Wolfgram, 2004, em signatures of vhms, cu zn-workshop
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Wolfgram, 2004, em signatures of vhms, cu zn-workshop
- 1. EM Signatures of VHMS in WA Peter Wolfgram, Cu Zn Workshop Perth, October 2004 EM Signatures of Volcanic Hosted Cu Zn Deposits and Implications for Exploration in WA Peter Wolfgram Fugro Airborne Surveys October 2004
- 2. EM Signatures of VHMS in WA Peter Wolfgram, Cu Zn Workshop Perth, October 2004 • Minotaur Resources • Fugro Airborne Surveys Acknowledgements
- 3. EM Signatures of VHMS in WA Peter Wolfgram, Cu Zn Workshop Perth, October 2004 Conductor Location . . . . . . .. . . . . . . . . . . . . v v v v v v vv v v v v v v v v v / / / / / / / Structural Mapping What can we see with EM? Conductivity (S/m) • Type of rock • Water content, salinity • Type of mineral & grade • Connectivity
- 4. EM Signatures of VHMS in WA Peter Wolfgram, Cu Zn Workshop Perth, October 2004 τ2τ1 Time Constant: time when signal decayed 1/3 Good Conductor Moderate Conductor Transmitter Pulse Receiver Listening Time Ground EM: 1 sec Airborne EM: 0.02 sec How do we measure conductivity? Nickel: seconds - minutes (Voisey’s Bay) VMS: 0 - 0.1 sec Regolith: 0 - 0.1 sec Longer time constant for larger bodies ...
- 5. EM Signatures of VHMS in WA Peter Wolfgram, Cu Zn Workshop Perth, October 2004 Conductivity of Common Rocks and Minerals Sphalerite 10 3 10 1 10 -1 10 -3 10 -5 10 -7 10 -9 Igneous Rocks Metamorphic Rocks Sedimentary Rocks Unconsolidated Sediments 100% Chalcopyrite 100% Arsenopyrite 50% 50% 50% 100% Graphite 100% Galena 100% Pyrrhotite 100% Pyrite 100% Hematite 100% Magnetite Conductivity (S/m) 10 6 VMS
- 6. EM Signatures of VHMS in WA Peter Wolfgram, Cu Zn Workshop Perth, October 2004 Table from Bishop and Emerson, 1999, Australian Journal of Earth Sciences 46, 311-328 Conductivities of some VMS Deposits in WA DEPOSITS NOT CONDUCTIVE
- 7. EM Signatures of VHMS in WA Peter Wolfgram, Cu Zn Workshop Perth, October 2004 Target Position Target Position Overburden: 20 m, 1 S/m Target: 100 S Basement: 0.003 S/m 50 m Z X How do we measure structure?
- 8. EM Signatures of VHMS in WA Peter Wolfgram, Cu Zn Workshop Perth, October 2004 Figures from: Fritz & Sheehan, 1984, Exploration Geophysics 15, 127-142, Annan & Lockwood, 1991, Exploration Geophysics 25, 5-12, Boyd & Frankcombe, 1994, Geophysical signatures of Western Australian Mineral Deposits, 133-144, and Huston et al, 2000, Australian Journal of Earth Sciences 47, 217-230, and Structure of some VMS Deposits in WA Salt Creek Mons Cupri Whim Creek ScuddlesTeutonic Bore DEPOSIT NOT CONDUCTIVE DEPOSIT NOT CONDUCTIVE Freddie Well
- 9. EM Signatures of VHMS in WA Peter Wolfgram, Cu Zn Workshop Perth, October 2004 Example: Teutonic Bore Figures from Buselli, 1980, Exploration Geophysics 11 SIROTEM coincident loop field data (left) and scale model curves (below) for 12 m thick body with 4 S/m conductivity. The time constant of the decay is 2.4 ms.
- 10. EM Signatures of VHMS in WA Peter Wolfgram, Cu Zn Workshop Perth, October 2004 Example: Scuddles SIROTEM coincident loop field data. 300 m Approximate position of the Scuddles deposit. Figure from Robinson & Belford, 1991, Exploration Geophysics 22, 315-320
- 11. EM Signatures of VHMS in WA Peter Wolfgram, Cu Zn Workshop Perth, October 2004 Example: Freddie Well Figure from Annan and Lockwood, 1991, Exploration Geophysics 22, 5-12 GEOTEM 75 Hz profile flown in 1990 Freddie Well Surficial 0 1000 2000 3000 4000 5000 6000 7000 Freddie Well Surficial 0 1 2 3 4 5 Millisecs after Turn-off dBx/dt(ppm) GEOTEM Decay GEOTEM 75 Hz decay curves
- 12. EM Signatures of VHMS in WA Peter Wolfgram, Cu Zn Workshop Perth, October 2004 Tables from Bishop and Emerson, 1999, Australian Journal of Earth Sciences 46, 311 Historic Lessons on VMS Targets
- 13. EM Signatures of VHMS in WA Peter Wolfgram, Cu Zn Workshop Perth, October 2004 White Roo Prospect in SA
- 14. EM Signatures of VHMS in WA Peter Wolfgram, Cu Zn Workshop Perth, October 2004 White Roo Prospect in SA White Roo, SIROTEM, Line 499550E 6464200 6464400 6464600 6464800 6465000 6465200 Station 0.01 0.1 1 10 100 1000 0 10 20 30 40 Delay Time (ms) TEMAmplitude(uV/A) STN = 6464700 tau = 12.0 ms
- 15. EM Signatures of VHMS in WA Peter Wolfgram, Cu Zn Workshop Perth, October 2004 • Use wideband EM systems with dense sampling • Include other data such as magnetics • Airborne methods preferable • Include structural mapping in survey objectives • Allow for an EM interpretation phase CONCLUSIONS EM SIGNATURES• Low - moderate conductivities • Short - moderate time constants • Conductive plate responses • Conductive cover screening • Variety of EM signatures • Can be difficult EM targets • Risk of missing a target IMPLICATIONS
Editor's Notes
- Figure 2-4 shows two groups of EM applications: conductor location and structural mapping. Conductor location initially caused the success of EM methods in base metal exploration. During the last two decades EM has however evolved from a mere conductor finding tool to a quantitative technique, where top to conductor, strike and dip were to be extracted from the field data. Recently, structural mapping applications such as in locating salinity problems in agricultural regions required a different way of looking at airborne EM data: The subsurface is no longer assumed "empty" apart from a conductor and an overburden but it is a continuous medium which is divided into regions of different electrical conductivity that correspond to structural units and facies changes. These two different applications of EM require different ways of interpreting field data. The principles behind that are illustrated in two separate chapters (3 and 4) each starting with a look at the typical electrical resistivities.
- Figure 3-1:Resistivities of common rocks and minerals in mining exploration. (Adapted from Grant & West, 1965; and Telford et al., 1990) Figure 3-1 (and the tables in appendix C) illustrate how almost all of the ore minerals are electrically much more conductive than barren rocks. During the last two decades ground EM has evolved from a mere location tool to an analysis tool providing depth to top, strike, dip and conductivity-thickness product of conductive orebodies. This chapter analyses the response of an electromagnetic prospecting system to a conductive target in the ground.