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Admiralty Island Alaska albite altered bed rock Berners Bay black slates black-slate band calcite cleavage considerable contain country rock Cretaceous crosscut crystals deposits depth developed dikes diorite distance district Douglas Island Ebner exposed feldspar fractures gabbro Gastineau Channel geologic Glacier Gold Creek gravel greenstone Gulch hornblende igneous impregnated Inlet intrusive Juneau belt Kowee Creek lava ledge limestone located Lynn Canal main lode system mainland masses material metamorphism miles mill mineral Mining Company Mount Juneau mountain nearly northeast northwest observed occur original outerops parallel portion present prospecting pyrite pyrrhotite quartz quartz veins region River schist schistose secondary Sheep Creek shore side Silver Bow basin slate band slate-greenstone slates and greenstones slaty slope small amounts southeast southeastern Alaska sphalerite strata stream strike stringer leads structure sulphides surface Treadwell mines tunnel upper valley vein stuff veinlets vicinity wall waters zone
Page vii - Reconnaissances in the Cape Nome and Norton Bay regions, Alaska, in 1900,
Page 112 - The assay charts show the ore in the lowest levels to be as good as in the upper workings, and it is evident that variations along the dip are not greater than those observed from place to place along the strike. It is true that the mine records for a period of years show a gradual decrea.se in the per ton value of the material which has been treated.
Page vi - Fortymile quandrangle; scale, 1:250000. EC Barnard. Grand Central Special, Seward Peninsula; scale, 1:62500. TG Gerdine. Juneau Special quadrangle; scale, 1:62500. WJ Peters. Nome Special, Seward Peninsula; scale, 1:62500. TG Gerdine. The following maps are included as illustrations of...
Page 109 - ... structure of the deposits it is evident that the dikes were subjected to pressure which caused fracturing, whereby they became porous, affording channels of easy circulation for underground waters. The minerals in the ores and their mutual relations suggest that carbonated and mineral-bearing solutions found the broken dikes and continued to move through them for a very long period. In transit these waters attacked the minerals of the albite-diorite, decomposing them, and in some cases effecting...
Page 100 - ... gabbro by Becker, who regarded it as later than the rock of the ore bodies, but there is now sufficient evidence to establish the opposite age relation, and reasons exist for doubting its intrusive nature. The inclusions of light-colored rock fragments in the greenstone, which form the basis of Becker's conclusions, are represented in his collection by a specimen and a thin section, showing a distinctly outlined fragment of grayish granitoid rock inclosed in greenstone...
Page 109 - Treadwell ore bodies are dikes of albite-diorite filled with reticulating veinlets of quartz and calcite and permeated with metallic sulphides carrying small amounts of gold. From the structure of the deposits it is evident that the dikes were subjected to pressure which caused fracturing, whereby they became porous, affording channels of easy circulation for underground waters. The minerals in the ores and their mutual relations suggest that carbonated and mineral-bearing solutions found the broken...
Page 140 - ... recrystallized as to yield micaschists or phyllites. The more sandy shales afford varieties that break irregularly and that lack homogeneity, but tough and even slates result from homogeneous clays and are among the most remarkable of rocks. The distinctive feature of slates as against shales is the possession of a new cleavage that may lie at any angle with the original bedding of the rock, and that has no definite relation to it. The cleavage has been developed by dynamic strains that have,...
Page 112 - It seems likely that a rearrangement of values by relatively recent circulation has been going on, and the course of the currents may well have been controlled by the zone of sheeting in which the dikes occur; but secondary migration of this sort must be distinguished from the original mineralization, the extensive results of which in the neighborhood are entirely beyond comparison with the effects directly or indirectly attributable to a pair of narrow dikes of this sort.
Page 100 - The cleavage of the slates is regarded as having been produced before the diorite intrusions, the direction of which it largely controls. In this respect the secondary structure corresponds with that of the sedimentary rocks of the general region, all of which were tilted and metamorphosed before the diorites of the Coast Range were intruded. The slates are not altered by contact metamorphism next to the intrusive dikes of diorite.