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A magnetotelluric (MT) survey across the Kimberley Craton, northern, Western Australia

A magnetotelluric (MT) survey across the Kimberley Craton, northern, Western Australia . J Spratt 1 , MC Dentith 2 , S Evans 3 , A Aitken 2 , M Lindsay 2 , JA Hollis, IM Tyler, A Joly 2 , J Shragge 4

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A magnetotelluric (MT) survey across the Kimberley Craton, northern, Western Australia

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  1. A magnetotelluric (MT) survey across the Kimberley Craton, northern, Western Australia J Spratt1, MC Dentith2, S Evans3, A Aitken2, M Lindsay2, JA Hollis, IM Tyler, A Joly2, J Shragge4 1 - Consultant, Wakefield, Quebec, Canada2 - Centre for Exploration Targeting, The University of Western Australia3 - Moombarriga Geoscience 4 - Centre for Petroleum Geoscience & CO2 Sequestration, The University of Western Australia

  2. Magnetotellurics • Passive, frequency domain, electromagnetic method • Maps electrical conductivity (conductance) • Deep penetrating, • Depth penetration depends on recording time • Routinely used to map entire lithosphere • Established ‘academic’ tool, emerging exploration applications •  Cheap •  Can see deep •  Limited resolution •  Controls on deep cond’ poorly understood Olympic Dam Location (X) Deposit MT Stations 0 Proterozoic Sediments Proterozoic Sediments Granite Batholith 10 Resistive Middle Crust 20 Resistive F a Depth (km) u l Archaean t / S u Crust Conductive Crust - t u 30 r Due to Mineralising Fluids? e Resistivity ( W m) 1826 40 350 0 20 KILOMETRES 20 50

  3. Magnetotellurics • Record a time series – electric (E) and magnetic (H) fields • Convert to frequency dependent apparent resistivity and phase (Ex and Hy; Ey and Hx) • Combine data from multiple stations Ex TE App’ Res’ Ey TM Hx Phase Hy Period 10 Minutes Increasing Depth

  4. Magnetotellurics • Stn data used to create pseudo, para and cross sections • Dimensionality, strike direction Apparent Resistivity r r 1 2 r 1 Period r Depth Period 2 Section Location Location Period Period 9 -9 0 Skew r r > 2 1 50 km

  5. Magnetotellurics • Station data are used to create pseudo, para and cross sections • Static effects • Conductivity variations in the near surface • Modelling  sub-surface electrical conductivity distribution • Forward modelling (Model  data) - manual • Inverse modelling (Data  model) – semi-automated • 2D routine, 3D now becoming available Location Apparent Resistivity r r 1 2 Static Conductive Shifts Zone Period y c n r e 1 u q e r F g r o L 2 Section r r > 2 1 50 km

  6. Why So Deep? Cu, Au (Escondida, Yanacocha) Cu/Au, Au (Olympic Dam, Muruntau) NiS, Pb-Zn (W Musgraves, Mt.Isa) “At the scale of the Yilgarn Craton … the gold deposits … and nickel sulphide deposits … the major architecture controlling these systems are lithospheric in nature adjacent to paleocraton margins” (McCuaig et al, 2010, Ore Geology Reviews) NiS (Norilsk) Diamonds (Begg et al. 2010, Economic Geology) Plume impacted on base of lithosphere

  7. GSWA-CET MT Surveys • Government funded MT surveys (though CET) • Southern Yilgarn (Hyden-Norseman) ++ extension & LP survey • Musgrave Province • Yilgarn-Fraser (Kambalda-Balladonia) + extns • East Capricorn • Kimberley • Capricorn (SIEF) • All areas with major suture zones

  8. Kimberley MT survey • Funding from Kimberley Science & Conservation Strategy via GSWA • Basement to the Kimberley Basin? • Map structures beneath the Basin • Establish large-scale geometry of the surrounding Orogens and Basins • 7 transects • Combination of broad band, long period MT and time-domain EM NORTH EAST CENTRAL CROSS 4 Kimberley & Speewah Basins WEST CROSS 2 CROSS 3 Ord Basin KLO CROSS 1 HCO Canning Basin Birrindudu Basins KLO – King Leopold Orogen HCO – Halls Creek Orogen Lamboo Province 100 km

  9. MT Deployment • Broadband MT • 100 m electric dipoles • Hx-y-z B field (coils) • 5-20 km stn spacing • ~40 hrs recording • Long period MT • 50 m electric dipoles • Hx-y-z B field (magnetometers) • 20-50 km stn spacing • Time-domain EM • Soundings to measure static shift

  10. Rationale • Interpretation of potential field data • Gunn & Meixner (1998)

  11. Rationale • A series of ‘zones’ in the basement to the Kimberley Basin • Ophiolites in the west • Deep penetrating structures trending NE-SW

  12. Rationale • Halls Creek Orogen site of palaeosubductionzone? EAST CROSS 4 KLO HCO KLO – King Leopold Orogen HCO – Halls Creek Orogen Lamboo Province 100 km

  13. 0- 20- 40- ) m k 60- ( h t p e 80- D 100- 120- Strike = 57 Strike = -65 5 0 km RMS = 2.30 1 10 140- RMS = 2.70 4 Kimberley MT - NORTH S N G & M (1998) G & M (1998) G & M (1998) Zone C Zone D Zone E Kimberley Basin Resistive lower crust Moho? Rho (ohm-m) 5 10 Lateral change in 4 10 mantle resistivity 3 10 2 10 Change in Modelled NORTH Strike Direction

  14. 0- ?? 20- 40- ) m k 60- ( h t p e 80- D 100- TMTEHZ 120- pha e.f.= 5% RMS = 2.66 50 km pho e.f.= 20% Strike = -52 140- Kimberley MT - WEST CROSS 1 CROSS 2 CROSS 3 SW NE Kimberley Basin Canning Basin KLO Resistive upper crust Dipping more Resistive conductive zones - faults? More conductive lower crust lower crust Moho? Rho (ohm-m) Lateral changes in mantle resistivity 5 10 4 10 3 10 2 10 1 10 4 WEST

  15. CROSS 1 CROSS 2 CROSS 3 Kimberley MT – CROSS (KLO) SE WEST WEST WEST SE SE NW NW NW 0- 0- 0- Rho (ohm-m) 5 10 20- 20- 20- ) ) ) m m m 4 10 k k 40- 40- 40- k ( ( ( h h h t t 3 t 10 p p p e e 60- 60- 60- 5 0 km e D 5 0 km D D 2 10 5 0 km 80- 80- 80- Strike = 94 Strike = -47 1 Strike = -20 10 RMS = 3.20 RMS = 2.99 4 100- 100- 100- C ROSS 2 C ROSS 1 C ROSS 3

  16. W E Kimberley Basin 0- 20- Resistive lower crust 40- Moho? ) Rho m (ohm-m) k 60- ( 5 10 h t Lateral change in p mantle resistivity e 80- 4 D 10 100- 3 10 TMTEHZ 120- 2 10 pha e.f.=5% Strike = -45 50 km pho e.f = 20% 1 10 140- RMS = 1.85 4 CENTRAL Kimberley MT - CENTRAL

  17. 0- 20- 40- ) m k 60- ( h t p e 80- D 100- TMTEHZ 120- pha e.f.=5% Strike = 38 pho e.f = 20% 50 km RMS = 2.98 140- Kimberley MT – EAST, CROSS (HCO) S N W E CROSS 4 EAST Kimberley Basin HCO 0- Kimberley Basin Phanerozoic 20- sediments? ) More conductive lower crust m 40- k ( Moho? h Rho t p (ohm-m) 60- 5 0 km e Subducted 5 10 D Lateral change in lithosphere? mantle resistivity 80- Strike = 60 4 10 RMS = 3.27 100- 3 10 C ROSS 4 2 10 1 10 4 EAST

  18. Kimberley MT • Provisional (2D modelling) geological conclusions • The Kimberley Basin sedimentary and volcanic sequences are a thin conductive unit with a maximum thickness of 5 km. Phanerozoic sediments are also conductive. • The upper crust (resistivity >10, 000 Ohm·m), consistent with the presence of Archean rocks • The lower crust is conductive and the depth to its base is in reasonable agreement with Moho depth estimates of 38 to 45 km from seismic data.

  19. Kimberley MT • Provisional (2D modelling) geological conclusions • The upper crust is cut by several less resistive, steeply-dipping structures – faults? It is unclearif these penetrate to the mantle. • Lateral changes in the conductivity of the lower crust (and mantle?) may be associated with the boundaries of major crustal blocks. • At the western margin of the Kimberley Basin, there appear to be important along-profile conductivity signatures that define boundaries between different structural blocks. • At the eastern margin of the Kimberley Basin, a northwestward dipping resistive feature is interpreted as ancient subducted lithosphere. • The boundaries recognized from gravity and magnetic data beneath the Kimberley Basin do not coincide with major features in the upper crust, and by implication are probably not major structures.

  20. Acknowledgments • Kimberley Science & Conservation Strategy • Landowners • Field crew • Ray Addenbrooke, Nick Mann, Cody Graco, Dean Clinnick and Christian Anzenhofer

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