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Physico-chemical factors controlling magmatic differentiation and vein formation
No abstract available.
Some problems of the Rocky Mountain phosphate field
No abstract available.
Rôle of water conditions in the formation and differentiation of common (banded) coals
No abstract available,
Structural history of the fracture systems at Grass Valley, California
In the foregoing outline of the structural history of the Grass Valley intrusive the vein fractures are regarded as elements of the internal tectonics of the intrusive body itself. They originated in an early stage of the structural development of the region, and are equivalent to the marginal thrusts observed in the Yose-mite and along the Mother Lode system . As such, the vein fractures are of local origin, since they are entirely restricted to the neighborhood of intrusive contacts. As the area of consolidation within the Sierra Nevada batholith increased, larger units were affected by tectonic stresses, and joints were formed which belong to a system of regional extent. As this system has been observed over an area of at least 7500 square miles, its regional character seems proven. Basic dikes and aplites follow the crossing system , and thus indicate that the earliest crossings were formed when magma to fill them was still available. Both fissure systems repeatedly served for adjustment of later stresses, as can be proved by relative mutual displacements and by the repeated fracturing and renewed deposition of quartz within the vein fissures. It is probable that these later stresses were regional in character.
Zoning of ore deposits in and adjoining the Leadville District, Colorado
The ore deposits in the heart of the Leadville district , Colorado , are divisible, as shown by Emmons, Irving, and Loughlin,2 into two main classes on the basis of mineral composition: "contact-metamorphic" silicates accompanied by iron oxides, which adjoin an obscure igneous stock, and mixed sulphides accompanied by manganosiderite and quartz gangue, which cut the "contact-metamorphic" deposits and form a broad zone extending for 2 1/2 miles to the west and for shorter distances in other directions. A detailed consideration of the mineralogy of the mixed sulphide ore deposits clearly shows that they in turn are divisible into groups, referred to as hotter and intermediate mesothermal deposits . These groups, however, because of complications in structural control, are not ideally arranged from the standpoint of zoning .Emmons, Irving, and Loughlin noted that barite was a conspicuous gangue mineral in certain outlying deposits on the west slope of the Mosquito Range. The junior author during 1928-1931 had an opportunity to study the outlying deposits over an area of twenty square miles, which includes mines near the heads of Iowa Gulch and Empire Gulch, and the Weston Pass district (Fig. 1), and found that these baritic deposits form a somewhat distinct outer zone in which the primary ore consists of galena and light-colored zinc blende, with small amounts of silver and practically no gold, in a gangue of barite, jasperoid, and crystalline quartz. There is also recognized a still more remote though poorly defined zone in which the same sulphides are present but jasperoid is less conspicuous and barite is practically absent, whereas dolomite is the characteristic gangue mineral. The deposits in these two outer zones are fissure-fillings and comparatively small replacement bodies. They contrast strongly with the sulphide deposits in the heart of the Leadville district and are assigned to the outer mesothermal and epithermal zones. As they have not been described before, general features of these deposits are given in some detail, followed by their relations to the deposits within the Leadville district . Analogies are also drawn with similar deposits in other districts. In the preparation of this paper the authors have been impressed with the confusion that may result from too simple a conception of zoning . For a proper conception of Leadville , the geologist must have sufficient regard for structural conditions, must make due allowance for successive stages of mineral deposition within a small block of ground or even within a single ore shoot, and must also allow for the reactions between minerals formed at an early stage and solutions introduced during a late stage. The effects of such complications in the Leadville district might give the impression that certain minerals found in one of the inner zones were formed at relatively high temperatures, but close study of paragenesis indicates late deposition at relatively low temperatures. Again, as already stated, there is sufficient inconsistency in the distribution of certain varieties of deposits to give a confused impression of zonal arrangement unless the influence of structure on the circulation of ore -forming solutions is appreciated. The field facts give a far more complex picture than might be anticipated from a simple statement of the theory of zoning .
Progressive regional metamorphism of the lower kittanning coal bed of western Pennsylvania
No abstract available.
Occurrence of enargite and wulfenite in ore deposits of northern Arkansas
One of the several contrasting features between zinc and lead deposits of the Mississippi Valley type and those of the Cordilleran type is the mineralogic simplicity of the Mississippi Valley ores. Because the usual ore and gangue minerals are few in kind and are those that can conceivably be carried in solution by circulating ground waters, special theories of origin that have prevailed for the Mississippi Valley deposits have carried more weight than might be the case if the ores were chemically and mineralogically more complex. In summarizing the characters of the Mississippi Valley type, Lindgren 2 points out that the elements gold, antimony, arsenic and molybdenum are almost wholly absent. In view of this long recognized dearth of certain elements, it is believed that the discovery of two of them in typical Mississippi Valley lead and zinc ores of northern Arkansas may be of some interest.
The Battle Branch gold mine, Auraria, Georgia
The Battle Branch mine, in north-central Georgia, is well known locally for its pockets of exceptionally rich gold ore. During the period from May 24, 1934, to May 20, 1935, 781.97 ounces of bullion, of an average fineness of about 850, was shipped to the mint. The deposit is of the lode type; it consists of many quartz stringers and lenses grouped in three zones in a schistose phase of the Carolina gneiss. Three stages of mineralization are recognized. An early high-temperature stage produced coarsely crystalline silicates, such as garnet, kyanite, tourmaline, biotite, and muscovite, which were probably formed by a recrystallization of the minerals in the country rock. During the intermediate stage ankerite, quartz, and pyrrhotite were formed. The last stage was marked by chlorite, galena, and gold. Throughout the period of mineralization minor amounts of movement in and across the lode occurred. The distribution of the ore shoots is dependent to a large extent upon the distribution of the cross fractures. The ore shoots are pod-shaped bodies, which pinch and swell abruptly. The gold in the rich shoots is contemporaneous with galena and is clearly of hypogene origin.