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Scientific Investigations Report 2010–5070–G

Descriptive and Geoenvironmental Model for Cobalt-Copper-Gold Deposits in Metasedimentary Rocks

Edited by John F. Slack

Chapter G of
Mineral Deposit Models for Resource Assessment

Thumbnail of and link to report PDF (16.5 MB)Abstract

A compilation of global data on cobalt-copper-gold (Co-Cu-Au) deposits hosted by metasedimentary rocks refines previous descriptive models for their occurrence and provides important information for mineral resource assessments and exploration programs. As defined herein, the Co-Cu-Au deposits contain 0.1 percent or more by weight of Co in ore or mineralized rock, consisting of disseminated to semi-massive Co-bearing sulfarsenide and sulfide minerals with associated Fe- (iron) and Cu-bearing sulfides, and local gold, concentrated predominantly within rift-related, siliciclastic metasedimentary rocks chiefly of Proterozoic age. Most of the deposits contain abundant As (arsenic) and have atomic Co/As ratios that approach 1. Some deposits have appreciable Ag ± Bi ± W ± Ni ± Y ± REE ± U (silver, bismuth, tungsten, nickel, yttrium, rare earth elements, and uranium). 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, but is absent in some deposits. Common gangue minerals include quartz, biotite, muscovite, K-feldspar, albite, chlorite, and (or) scapolite; many deposits contain minor to major amounts of tourmaline. Altered wall rocks generally contain abundant biotite or albite. Based on geoenvironmental data for the Blackbird district in central Idaho, weathering of minerals containing abundant Fe (iron), S (sulfur), As, Co, and Cu in the deposits produces acidic waters, particularly in pyrite-rich deposits. Mine runoff has concentrations of Fe, Cu, and Mn (manganese) that exceed U.S. drinking water or aquatic life guidelines.

First posted December 30, 2013

Revised March 14, 2014

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:

Slack, J.F., ed., 2013, Descriptive and geoenvironmental model for cobalt-copper-gold deposits in metasedimentary rocks, chap. G of Mineral deposit models for resource assessment (ver. 1.1, March 14, 2014): U.S. Geological Survey Scientific Investigations Report 2010–5070–G, 218 p., http://dx.doi.org/10.3133/sir20105070G.

ISSN 2328-0328 (online)



Contents

1. Introduction

2. Deposit Type and Associated Commodities

3. Historical Evolution of Descriptive and Genetic Knowledge and Concepts

4. Regional Environment

5. Physical Description of Deposits

6. Geophysical Characteristics

7. Hypogene Ore and Gangue Characteristics

8. Hydrothermal Alteration

9. Supergene Ore and Gangue Characteristics

10. Weathering/Supergene Processes

11. Geochemical Characteristics

12. Petrology of Associated Igneous Rocks

13. Petrology of Associated Sedimentary Rocks

14. Petrology of Associated Metamorphic Rocks

15. Theory of Deposit Formation

16. Exploration/Resource Assessment Guides

17. Geoenvironmental Features and Anthropogenic Mining Effects

18. Knowledge Gaps and Future Research Directions

Appendix 1. Database for Co-Cu-Au Deposits Included in This Report

Appendix 2. Database for Co-Cu-Au Deposits Not Included in This Report


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