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Lookout Pass Thallium prospect, Little Valley, Sheeprock Mts, Tooele Co., Utah, USA

This page kindly sponsored by Norman King
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Latitude & Longitude (WGS84): 40° 4' 58'' North , 112° 35' 48'' West
Latitude & Longitude (decimal): 40.08278,-112.59694
GeoHash:G#: 9rpujbc7j
Locality type:Prospect
Köppen climate type:BSk : Cold semi-arid (steppe) climate

The mineral collecting area is not really at Lookout Pass, which is located along Pony Express and Overland Stage Route, but is 3-4 miles south of the pass itself, along a dirt road through Little Valley that requires high vehicle clearance and perhaps an ax or hatchet to clear tree limbs growing across it. The N-S trending Little Valley is labeled on the USGS topographic map, but “Little Valley” has hardly ever been used to refer to this site. West Lookout Hills and East Lookout Hills are on either side of the Valley, but those terms are not on the USGS topographic map. All of these features – East and West Lookout Hills and Little Valley – exist only from the Pony Express and Overland Stage Route for about four miles southward to where the hills and the valley between them lose definition. The area of the prospect presently (August 2014) lies between Lat. 40.078 to 40.088 and Long. -112.594 to -112.600, and is in Secs. 26 and 35, T. 8 S., R. 7 W., Tooele County, Utah.

The thallium minerals have been reported from the west side of Little Valley, in foothills forming the northernmost Sheeprock Mountains. Sheeprock Mountains are a horst-block mountain range in the Basin and Range physiographic province. A thick sequence of Paleozoic sedimentary strata ranging in age from late Precambrian to Mississippian is exposed in this area, dipping generally toward the north. The Cambrian Ajax Dolostone, including shallow-water facies such as oolitic, stromatolitic, and mud-cracked carbonates rich in burrow and crawling marks, forms the host rock. Tertiary decalcification of the carbonates led to extensive brecciation, followed by metasomatic silicification to form a variety of dark-colored (organic-rich) chert known as “jasperoid.” Silicification, mineralization, and concentration of organic material occurred along fault and/or shear zones that cut the strata. The structure in Paleozoic strata is dominated by a conjugate set of shears trending NW-SE and NNE-SSW (green to brown outcrop areas in the geologic map below). The drainage pattern, visible in topographic maps, reflects these trends. The recently active quarry is very near the major, southernmost shear shown on this map.

Thallium minerals in jasperoid were discovered here in 1987 near the site shown by the Mindat icon, but closer to the road through Little Valley. Organic-rich jasperoid containing thallium minerals, radiating sprays of stibnite and stibiconite, and arsenian pyrite suggests this is a Carlin-type deposit, and potential for commercial gold production here has inspired occasional exploratory work. A road for access by drilling rigs was constructed near the original site of specimen collection. Drill-hole cores taken there registered as high as 0.02 oz gold/ton (reported by Miranda Gold Corporation–see reference). Silver was detected by EDS analyses in two samples collected by Norman King in 2011, but no visible silver has been found.

Parapierrotite is the most conspicuous mineral in the dark-gray jasperoid, occurring in isolated, stubby, black crystals up to 2 mm in maximum length. These produce brilliant silvery reflections from cleavage surfaces in bright sunlight. Stibnite is present in similar-appearing cleaved crystals, but these are very elongated and form radiating sprays. Many of the latter are altered to cream-colored stibiconite. Parapierrotite alters initially to inconspicuous reddish-brown kermesite, and later, more alteration produces conspicuous cream-colored hydroxycalcioroméite pseudomorphs that mimic the parapierrotite crystals.

Surfaces of the jasperoid are commonly covered by dull back coatings of the thallium oxide avicennite--another product of the weathering of parapierrotite (thallium is lost during formation of kermesite and hydroxycalcioroméite). The jasperoid is oxidized in places to reddish-brown color from iron oxides and kermesite, and color banding in the oxidized jasperoid produces what lapidarists refer to as "wonderstone." Remnants of still-unaltered parapierrotite (metallic cleavages still visible) can be found locally in the oxidized jasperoid. Generally, the darker color, plus brilliant reflections from parapierrotite cleavage planes, are keys to locating the mineralized zones.

NOTE: Lookout Pass thallium-bearing mineral specimens have a history of little analytical work. Collectors should, therefore, be wary of identities listed on labels.

Norman King, rev. December 2017

Mineral List

27 valid minerals.

Rock Types Recorded

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Alphabetical List Tree Diagram

Regional Geology

This geological map and associated information on rock units at or nearby to the coordinates given for this locality is based on relatively small scale geological maps provided by various national Geological Surveys. This does not necessarily represent the complete geology at this locality but it gives a background for the region in which it is found.

Click on geological units on the map for more information. Click here to view full-screen map on Macrostrat.org

Quaternary - Miocene
0 - 23.03 Ma

ID: 3185380
Cenozoic sedimentary rocks

Age: Cenozoic (0 - 23.03 Ma)

Lithology: Sedimentary rocks

Reference: Chorlton, L.B. Generalized geology of the world: bedrock domains and major faults in GIS format: a small-scale world geology map with an extended geological attribute database. doi: 10.4095/223767. Geological Survey of Canada, Open File 5529. [154]

Pleistocene - Pliocene
0.0117 - 5.333 Ma

ID: 2962586
Alluvial materials, Axtell and Harkers Formations

Age: Cenozoic (0.0117 - 5.333 Ma)

Stratigraphic Name: Axtell Formation; Harkers Formation

Lithology: Major:{unconsolidated alluvium}

Reference: Horton, J.D., C.A. San Juan, and D.B. Stoeser. The State Geologic Map Compilation (SGMC) geodatabase of the conterminous United States. doi: 10.3133/ds1052. U.S. Geological Survey Data Series 1052. [133]

Data and map coding provided by Macrostrat.org, used under Creative Commons Attribution 4.0 License

This page contains all mineral locality references listed on mindat.org. This does not claim to be a complete list. If you know of more minerals from this site, please register so you can add to our database. This locality information is for reference purposes only. You should never attempt to visit any sites listed in mindat.org without first ensuring that you have the permission of the land and/or mineral rights holders for access and that you are aware of all safety precautions necessary.


Sort by Year (asc) | by Year (desc) | by Author (A-Z) | by Author (Z-A)
Rocks & Minerals (1993) 68(6): 381-393.
King, N.R. (2014) The Mineral Assemblage from Lookout Pass Area, Sheeprock Mountains, Tooele County, Utah, in 2014 GSA Annual Meeting in Vancouver, British Columbia (October 2014).

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