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Professional Paper 1763

Prepared in cooperation with Kennecott Greens Creek Mining Company

Geology, Geochemistry, and Genesis of the Greens Creek Massive Sulfide Deposit, Admiralty Island, Southeastern Alaska

Edited by Cliff D. Taylor and Craig A. Johnson

Abstract

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In 1996, a memorandum of understanding was signed by representatives of the U.S. Geological Survey and Kennecott Greens Creek Mining Company to initiate a cooperative applied research project focused on the Greens Creek massive sulfide deposit in southeastern Alaska. The goals of the project were consistent with the mandate of the U.S. Geological Survey Mineral Resources Program to maintain a leading role in national mineral deposits research and with the need of Kennecott Greens Creek Mining Company to further development of the Greens Creek deposit and similar deposits in Alaska and elsewhere. The memorandum enumerated four main research priorities: (1) characterization of protoliths for the wall rocks, and elucidation of their alteration histories, (2) determination of the ore mineralogy and paragenesis, including metal residences and metal zonation within the deposit, (3) determination of the ages of events important to ore formation using both geochronology and paleontology, and (4) development of computer models that would allow the deposit and its host rocks to be examined in detail in three dimensions.

The work was carried out by numerous scientists of diverse expertise over a period of several years. The written results, which are contained in this Professional Paper, are presented by 21 authors: 13 from the U.S. Geological Survey, 4 from Kennecott Greens Creek Mining Company, 2 from academia, and 2 from consultants.

The Greens Creek deposit (global resource of 24.2 million tons at an average grade of 13.9 percent zinc, 5.1 percent lead, 0.15 troy ounce per ton gold, and 19.2 troy ounces per ton silver at zero cutoff) formed in latest Triassic time during a brief period of rifting of the Alexander terrane. The deposit exhibits a range of syngenetic, diagenetic, and epigenetic features that are typical of volcanogenic (VMS), sedimentary exhalative (SEDEX), and Mississippi Valley-type (MVT) genetic models. In the earliest stages of rifting, formation of precious-metal-rich silica-barite-carbonate white ores began at low temperature in a shallow, subaqueous setting, probably a thin carbonate shelf on the flanks of the Alexander landmass. Epigenetic carbonate replacement textures in the footwall dolostones are overlain by stratiform silica-carbonate-barite-rich ores and indicate that early mineralization formed at and just beneath the paleo sea floor by mixing of a reduced, precious-metal-rich, base-metal-poor hydrothermal fluid with oxygenated seawater. As rifting intensified, the shelf was downfaulted and isolated as a graben. Isolation of the basin and onset of starved-basin shale sedimentation was concurrent with emplacement of mafic-ultramafic intrusives at shallow levels in the rift, resulting in an increasingly higher temperature and progressively more anoxic ore-forming environment. The formation of the main stage of massive sulfide ores began as the supply of bacterially reduced sulfur increased in the accumulating shales. As the main-stage mineralization intensified, shale sedimentation inundated the hydrothermal system, eventually forming a cap. Biogenic sulfate reduction supplied reduced sulfur to the base of the shales where mixing occurred with hot, base-metal-rich hydrothermal fluids. Ore deposition continued by destruction and epigenetic replacement of the early white ores in proximal areas and by inflation and diagenetic replacement of unlithified shale at the interface between the white ores and the base of the shale cap. Ore deposition waned as the shales became lithified and as the supply of bacterially reduced sulfur to the site of ore deposition ceased. The final stages of rifting resulted in the emplacement of mafic-ultramafic intrusive rocks into the Greens Creek system and extrusion of voluminous basaltic flows at the top of the Triassic section. Greenschist facies metamorphism during the Jurassic-Cretaceous accretion of the Alexander terrane to the continental margin resulted in recrystallization, remobilization, and significant upgrading of the Greens Creek orebody.

Current ore-genetic models are inadequate to explain the diverse characteristics of the Greens Creek deposit. We suggest that Greens Creek represents a transitional type of deposit that formed as a result of its evolving metallogenic setting in a propagating intra-arc rift. It is therefore a hybrid deposit within a spectrum of exhalative to replacement-style deposits.

First posted September 8, 2010

For additional information contact:

USGS Central Mineral and Environmental Resources Science Center
Box 25046, Mail Stop 973
Denver, CO 80225

http://minerals.cr.usgs.gov/

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Suggested citation:

Taylor, C.D., and Johnson, C.A., editors, 2010, Geology, geochemistry, and genesis of the Greens Creek massive sulfide deposit, Admiralty Island, southeastern Alaska: U.S. Geological Survey Professional Paper 1763, 429 p., 7 plates on CD.



Contents

1. Introduction and Overview of the U.S. Geological Survey–Kennecott Greens Creek Mining Company Cooperative Applied Research Project at the Greens Creek Mine

By Cliff D. Taylor and Craig A. Johnson

2. The Late Triassic Metallogenic Setting of the Greens Creek Massive Sulfide Deposit in Southeastern Alaska

By Cliff D. Taylor, John Philpotts, Wayne R. Premo, Alan L. Meier, and Joseph E. Taggart, Jr.

3. The History of Greens Creek Exploration

By Andrew W. West

4. Geology of the Greens Creek Mining District

By Norman A. Duke, Paul A. Lindberg, and Andrew W. West

5. The Airborne Geophysical Survey of the Greens Creek Area

By James Fueg

6. Geology and Metal Zoning of the Greens Creek Massive Sulfide Deposit, Southeastern Alaska

By Cliff D. Taylor, Andrew W. West, Kerry G. Lear, Tim E. Hall, and John M. Proffett

7. Geologic Structure of the Greens Creek Mine Area, Southeastern Alaska

By John M. Proffett

8. Geochemistry of Metasedimentary Rocks in the Hanging Wall of the Greens Creek Massive Sulfide Deposit and of Shales Elsewhere on Admiralty Island

By Craig A. Johnson, Cliff D. Taylor, Joel S. Leventhal, and Katja Freitag

9. Mineralogical, Textural, and Metal Residence Studies of Primary, Recrystallized, and Remobilized Ores of the Greens Creek Deposit

By Cliff D. Taylor, Steven J. Sutley, and Frederick E. Lichte

10. Sulfur and Lead Isotope Characteristics of the Greens Creek Polymetallic Massive Sulfide Deposit, Admiralty Island, Southeastern Alaska

By Cliff D. Taylor, Wayne R. Premo, and Craig A. Johnson

11. Microfossil and Radioisotopic Geochronological Studies of the Greens Creek Host Rocks

By Wayne R. Premo, Cliff D. Taylor, Lawrence W. Snee, and Anita G. Harris

12. Radiogenic Isotopic Characterization and Petrogenesis of Host Rocks to the Greens Creek Deposit

By Wayne R. Premo, and Cliff D. Taylor

13. Structure of the Lower Southwest Orebody, Structural Comparison to Neighboring Orebodies, and Tectonic Model for the Greens Creek Deposit

By Katja Freitag

14. Three-Dimensional Modeling and Visualization of Greens Creek Drill-hole Data

By Gregory K. Lee, and Cliff D. Taylor

15. A Genetic Model for the Greens Creek Polymetallic Massive Sulfide Deposit, Admiralty Island, Southeastern Alaska

By Cliff D. Taylor, Kerry G. Lear, and Steven R. Newkirk


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