposits in central Wyoming, lie north of the Jeffrey
City almost ghost town.

JASPER - Some things you should know about jasper: (1) Do you know how many different varieties of jasper are recognized? (2) What rock formations in America contain chert (agate, jasper, flint, chalcedony, etc)? (3) Where is jasper found? (4) Do you know how many times jasper is mentioned in the Holy Bible? (5) Everything you need to know about jasper; (6) Pick a state in the US where jasper is found; (7) GemHunter's guide to Agate and Jasper.

TIN CUP DISTRICT - Just how much jasper is in the Tin Cup district? Don't know, but suspect it may be one of the larger deposits in North America! When you visit scatter prospects in the district, it is clear most are likely interconnected. When you walk over the ground in-between prospects, you will find thin soil cover with periodic chips, fragments and pieces of the lapidary mineral in the dirt. SO, potentially, there could be considerable tonnage buried under just inches to a few feet of soil. 

The GemHunter, former research geologist for the Wyoming
Geological Survey at the University of Wyoming, holds cobbles
of jasper in hands from the Tin cup district, Wyoming, while
Wayne Sutherland takes photo.
During initial reconnaissance of the district some years ago, I was intrigued by the impressive gold assays reported by Henry C. Beeler, geologist, in his 1907 publication (see below). Beeler's report suggested possibilities for significant gold occurrences, but samples I later collected for assay, did not detect any anomalous gold! Even so, the area should be sampled in detail, as there are many small prospects, several shear zones, and a few large  shear zones (Tin Cup Mountain shear, and Sweetwater shear) that are readily visible on aerial photography. There are also numerous tight and isoclinal fold closures in these rocks (42°38'57"N; 107°53'15"W) that are outlined by white granitic dikes and pegmatites in dark schist. The fold noses are potential sites for mineral enrichment, if any mineralizing fluids were released during regional metamorphism of the Granite Mountains. The district is also extensively faulted and fractured with a number of interesting minerals found nearby. While exploring for jade and corundum in this area, examples of distinct hydrothermal alteration was recognized with most in-situ jade deposits. So, there is potential for hydrothermal and metamorphogenic gold in the Granite Mountains, and there still is a lot of geology to examine in the region! 

Photo by Wayne Sutherland showing massive to brecciated
jasper in the wall of the prospect, with geologist for scale. 
The nearby Sweetwater shear zone (42°37'5"N; 107°54'16"W) forms a N65oE-trending shear structure that can be traced for 3 miles on trend. Locally, it is as much as 0.25 to 0.5 mile wide with numerous shear fractures, foliation, and rock cleavage. These all potentially provide permeable pathways for mineral fluids mobilized during regional metamorphism (2.8 Ga) of the Granite Mountains.

Another shear structure is visible in a granitic inselberg west of Tin Cup Mountain, where the N55oW-trending, Tin Cup shear (42°39'13"N; 107°55'42"W) is visible on aerial photography. The region has potential, but is mostly unexplored even to this day.

JADE - Other interesting minerals in the region include nephrite-jade pseudomorphs after quartz. Nephrite is a calcium-, magnesium-, iron-rich silicate (amphibole) of the formula: Ca2(Mg,Fe)5Si8O22(OH)2 that forms masses while crystallizing in the monoclinic crystal system. While exploring this region, specimens of nephrite jade with interesting crystal habits were collected. These include jade specimens with quartz (a hexagonal mineral).


Emerald green nephrite jade (monoclinic) with hexagonal quartz crystals in matrix. Tin Cup
district, Wyoming.

Grass-green nephrite jade with pyramidal (hexagonal) quartz,
Tin Cup district, Wyoming
These include rare specimens of nephrite jade (verified by XRD by Robert Gregory at the Wyoming Geological Survey) that exhibit, distinct, hexagonal crystal habit. Since primary jade does not have hexagonal habit, it is apparent these specimens are pseudomorphs of original, hexagonal quartz crystals replaced by jade. During the regional metamorphic event (dramatic changes in pressure and temperature) in the Granite Mountains in the Archean, some quartz was replaced by nephrite which inherited the original quartz crystal habit. These secondary crystals generally do not have high-quality nephrite like those found further south in the Crooks Gap and Green Mountain uranium district by jade prospectors during the 1930s through the 1960s. The jade pseudomorphs in the Tin Cup region have grass-green, light-green to white colors. Still, these specimens are rare.

Hexagonal jade pseudomorph after quartz, Granite Mountains,
Wyoming.



Crystals of nephrite jade mimicking crystal habit of quartz. These hexagonal specimens
likely formed by nephrite replacing a vug of quartz crystals (Tin Cup district, Wyoming).

Reported jade localities in Wyoming. Jade is found primarily within the Granite Mountains where
detrital rocks were shed in alluvial fans south of the Granite Mountains during erosional events in the
geological past. Other jade localities reported in Wyoming have not been verified, and some are likely
serpentinite or chlorite schist mistaken for jade, such as those in the Big Sandy Opening in the Wind
 River Mountains, and those found near Deer Creek in the northern Laramie Mountains.


Tin Cup jasper

REFERENCES:
Beeler, H.C., 1907, Prospecting in the Black Canyon-Long Creek vicinity near the Sweetwater River, Fremont County, Wyoming: Wyoming State Geologist Report, Cheyenne, Wyoming, 12 p.

Hausel, W.D., 2014, A Guide to Finding Gemstones, Gold, Minerals and Rocks: GemHunter Publications, 368 p.

Hausel, W.D., and Hausel, E.J., 2011, Gold A Field Guide for Prospectors and Geologists (Wyoming Examples)GemHunter Publications, 366 p.

Love, J.D., 1970, Cenozoic Geology of the Granite Mountains area, Central Wyoming: US Geological Survey Professional Paper 495-C, 154 p.


Tin Cup Hilton. Don't know if anyone else experienced
strangers in the night in Wyoming? While camping and 
mapping in the Granite Mountains, Seminoe Mountains, and
South Pass, my tent was periodically visited at night by a 
very fast critter that would run around my tent much faster than
any coyote could run. Each time I stepped out my front 
door with my flashlight and gun in hand, there was never anything
to find? Still don't know what it was to this day.





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