SK Star Claim #2, Thomas Range, Juab County, Utah, USAi

Regional Level Types
SK Star Claim #2	Claim (Active)
Thomas Range	Mountain Range (Active)
Juab County	County
Utah	State
USA	Country

The SK Star Claim #2 is a small BLM mine claim that is, as far as the owner as well as geologists that studied the site can tell, the only place on Earth that consistently produces black topaz. Originally discovered by Sean Rasmussen and Krisann Morrill in 2015 after they discovered some black material that was thought to be obsidian at first. Further investigation revealed it to in fact to be euhedral crystals of black topaz, and upon realizing this, Rasmussen and Morrill immediately struck a claim. The pair continues to work the claim by hand, unearthing more specimens each year.

The mine itself is a trench, dug along a NW trending fault, which hosts the mineralization. The host rock is a vuggy section of the Topaz Mountain Rhyolites, which are associated with volcanism that occurred during the Miocene. The minerals found there include the black topaz, a mix of uncommon metal oxide minerals including wolframite and pseudobrookite, "ram's horn" calcite, uranium-bearing chalcedony, and rarely native gold.

The topaz from the site is consistently euhedral and partially or completely black. Most samples are very small, less than 3 cm in length, but occasionally much larger crystals are found. Twinning and double termination are both quite common at the site. The crystals are black due to hundreds of microscopic inclusions that measure on average about 30 micrometers across. They are mostly wolframite or biotite, with compositional zonations observed inside some crystals.

Geologists at Western Carolina University, as part of a senior thesis by Patrick Lahr working with Sean Rasmussen, attempted to try to figure out a formational model. In the process, they were able to get XRF and ICP-MS measurements, as well as conduct a thin section petrographic analysis on samples from the site. Based on the data available, the working theory is that a fault ruptured during crystallization creating a pathway for fluids to interfere with the crystallization, depositing incompatible elements onto the surface of the crystal. The incompatible elements were subsequently incorporated into the crystal structure, which turned the crystals black in colour. This is still an ongoing project by Patrick Lahr, though now as independent research due to graduating. Samples from this project are maintained within the Marsh Fox Collection.

Additionally, the site has produced rather fine samples of highly fluorescent uranium-bearing chalcedony. XRF conducted by Western Carolina University measured the uranium content to be 269 ppm.

Other Regions, Features and Areas containing this locality

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