Found in a single location on the earth’s surface, McNulty rhyolite is a comparatively rare gem rock (geology) quality U.S. variety of rhyolite rock. McNulty rhyolite appears in the official U.S. Department of Interior, United States Geological Survey Lexicon of Geological Names of the United States.
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Differentiated from common rhyolite
Rhyolite is an igneous, volcanic rock of felsic. It often resembles granite and has the same basic mineral makeup; however, rhyolite’s origin is extrusive and not plutonic. Because it is so rich in silica, its melts or lavas, unlike those of basalt, are highly viscous. In volcanic extrusion, rhyolite tends not to flow, but, rather to plug expanding gases until thrown off in eruption in release of the pent up force of the expansion. In the U.S. today, rhyolites are most commonly found in the west and southwest. Unlike granite, some few varieties of rhyolite, like McNulty rhyolite, are coveted by lapidaries as gem rocks, because of their identifying mineral aggregates, interesting colors and flow banding. Other examples of varieties of rhyolite, suitable for use as gem rock (though both far, far more common than McNulty rhyolite), are “Wonderstone” and “Hichoryite”, which are together found in Mexico, Nevada, Utah and New Mexico.
Brief description and source
McNulty rhyolite (as per S.F. Simmons 1898) or McNulty Gulch rhyolite (as per W. Cross 1886), described by Cross as “Light-colored; numerous slightly pinkish quartz crystals, white glassy feldspars, and brown blotlte leaves, with subordinate ashen gray ground mass btw. them.”, is found only in McNulty Gulch in Colorado, United States. McNulty Gulch is a feature of granite porphyry near Leadville, Colorado in the U.S. Ten Mile Mining District. The igneous rhyolites of McNulty Gulch formed in the mountain of Colorado’s Ten Mile Mining District later than the diorite and Lincoln porphyry there but earlier than the once rich silver ore deposits of the district’s Grand Union or Silver Bowl mine. McNulty Gulch is an integral part of the western United States’ and Colorado’s gold rush history, being the 1861 site of one of the earliest gold finds in Colorado. The first commercial gold placers in Colorado were only discovered by George A. Jackson in nearby areas of Colorado 2 years earlier in 1858. These discoveries precipitated a rush of prospectors into the surrounding mountains and the Colorado Gold Rush commenced. During 1858-67, Colorado produced about $14,924,000 in placer gold and about $10 million in lode gold at mid 19th century values.
Detailed description
In the United States Geological Survey description of the Ten Mile District Quadrangle, McNulty rhyolite rock is described thus “McNulty rhyolite. This is a fine-grained porphyritic rhyolite, light gray in color, with many small white feldspars and locally some small smoky quartz crystals. It is probably allied to the nevadite in time of eruption, or it may be later. It cuts both the Lincoln and the Quail diorite-porphyry. It occurs in small irregular masses in McNulty Gulch, and extends southward beyond the present quadrangle. In one mass above the Railroad Boy tunnel (45)* are small drusy cavities containing little tablets of tridymite.” [1]