breccia - brecciated rhyolite associated with gold mineralization - hand specimen
Textbook hand specimen of breccia in rhyolite from the Monte Cristo Range in Nevada. Initially a light tan, this rhyolite was brecciated by faulting and then hydrothermally altered, with the mineralizing fluids rising through fractures in the breccia and permeating the rhyolite.
This breccia was associated with gold mineralization, and was exposed during development of the Boss Mine, which recovered gold by heap leaching cyanidation. Specimens like these are uncommon and spectacular.
A breccia, from the Italian for broken, is a rock composed of angular broken fragments that have later been cemented. The fracturing is usually a result of faulting, though there are some mega-breccias that are the result of the downslope sliding of whole mountain ranges during Cenozoic extension of the Basin and Range province.
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