Mineral resources of Alaska, report on progress of investigations in 1910--Geologic features of Alaskan metalliferous lodes
Thirteen years of systematic investigation of the mineral resources of Alaska by the Geological Survey has yielded a large amount of information relating to the geology of the mineral deposits. This has been used for the most part only in the descriptions and discussions of the economic geology of the individual districts about which reports have been published. Heretofore relatively little attention has been paid to the relation of the mineral deposits of different parts of the Territory to
... the Territory to one another or to other broader problems of economic geology. Practically the only exception is the geology of the coal fields, which has been briefly summarized. 1 Therefore anyone desiring information about the areal distribution and geologic occurrence of the metalliferous deposits must seek it in nearly two score publications.2 It is proposed to summarize briefly in this paper the salient geologic features of the metalliferous deposits of Alaska, using the results obtained by the many geologists who have worked in this field. As the facts and their interpretation are taken largely from published reports, it will be evident that this article can lay no claim to being an original contribution to the geology of the ore deposits. It is hoped that the matter presented will be useful, not only to those who are developing the metallic wealth of the Territory, but also to those who may desire a convenient outline of the more purely scientific results which have been achieved. Alaska is rich in metallic wealth. Its lode' and placer mines up to the close of 1910 have produced 3 about 8,411,467 fine ounces of gold, valued at $178,789,171°; 2,130,199 fine ounces of silver, valued at $1,286,678; 33,795,108 pounds of copper, valued at $5,338,709; and about $100,000 worth of tin. The rapid advance of the mining 1 Brooks, A. H., The coal resources of Alaska: Twenty-second Ann. , but there has been a small output from lodes. The above statistics of the output of precious metals serve only to indicate the advancement in productive mining. Of more importance to the future of the industry is the large amount of work accomplished during the last few years in prospecting and developing'lode deposits. This is true not only of long-established lode-mining camps on the Pacific seaboard but also of many inland districts. These developments are constantly adding new facts bearing on the occurrence of metalliferous lodes, and hence a paper like the present one can be regarded only in the nature of a progress report, which should be succeeded by others after more exhaustive studies of the several districts have been made. GEOGRAPHIC AND GEOLOGIC DISTRIBUTION OF METALLIFEROUS LODES. INTRODUCTION. The distribution of the metalliferous lodes in Alaska is indicated on the accompanying map (PI. I, in pocket), on which the localities of metal deposits are indicated by symbols. It would undoubtedly add much value to this paper if the distribution of the metals could have been shown on a geologic map, but such a map was not practical in this report, intended for early publication, because of the time needed both to compile the geologic data and to publish a map in colors. The larger features of the areal geology of some of the more important parts of the Territory are represented on Plates II, III, and IV, which will be supplemented by brief verbal descriptions of the geology of the entire province. The general map (PL I).strikingly illustrates the very wide distribution of gold in the Territory. It will be shown that the gold is so widespread because the conditions for its deposition in the bedrock existed over large areas during Mesozoic time, rather than because there are many types of auriferous deposits or because they were formed during many epochs of geologic history. Copper, too, occurs in several widely separated districts, but there is less uniformity in the form of its occurrence than in that of the gold deposits. GEOLOGIC FEATURES OF METALLIFEROUS LODES. 45 The copper deposits are probably also chiefly of Mesozoic age. The tin deposits are practically all limited to one district and to one general epoch of formation. The marked parallelism of the larger bedrock structures which prevails throughout much of Alaska has had a wide influence on the areal distribution of many of the stratigraphic subdivisions, and hence on the mineral deposits -associated with them. These structures parallel the Pacific seaboard of Alaska, trending northwestward to about the one hundred and forty-ninth meridian, thence swinging to the west and southwest. As the position of the dominant mountain ranges .and systems has been determined by these structures, their axes have the same trends. The structures above noted have determined to a large extent the areal distribution of the bedrock formations, especially of those older than Upper Cretaceous. As a result of these tectonic lines, many of the geologic subdivisions occur in belts which parallel the Pacific seaboard. As it is impossible to present here a complete summary of the geology of Alaska, its larger features can probably best be elucidated by describing briefly the successive zones of rocks, similar in general lithology or age, which form the larger stratigraphic units, together with the metal deposits associated with them. These formations or zones will be described, so far as possible, in their geographic sequence from south to north. The order of treatment will have to be modified to a certain extent and some of the local geology will be presented by provinces, defined by drainage basins. PACIFIC COAST REGION.