| Summary: | Geology of the vermiculite deposits, Gold Butte mining district, southern Virgin Mountains, Nevada: Vermiculite deposits lie within a one mile square area in the Gold Butte mining district, Southern Virgin Mountains, Nevada. An integrated field and laboratory study of the deposits was undertaken in an attempt to determine their origin. Particular attention was devoted to the mineralogy of the vermiculite and associated minerals and the geochemistry of vermuculitization. The deposits occur in ultramafic sheets and lenses of probable pre-Cambrian age. Peridotites and perknites have intruded pre-Cambrian migmatites, granulites, and quartzites, and now crop out in the interior of an elliptical dome. Diverse ages of ultramafic intrusives, demonstrated by crosscutting relations, suggest multiple intrusion. Intrusion and formation of the domical structure are believed essentially coincident in time, with intrusion outlasting the severe deformation associated with the forceful injection of an ultramafic crystal mush. During an early Cenozoic(?) orogeny the Gold Butte granite porphyry was emplaced. This granite was the probable source of most granitic pegmatites which are abundant in the area. Hydrothermal solutions supplied from the granitic magmas circulated through the ultramafic rocks, profoundly altering them. Vermiculite is widely distributed in altered ultramafic rocks, occurring in veins, stringers, pockets, and as scattered flakes. A study of X-ray, chemical, differential thermal and heat exfoliation data indicate two distinct types of vermiculite mixtures are present. All vermiculites examined were one of these two types or gradations between them. All must be considered varieties of hydrobiotite, for K2O is present in every case and represents a contamination by biotite layers. One type is a vermiculite-biotite mixture with the approximate ratio of 2:1, respectively. The second type is essentially biotite, containing approximately four per cent vermiculite. Both types show a marked degree of heat exfoliation, ...
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