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Scientific Investigations Report 2013–5091

A Deposit Model for Magmatic Iron-Titanium-Oxide Deposits Related to Proterozoic Massif Anorthosite Plutonic Suites

By Laurel G. Woodruff, Suzanne W. Nicholson, and David L. Fey

Thumbnail of and link to report PDF (8.35 MB)Abstract

This descriptive model for magmatic iron-titanium-oxide (Fe-Ti-oxide) deposits hosted by Proterozoic age massif-type anorthosite and related rock types presents their geological, mineralogical, geochemical, and geoenvironmental attributes. Although these Proterozoic rocks are found worldwide, the majority of known deposits are found within exposed rocks of the Grenville Province, stretching from southwestern United States through eastern Canada; its extension into Norway is termed the Rogaland Anorthosite Province. This type of Fe-Ti-oxide deposit dominated by ilmenite rarely contains more than 300 million tons of ore, with between 10- to 45-percent titanium dioxide (TiO2), 32- to 45-percent iron oxide (FeO), and less than 0.2-percent vanadium (V).

The origin of these typically discordant ore deposits remains as enigmatic as the magmatic evolution of their host rocks. The deposits clearly have a magmatic origin, hosted by an age-constrained unique suite of rocks that likely are the consequence of a particular combination of tectonic circumstances, rather than any a priori temporal control. Principal ore minerals are ilmenite and hemo-ilmenite (ilmenite with extensive hematite exsolution lamellae); occurrences of titanomagnetite, magnetite, and apatite that are related to this deposit type are currently of less economic importance. Ore-mineral paragenesis is somewhat obscured by complicated solid solution and oxidation behavior within the Fe-Ti-oxide system. Anorthosite suites hosting these deposits require an extensive history of voluminous plagioclase crystallization to develop plagioclase-melt diapirs with entrained Fe-Ti-rich melt rising from the base of the lithosphere to mid- and upper-crustal levels. Timing and style of oxide mineralization are related to magmatic and dynamic evolution of these diapiric systems and to development and movement of oxide cumulates and related melts.

Active mines have developed large open pits with extensive waste-rock piles, but because of the nature of the ore and waste rock, the major environmental impacts documented at the mine sites are reported to be waste disposal issues and somewhat degraded water quality.

First posted August 22, 2013

For additional information contact:
Director, Central Mineral and Environmental Resources Science Center
U.S. Geological Survey
Box 25046, Mail Stop 973
Denver, CO 80225
http://minerals.cr.usgs.gov/

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Suggested citation:

Woodruff, L.G., Nicholson, S.W., and Fey, D.L., 2013, A deposit model for magmatic iron-titanium-oxide deposits related to Proterozoic massif anorthosite plutonic suites: U.S. Geological Survey Scientific Investigations Report 2013–5091, 47 p., http://pubs.usgs.gov/sir/2013/5091/.



Contents

Abstract

Introduction

Deposit Type and Related Commodities

Historical Evolution of Descriptive and Genetic Knowledge and Concepts

Regional Environment

Physical Description of Deposit

Geophysical Characteristics

Hypogene Ore Characteristics

Gangue Mineral Characteristics

Hydrothermal Alteration

Supergene Ore and Gangue Characteristics

Weathering and Supergene Processes

Geochemical Characteristics

Petrology of Associated Igneous Rocks

Petrology of Associated Sedimentary Rocks

Petrology of Associated Metamorphic Rocks

Theory of Deposit Formation

Ore Deposit System Affiliation

Sources of Ti-Fe-P-Ore Components

Mechanisms that Concentrate Ore

Summary of the Origin of Magmatic Fe-Ti-Oxide Deposits

Geological Assessment Guides

Attributes Required for Inclusion in Permissive Tract at Various Scales

Geochemical Considerations

Geophysical Attributes

Geoenvironmental Features and Anthropogenic Mining Effects

Acknowledgments

References Cited

Appendix


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