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Open-File Report 2010-1212

Co-Cu-Au Deposits in Metasedimentary Rocks—A Preliminary Report

By J.F. Slack,1 J.D. Causey,2 R.G. Eppinger,3 J.E. Gray,3 C.A. Johnson,4 K.I. Lund,3 and K.J. Schulz1

1U.S. Geological Survey, National Center, MS 954, Reston, VA 20192
2U.S. Geological Survey, 904 West Riverside Ave., Spokane, WA 99201
3U.S. Geological Survey, Denver Federal Center, Box 25046, MS 973, Denver, CO 80225
4U.S. Geological Survey, Denver Federal Center, Box 25046, MS 963, Denver, CO 80225

ABSTRACT

Thumbnail of and link to report PDF (1.00 MB)

A compilation of data on global Co-Cu-Au deposits in metasedimentary rocks refines previous descriptive models for their occurrence and provides important information for mineral resource assessments and exploration programs. This compilation forms the basis for a new classification of such deposits, which is speculative at this early stage of research. As defined herein, the Co-Cu-Au deposits contain 0.1 percent or more by weight of Co in ore or mineralized rock, comprising disseminated to semi-massive Co-bearing sulfide minerals with associated Fe- and Cu-bearing sulfides, and local gold, concentrated predominantly within rift-related, siliciclastic metasedimentary rocks of Proterozoic age. Some deposits have appreciable Ag ± Bi ± W ± Ni ± Y ± rare earth elements ± U. Deposit geometry includes stratabound and stratiform layers, lenses, and veins, and (or) discordant veins and breccias. The geometry of most deposits is controlled by stratigraphic layering, folds, axial-plane cleavage, shear zones, breccias, or faults. Ore minerals are mainly cobaltite, skutterudite, glaucodot, and chalcopyrite, with minor gold, arsenopyrite, pyrite, pyrrhotite, bismuthinite, and bismuth; some deposits have appreciable tetrahedrite, uraninite, monazite, allanite, xenotime, apatite, scheelite, or molybdenite. Magnetite can be abundant in breccias, veins, or stratabound lenses within ore or surrounding country rocks. Common gangue minerals include quartz, biotite, muscovite, K-feldspar, albite, chlorite, and scapolite; many deposits contain minor to major amounts of tourmaline. Altered wall rocks generally have abundant biotite or albite. Mesoproterozoic metasedimentary successions constitute the predominant geologic setting. Felsic and (or) mafic plutons are spatially associated with many deposits and at some localities may be contemporaneous with, and involved in, ore formation. Geoenvironmental data for the Blackbird mining district in central Idaho indicate that weathering of abundant Fe, S, As, Co, and Cu in sulfide minerals of the deposits produces acidic waters, especially in pyrite-rich deposits; mine runoff has high concentrations of Fe, Cu, and Mn that exceed U.S. drinking water or aquatic life standards.

First posted September 2010

For additional information contact:
John F. Slack
U.S. Geological Survey
12201 Sunrise Valley Drive, Mail Stop 954
Reston, VA 20192
(703)-648-6337

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Suggested citation:

Slack, J.F., Causey, J.D., Eppinger, R.G., Gray, J.E., Johnson, C.A., Lund, K.I., and Schulz, K.J., 2010, Co-Cu-Au deposits in metasedimentary rocks—A preliminary report: U.S. Geological Survey Open-File Report 2010–1212, 13 p. (Available at http://pubs.usgs.gov/of/2010/1212.)



Contents

Abstract

Introduction

Tonnage and Grade Variations

Regional Environment

Deposit Features

Mineralogy

Hydrothermal Alteration

Geochemical Characteristics

Metasedimentary Host Rocks

Petrology of Related Igneous

Genetic Models

Geophysical Characteristics

Geoenvironmental Features and Anthropogenic Mining Effects

Acknowledgments

References Cited



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