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Origin of Magnetite in the Zoned Ultramafic Complexes of Southeastern Alaska1

By
Hugh P. Taylor, Jr.
Hugh P. Taylor, Jr.
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James A. Noble
James A. Noble
H. P. Taylor, Division of Geological Sciences, California Institute of Technology, Pasadena, California
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Published:
January 01, 1969

Abstract

Appreciable concentrations of titaniferous magnetite occur in several of the ultramafic complexes that are exposed along a linear belt 350 miles in length in southeastern Alaska. Four of the larger bodies, at Klukwan, Snettisham, Union Bay, and Duke Island, are of economic interest. Large magnetite concentrations (15–20 percent of the rock by volume) of very uniform grade occur in the hornblende pyroxenite units of these complexes.

In several of the complexes (including Union Bay and Duke Island) a consistent zoning of ultramafic rock types occurs, with dunite, peridotite, olivine pyroxenite, and hornblende pyroxenite occurring in sequence from the center outward to the margin. Only the hornblende pyroxenite zone is exposed at Klukwan and Snettisham, but the mineralogy of this zone is identical in all the complexes. The minerals are diopsidic augite (Di85Hed15 to Di70Hed30), hornblende (abnormally high in A12O3 and low in SiO2 relative to most igneous hornblendes), magnetite, and accessory ilmenite and hercynitic spinel. Accessory amounts of olivine may occur in the interior parts of the magnetite pyroxenite zones of the complexes that contain olivine pyroxenite and/or dunite, but the magnetite concentrations drop abruptly as the amount of olivine increases; magnetite is largely absent from the olivine pyroxenite zones.

The many common features of these ultramafic bodies indicate that they formed by a single mechanism, namely by crystallization of ultramafic magmas. Evidence for this includes: (1) graded layering in rhythmically-bedded olivine pyroxenite and magnetite pyroxenite; (2) significant contact metasomatism and metamorphism at the margins of the bodies; and (3) good correspondence between observed rock types and experimentally studied liquids in the system diopside-olivine-iron oxide. It is suggested that the complexes formed by a combination of fractional crystallization and multiple intrusion of ultramafic magmas, and that the magnetite-hornblende pyroxenite zones in particular crystallized from a liquid roughly corresponding to their present chemical composition. This liquid must have had a total Fe content of about 15–18 percent by weight.

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Contents

Society of Exploration Geophysicists Geophysical Monograph Series

Magmatic Ore Deposits

H. D. B. Wilson
H. D. B. Wilson
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Society of Economic Geologists
Volume
4
ISBN electronic:
9781934969991
Publication date:
January 01, 1969

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