Preliminary Report of the Dakota School of Mines Upon the Geology, Mineral Resources, and Mills of the Black Hills of Dakota

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Seite 105 - Wetherill ; a paper read before the American Institute of Mining Engineers, at the October meeting of 1876, in Philadelphia ; rewritten by the author for this report.
Seite 114 - ... presence at that time is attested by the large quantities of carbonaceous matter yet present in these beds. This proto-sulphide of iron was not, at least, not to any great extent, gold-bearing. Subsequently, by the action of ferric salts, in solution, this proto-sulphide was changed to bi-sulphide, and the iron solutions which wrought this change also brought the gold which is now found in these deposits. I have adopted these views for the following reasons: The presence of such quantities of...
Seite 135 - They vary as much in breadth as they do in length. Some are only a few inches thick, while others exceed 100 feet. The amount of cassiterite present varies. It is never evenly distributed throughout the veins from wall to wall, but lies in zones or sheets. With the exception of being usually parallel to the bedding and not cutting across, these veins have all the characteristics of true fissure veins, and occasionally even this feature is not wanting, two having been observed which cut across the...
Seite 113 - That these deposits were originally laid down as beds, I have no doubt; but it is also certain that they were subsequently much altered. My view of their formation is as follows: At the time when the enclosing rocks were laid down, large quantities of proto-sulphide of iron were formed through the agency of...
Seite 141 - The greatest quantities of cassiterite occur in veins consisting of mica and quartz alone. The micas seem always to be ordinary muscovite. They have not been tested for fluorine. Fluorspar has not been found. No pseudomorphs of cassiterite, as in Cornish veins, have been observed. Orthoclase occurs very sparingly ; the variety, microcline, much more frequently. Albite is everywhere the predominating feldspar. Analyses of Cassiterite from the Veins. — The following analyses of Black Hills cassiterite...
Seite 138 - Tantalic acid is always present in varying quantities, but never to such a degree as to be the predominating acid. The crystal form is always tabular, while the habitus of tantalite is prismatic. A sample, however, from the Etta mine, analyzed by Professor Schaeffer, was pronounced...
Seite 115 - The sheets of igneous rock in the Homestake are porous and much decomposed. Careful assays showed no gold. The sheets east of these, near the pump-shaft, are pyritiferous and' assay from $1 to $2 per ton. From what I have stated, I am led to the conclusion that the porphyry has had a two-fold benefit: First — It has rendered the ores more free milling.
Seite 115 - The percentage of pyrite impregnating the slates and schists and forming the "ore" is never large. The yield upon concentration is seldom 20 per cent. The average is probably not more than 7 per cent. There are no solid bodies of pyrite. The influence of the porphyry upon the lode seems to be good, but whether it produced an enrichment of the bed, or simply rendered it, by some action, oxydizing or otherwise, more free-milling, I have not determined definitely.
Seite 11 - Elevated as they are like an island above the surrounding sea of the plains, and separated by more than one hundred miles from the nearest spur or sub-range of the Rocky mountains, the Black Hills are a complete study in themselves. Exhibiting in the strata exposed, and in the general character of the elevation, most of the principal features of the geology of the Rocky mountains, they are a geological epitome of the neighboring portions of that great range.
Seite 114 - In the northern half — that is, the Deadwood-Terra and De Smet end — no porphyry is. found in the vein, but the section was once overlain by a felsite which yet remains as a capping to the ridge between Gold Run and Bobtail Gulch, and between Bobtail Gulch and Deadwood Creek. This porphyry was injected between the slates and the Potsdam...

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