Description

This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1903. Excerpt: ... more or less throughout the area by intrusives of various types, including gabbroic and dioritic masses (often covering considerable areas), and porphyritic intrusives. which are especially abundant in the Triassic shales. The latter intrusives may bear some genetic relation to the Chititu gold. COPPER OCCURRENCES. GENERAL STATEMENT. The copper deposits everywhere within the Chitina district are closely associated with the body of altered igneous rocks which has been described as the Nikolai greenstone. It is believed that the copper was originally disseminated in minute quantities throughout this formation and that the valuable deposits now recognized are concentrations, in various forms, from this original widely disseminated condition. These concentrations have been made at different horizons, but the richer ore bodies seem generally to have formed near the contact with the overlying limestone, usually in the upper part of the greenstone, but in one or two notable instances in the limestone itself, a short distance above the contact. This contact is easily followed and has been recognized by prospectors as marking an important economic horizon throughout the district. Its position, so far as it is at present known, is indicated on the accompanying map (Pl. IV). Owing to the limited amount of development in the region and the general character of the work done in the study of the copper deposits, it is not now possible to classify genetically the various occurrences or to give in detail the mineral association or the chemical processes that have accompanied the deposition of the various forms of copper ore. Such work must be left largely to the closer studies which may follow the mining developments of the future. For convenience of discussion, however,...

    The Mineral Resources of the Mount Wrangell District, Alaska

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