Geology & Mineralogy of the Virgin Valley Opal District
The precious opal in Virgin Valley is found "in-place" (where it was originally formed and deposited) within a hard, defined and traceable sub-surface horizon or zone of bentonite. The opal was deposited by hydrothermal fluids and silica rich water filling voids in the host bentonite matrix and as veins and fracture fillings of petrified wood. Bentonite has been defined as a mineral and consolidated clay rock derived from volcanic ash. Above and below the specific opal-producing zone, no commercially valuable deposit of opals are found to exist. The precious opals occur disseminated in-place throughout the opal bearing horizon or zone. This opal zone is followed by miners as a "lead." Because of "it's brittleness and low density, opal does NOT form economic placer deposits." See, The Geology of Gems by Eugenii Ya Kievlenko at page 124 (2003). "With the exception of some Brazilian material, gem opal is rarely found in alluvial gemstone placers, but is instead mined directly from its host rock." See, OPAL: The Phenomenal Gemstone, at page 11 (various contributors, Lithographie LLC 2007). "Alluvial opal deposits are rare and lie immediately on in situ deposits because opal cannot survive movement, as such, Placer opal deposits are unheard of." http://wy-opal.blogspot.com/.
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