Structural Evolution of the Keno Hill Ag-Pb-Zn mining district, Yukon

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1 Structural Evolution of the Keno Hill Ag-Pb-Zn mining district, Yukon Simon D. Craggs, David R. Lentz and Joseph C. White

2 Introduction The Keno Hill Ag-Pb-Zn mining district is located in the central Yukon, approximately 350 km north of Whitehorse. Between 1913 and 1990 the camp produced: > 6 M kg average grade 1,373 g/t 300,000T average grade 6.7% 195,000T average grade 4.1% Fault-controlled, vein-type deposit Ore is typically hosted in the Mississippian Keno Hill Quartzite of the Selwyn Basin. At least three deformation phases identified.

3 Location and Regional Geology Arctic Ocean Keno Hill Whitehorse Pacific Ocean

4 Location and Regional Geology Modified after Mair et al., 2006.

5 District Geology Modified after Murphy (1997), Boyle, (1965), and McOnie (2008).

6 District Geology Murphy, 1997

7 D 1 - Brittle Ductile Deformation Two phases (F 1 F 2 ) of subcoaxial, north to northwest verging isoclinal folding; North- to northwest-directed thrust faulting; Composite S 1-2 foliation development; and Greenschist grade metamorphism. S N Sime open pit

D 1 - Brittle Ductile Deformation F 1 axial trace F 2

2 m F 1 axial trace Equal Area (Schmidt) N +28S +26S

8 D 1 - Brittle Ductile Deformation F 1 axial trace F 2 axial trace SW F 2 axial trace Equal Area (Schmidt) N +14S +12S +10S +8S +6S +4S +2S E Axial N = 134 F 1-2 Fold axes K , m F 1 axial trace Equal Area (Schmidt) N +28S +26S +24S +22S SW +20S +18S +16S +14S +12S +10S +8S +6S +4S +2S E Axial N = 1293 S 1-2 foliation (poles)

9 D 2 D 3 Brittle Deformation Two sets of approximately orthogonal faults present in district. SW D 2 Northeast- to east northeast-striking, typically sinistral to sinistral-oblique faults; Moderate to steeply southeast-dipping Commonly associated with Ag-Pb-Zn mineralisation (galena, sphalerite, siderite); Sporadic quartz with minor associated Au. KHQ D 3 Northwest- to north-striking dextraloblique faults; Moderate to steeply southwest-dipping; Offset mineralised faults. F 3 folds: open, plunge shallowly southeast. Calumet open pit

10 D 2 Brittle Deformation - Mineralisation NE Equal Area (Schmidt) N Gnst D 1 thrust +10S +8S +6S +4S +2S E KHQ Axial N = 112 Onek open pit Sime open pit Two main populations of D 2 faults: Northeast-striking; North northeast striking.

Grade modelling of Hector-Calumet deposit High-grade ore shots plunge steeply towards the northeast.

11 Local Controls on Mineralisation Steep NE-plunge Long section Steep NE-plunge Plan view Grade shells 400 gpt/ag (green); 600 gpt/ag (Red) Mineralisation associated with NEtrending splay off main structure Plan view slice Grade modelling of Hector-Calumet deposit High-grade ore shots plunge steeply towards the northeast. High-grade shoots spatially associated with NW-trending splay off main WNWtrending structure, and proximal to fault intersection.

Local Controls on Mineralisation σ 1 Pb-Zn veins typically

Faults rotate into extension σ 3 D 2 - Galena Hill Great

(typically galena); σ 1 Red symbols are measured slip vectors

12 Local Controls on Mineralisation σ 1 Pb-Zn veins typically <20 cm width σ 3 Pb-Zn veins typically 20 cm to >1 m width Faults rotate into extension σ 3 D 2 - Galena Hill Great circles are D 2 faults with observed mineralisation (typically galena); σ 1 Red symbols are measured slip vectors on the D 2 faults; Triangles and contours are Pb-Zn veins (galena and sphalerite)

13 Brittle Deformation Post Mineralisation Equal Area (Schmidt) N D 3 structures are sporadically seen across the district. Northwest- to north striking faults; +10S +8S +6S +4S +2S E Slickenlines sporadically present but can give ambiguous slip vectors; Axial N = 142 Offset mineralised structures by up to 100 m; Offsets indicate dextral oblique movement Equal Area (Schmidt) N F 3 folds defined by warped foliation and plunge shallowly southeast. +10S +8S +6S +4S F 3 axial surfaces commonly sub-parallel to D 3 faults E +2S Axial N = 83

mineralisation, and sericite schist unit with a dextral oblique separation by at

14 Brittle Deformation Post Mineralisation NW Hanging wall of Mastiff Fault SE Trace of Mastiff Fault Bermingham open pit Bermingham open pit Mastiff Fault offsets mineralisation, and sericite schist unit with a dextral oblique separation by at least 100 m; Polyclinal to chevron folds abundant in hanging wall plunge southeast.

15 Brittle Deformation F 3 Foliation at Bermingham is folded around SE-plunging axis Foliation is relatively uniformly dipping across district, except around the Bermingham deposit.

16 Localisation of Deformation Helicopter Magnetic Survey - First Vertical Derivative

17 Localisation of Deformation Lineament Density

18 Localisation of Deformation Lineament Density with Location of Keno Hill Quartzite and Greenstone (after McOnie, 2008)

19 Localisation of Deformation Dilational jog

5 Ma), but isochrons considered to give most reliable dates K/Ar analysis by Sinclair et

20 Age of Mineralisation 6 40 Ar/ 39 Ar dates obtained from vein selvages. Ages range from 69.6 Ma to 81.1 Ma. Plateau ages give slightly older dates (72.1 to 85.5 Ma), but isochrons considered to give most reliable dates K/Ar analysis by Sinclair et al. (1980) give ages of 85.1 ± 10.2 Ma, and 88.9 ± 7.8 Ma. Hantlemann (2013) - Pb isotope model age 82 ± 31 Ma.

21 Age of Mineralisation

22 Conclusions Mineralisation focusing is controlled by complex interplay of lithology and structures. Mineralisation preferentially precipitated in packages of Keno Hill Quartzite and to a lesser extent greenstone. F 1-2 folds produce stratigraphic repetition and thicken packages of Keno Hill Quartzite and greenstone, and may produce sites for elevated mineralisation. Mineralising fluids were focused along sinistral-oblique D 2 faults. At left-stepping fault bends, where fault segments strike northeast to north northeast, faults become extensional, and provide dilational sites and wider zones of mineralisation. D 2 faults may have initiated as antithetic structures in response to the onset of orogen-parallel dextral displacement. Continued orogen-parallel displacement and a slight rotation of the local stress field produced D 3, post-mineralisation dextral-oblique faults that offset mineralised structures.

23 Acknowledgements Dave Lentz and Joe White (supervisors), Chris MacFarlane, and Adrian Park Al McOnie, Melanie Roberts, Stan Dodd, Dick Lippoth, Peter Read, Bruce Otto, Tim Hall, Riley Hall, Kathleen Gould, Genevieve Gay, Natasha Morris, Kristen Chislet, Linette MacInnis, Travis Murphy, Seymour Isles, John Nguyen, and Jared Chipman Don Murphy, Venessa Bennett, and Mike Burke Doug Archibald James Siddorn, Anna Fonseca, and Alison Harrington

Lecture 6 Folds, Faults and Deformation Dr. Shwan Omar

Lecture 6 Folds, Faults and Deformation Dr. Shwan Omar Fold: A fold is a bend or wrinkle of rock layers or foliation; folds form as a sequence of ductile deformation. Folding is the processes by which crustal forces deform an area of crust so that layers of