Geology of the vermiculite deposits, Gold Butte mining district, southern Virgin Mountains, Nevada. Ogives of the East Twin Glacier, Alaska--their nature and origin. Investigations in the Taku Glacier firn, Alaska

摘要

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, the second type to a lessor degree.Considerable variation between the ratio of vermiculite and biotite exists in a single vein. This heterogeneity is believed due to the high cation-exchange capacity of vermiculite.The fact that all samples showed either biotite interleaved with vermiculite, or non-expandable biotite along with hydrobiotite is one of the best evidences that vermiculite has altered from biotite, which most cases, is an intermediate product in the alteration of other ultramafic minerals.Vermiculitization of biotite involved (1) progressive subtraction of the alakies, (2) hydration and formation of loosely-bound interlayered water, and (3) progressive oxidation of ferrous to ferric iron. The process was largely accomplished by hydrothermal solutions. Zoning relations of vermiculite and other minerals in serpentine veins are especially indicative of hydrothermal activity. Still, other facts are suggestive of meteoric origin. Hence it is believed that meteoric solutions continued the process of vermiculitization after hydrothermal activity ceased, and perhaps, in some cases initiated it.Ogives of the East Twin Glacier, Alaska--their nature and origin: Investigations in the Taku Glacier firn, Alaska:The perfection, fine exposure, and unusual accessibility of ogives on East Twin Glacier, Alaska, make this an exceptional place in which to study the nature and origin of this phenomenon. The origin of ogives is discussed, and previous hypotheses are critically analyzed. With the exception of a hypothesis first suggested by R. T. Chamberlin, that ogives are the surficial expressions of shearing planes, no other hypothesis satisfactoriy accounts for the fact that these ogives are exposed edges of layers of denser and dirtier ice than the intervening layers. Concepts of glacier flow evolved by Demorest provide a reasonable mechanism for understanding their formation, namely, periodic obstructed extrusion flow down-glacier from an icefall. Debris which was originally basal is believed to become, by upthrusting and ablation, the surface manifestation of an ogive.