Linking structure & mineralisation in Laverton, with specific reference to Sunrise Dam & Wallaby John Miller

Transcription

1 Linking structure & mineralisation in Laverton, with specific reference to Sunrise Dam & Wallaby John Miller Centre for Exploration Targeting University of Western Australia

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3 W Swager (1997) regional D 2 Granny Smith Mineralised conjugate veins (E-W compression; Ojala et al. 1993) within mafic granodiorite (2665 +/- 4 Ma; Hill et al., 1992) E Photo from Granny Smith Geology report, Placer Dome Asia Pacific, also Ojala et al., (1993)

4 E Swager (1997) regional D 2 Sunrise Dam Mineralised conjugate pop-up (E-W compression) W BIF-style ore D 1 north-south lineation (some Au)

5 Nugus et al. (2005) D 3 Sunrise Dam (NNW-SSE σ 1 ). Late veins with distinct mineralogy that over prints D 2 and D 1 alteration D 3 vein Sunrise shear D 3 alteration over growing D 1 pyrite D 3 alteration

6 W Sunrise Dam section Cleo Upper SZ Western Shear Sunrise SZ E Watu As (Blue; Leapfrog model) 20 m slice, 99410N, 500m grid ULU SZ Photographer unknown

7 Sunrise D 3 (sinistral-slip model) Poster

8 Definitely late dextral movement sense (Tornatora 2002; D 4 of Nugus et al., 2005) - probably dextral transtension N S Stepped fibres indicating dextral movement

9 Wallaby early magmatic calcite veins (some Au ; Sue Drieberg) associated with syenites (Intrusive age: /- 3 Ma; Salier et al., 2004) N S Plunge of wallaby syenite pipe looks like radial extension (σ 2 =σ 3 )

10 Wallaby early magmatic calcite veins (some Au; Sue Drieberg) (NOTE: Au can occur with syenite e.g., 98 Ma Mt Dromedary, N.S.W.) N S Plunge of wallaby syenite pipe looks like radial extension (σ 2 =σ 3 )

11 Wallaby two main alteration styles associated with major phases of Au (similar σ 1 but different σ 2 & σ 3 ) Oxidised (Hem, py, Fe-dol, cc, qtz, ser): hematite-associated FIRST Reduced (py, dol, cc, qtz, ser): sinistral-slip SECOND (2650 +/- 6 Ma; Salier et al., 2004) S Sinistral-slip N Hematite-associated

12 SE Hematite-bearing Zone 50 NW Zone 50 Fault in footwall (Conjugate) Clast (grab sample 13.4 g/t)

13 Zone 50 N Zone 240 Horizontal slice through Wallaby pit highlights hematite-bearing Zone 50 (purple) being dragged and offset by sinistral movement along Zone 240 (green)

14 Wallaby pit (top-to-nw = sinistral oblique-slip) NW Zone 240 SE green is zone 240 No hematite infer more reduced fluid from isotope data in Y3 report = tapping of deeper reduced fluid reservoir NW SE SE

15 Post E-W (D 2 ) shortening Poster

16 Post E-W (D 2 ) shortening

17 Compilation/correlation diagram

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19 Model (excludes early D 1 gold) One major thermal event linked to change in magmatism (Champion & Cassidy 2002) and generation/infiltration of fluids. A series of stress switches change the fluid pathways through time. Results in fluids traveling up different pathways - different for compression vs. strike-slip. LOW STRAIN. Different time scales for above two processes Some gold associated with plutons (e.g., magmatic calcite veins at Wallaby)

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22 Infer regional NNW-SSE compression

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25 ~NW SE contraction Development of a restraining stepover Sinistral strike-slip movement Localized NW and SE thrusting and significant dilation within the restraining stepover Mantle derived intrusions Major gold event D 3 Au

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27 Some conclusions World-class deposits have gold lodes linked to more than one stress field Can correlate switches in paleostress to changes in observed alteration Appears gold is associated with oxidised fluids linked to intrusions and also deeply sourced reduced fluids relative importance and abundance of each fluid type would have changed through time Steep-dipping Sunrise Dam style D 3 lodes at strikechanges on early formed regional structures could be targeted