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Chapter 13 of Geology, Geochemistry, and Genesis of the Greens Creek Massive Sulfide Deposit, Admiralty Island, Southeastern Alaska Edited by Cliff D

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Chapter 13 of Geology, Geochemistry, and Genesis of the Greens Creek Massive Sulfide Deposit, Admiralty Island, Southeastern Alaska Edited by Cliff D Structure of the Lower Southwest Orebody, Structural Comparison to Neighboring Orebodies, and Tectonic Model for the Greens Creek Deposit By Katja Freitag Chapter 13 of Geology, Geochemistry, and Genesis of the Greens Creek Massive Sulfide Deposit, Admiralty Island, Southeastern Alaska Edited by Cliff D. Taylor and Craig A. Johnson Professional Paper 1763 U.S. Department of the Interior U.S. Geological Survey Contents Abstract .......................................................................................................................................................371 Introduction.................................................................................................................................................371 Structural Background..............................................................................................................................374 Methodology ...............................................................................................................................................375 Structure of the Lower Southwest Orebody .........................................................................................375 Dmineral Deformation ............................................................................................................................375 D0 Deformation..................................................................................................................................375 D1 Deformation..................................................................................................................................375 D2 Deformation..................................................................................................................................375 D3 Deformation..................................................................................................................................378 D4 Deformation..................................................................................................................................386 D5 Deformation..................................................................................................................................388 Structural Transition and Comparison to Neighboring Orebodies ....................................................389 Model of the Structural and Tectonic Evolution of the Greens Creek Deposit................................399 Summary and Conclusions .......................................................................................................................401 Acknowledgments .....................................................................................................................................401 References Cited........................................................................................................................................401 Figures 1. Location map of the Greens Creek deposit ..........................................................................372 2. Cross section 2400 of the Greens Creek deposit looking north ........................................373 3. Plan of Greens Creek orebodies (A) and Lower Southwest orebody located on the mine grid (B) ..................................................................................................................376 4. Drawing and photograph of typical structures in massive argillite beds in the stratigraphic hanging wall of the Lower Southwest orebody ...........................................377 5. Rib map of siliceous rock showing well-developed S2 and S3 foliation .........................377 6. Cross section 2500 of the Lower Southwest orebody ........................................................378 7. Plan map of the 224 level, Lower Southwest orebody........................................................379 8. Photomicrograph of a quartz-carbonate “ladder” vein offset along a stylolite ............380 9. Cross section illustrating early east-directed thrust faults in massive argillite beds after removing effects of F2 and F3 folding ................................................................380 10. Rib map showing argillite forming the core of a major isoclinal F2 syncline, Lower Southwest orebody ......................................................................................................380 11. Plan map of level 224 onto which the locations of F2 fold axes are projected to show the axis trends ............................................................................................................381 12. Cross section 2400 onto which the locations of F2 fold axes are projected to show the axis plunges .............................................................................................................383 13. Photomicrograph of S2 stylolitic foliation in stratigraphic footwall siliceous rocks .....383 14. Photomicrograph of white mica forming penetrative S2 foliation. S2 foliation is folded S3 crenulation cleavage ..............................................................................................383 15. Cross-sectional model showing the development of shears along F2 anticlines during progressive D2 deformation .......................................................................................384 16. Rib map of F3 folds ....................................................................................................................385 17. Schematic cross section illustrating the “ripple and fall” model of F3 fold distributions at the Greens Creek deposit ............................................................................386 18. Ramsay and Huber’s (1987) principal type-3 three-dimensional fold ...............................387 19. Photomicrograph of S3 stylolites in siliceous rock .............................................................388 20. Photomicrograph and sketch of spaced S3 crenulation cleavage in sericitic phyllite, defined by S3 cleavage domains and microlithons .............................................389 21. Rib map depicting intermediate to steeper sections along the upper thrust of the Lower Southwest thrust system ......................................................................................390 22. Rose diagram comparing strikes of left-slip faults (n=8) with right-slip high-angle faults .......................................................................................................................390 23. Map of the various terranes and their bounding regional strike-slip faults, southeast Alaska and the structural setting of the Greens Creek deposit .....................391 24. Schematic cross section illustrating the sequence of brittle deformation events .......392 25. Block diagram representing two fault blocks of massive sulfide of the Lower Southwest orebody offset across right-slip faults, with an intensely fractured fault zone ..................................................................................................................393 26. Schematic model for the transition from the Lower Southwest to the Upper Southwest and 224 West Bench orebodies based on F2 fold geometry .........................394 27. Lower hemisphere equal area projections of F3 fold axes of the (a) Lower Southwest and (b) Upper Southwest and 200 South orebodies .......................................396 28. Lower hemisphere equal area projections of L3 crenulation lineations of the (a) Lower Southwest (n=22) and (b) Upper Southwest (triangles; n=16), 224 West Bench (square; n=1), and 200 South (star; n=1) orebodies .....................................396 29. Lower hemisphere equal-area projections of poles to S2 foliation planes measured in footwall rocks of the (a) Lower Southwest (b) Upper Southwest, and (c) 224 West Bench orebodies ........................................................................................397 30. Lower hemisphere equal area projections of poles to S3 cleavage in the (a) Lower Southwest, 200 South, (b) Upper Southwest, and (c) 224 West Bench orebodies ...................................................................................................................................398 31. Schematic representation of polyharmonic folds developed in competent layers of varying thickness and incompetent host materials showing the effect of the thickness of the mineralized horizon during folding ...........................................................399 32. Model of the structural and tectonic evolution of the Greens Creek deposit ................400 Tables 1. Deformation nomenclature (taken from chap. 7) ................................................................374 2. Deformation nomenclature used in this study .............................................................................374 3. Comparison of orientations of structural features in the Lower Southwest, Upper Southwest, 224 West Bench and 200 South orebodies ..........................................395 4. Comparison of orebody thickness and orientations of structural features ....................395 Structure of the Lower Southwest Orebody, Structural Comparison to Neighboring Orebodies, and Tectonic Model for the Greens Creek Deposit By Katja Freitag Abstract southeastern Alaska (fig. 1), formed by structurally con- trolled hydrothermal activity in a back-arc basin environment. Greens Creek is a Zn-Pb-Ag-(Cu-Au) volcanogenic mas- Numerous sulfide orebodies are located discontinuously along sive sulfide deposit that formed in a back-arc basin
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