The Implications of the Mineral System Concept for Geophysical Exploration: A Perspective Mike Dentith michael.dentith@uwa.edu.au PDAC Toronto 2018
Traditional geophysical exploration strategy: Mapping and targeting form end members of a continuous spectrum Greater emphasis on responses from the immediate mineralised environment The Bump Smaller Scale Later Stages of Exploration Drilling Prospect Scale Few-10s km 2 District Scale 10s-100s km 2 The Map Larger Scale Regional Scale 100s-1000s km 2 Earlier Stages of Exploration Radiometric data, Ranger area, NT Greater emphasis on mapping geology
Mineral Systems all geological factors that control the generation and preservation of mineral deposits (Wyborn et al., 1994) Source-pathway-trap(physical throttle-chemical scrubber) A whole new set of targets! Source-pathway as well as the trap What will these look like associated with alteration? Source: Witherly (2014) Source the major change that is required is a shift from direct targeting to a staged process where geophysical approaches are used initially to help define the pathways. that carried mineralizing solutions
Mineral Systems Mineral systems processes occur on a scale of 100s to 1000s of km 3 Need geographically widespread datasets Scale is such that these are only likely to come from Government/ Geological surveys Need geophysical methods that can image source/pathway/(trap) at kms to mantle depths Academic methods
Mineral Systems Critical elements in a mineral system Source: McCuaig and Hronsky (2014) Fertility Ore Genesis Favourable Whole-lithosphere Architecture + Preservation of Primary Depositional Zone Source: G Begg (pers comm) Favourable (Transient) Geodynamics A whole new set of targets! Indirect inference of the fluid pathway
Deep Penetrating Geophysics Whole lithospheric architecture: Geophysical options? (Gravity) Magnetotellurics (MT) Active seismic methods Passive seismic methods
Magnetotellurics Southern Yilgarn Craton, Western Australia Major faults both intra- and inter-cratonic Mantle source zone?
Seismic Methods Capricorn Orogen, Western Australia Mapping cratonic margins/suture zones beneath thick cover
Seismic Methods Advantages passives surveys Do not require expensive artificial sources Drilling of shot holes Disadvantages passive surveys Lack resolution Long deployment times Weeks, months, years Options Ambient noise methods V s Teleseismic methods velocity contrasts Receiver functions, body wave tomography, H-k analysis From Distant Micro Seismic Events Moho Ambient Noise Data Source: Local explosion/ vibration Moho Active Source Data Teleseismic Data Moho From Distant Earthquakes
Mapping basement under thick cover H-k analysis of teleseismic arrivals Ambient noise derived Vs Commonconversionpoint stacking of teleseismic arrivals Vp/Vs Depth (km) Depth (km) Depth (km) Depth (km) 1.80 1.74 1.68 0 20 40 0 20 40 0 20 40 0 20 40 H-k 0 50 100 150 200 250 km Reflection Vs (km) 4.8 4.4 4.0 3.6 3.2 Amplitude Relative to Direct P-wave +6.0 +3.0-3.0-6.0 Vp/Vs=1.69 Glenburgh Terrane Ambient noise CCP stacking 13 T1 T2 Vp/Vs=1.75 Vp/Vs=?1.72 High Vs T3 Suture? Mantle Suture? Suture? 10
Mineral Systems Critical elements in a mineral system Source: McCuaig and Hronsky (2014) Fertility Ore Genesis Favourable Whole-lithosphere Architecture + Preservation of Primary Depositional Zone Favourable (Transient) Geodynamics Transient dynamics? Reservoir of over-pressured fluids below an impermeable barrier Barrier is periodically breeched (transient stress event) allowing fluid flow from the reservoir and deposition of metals
Palaeo-Reservoirs Reservoirs a useful camp-scale target? Relatively large and shallow targets Expect extensive and intensive alteration Allow detection-based exploration strategies in the gap between regional- and prospect-scale? Targeting Capability Greenstone belts Sedimentary basins Craton margins Regional Scale Palaeo-reservoirs? District/Camp Scale Mineralisation Alteration haloes Prospective contacts/lithotypes Prospect Scale Source: McCuaig and Hronsky (2014) Decreasing Exploration Scale
Palaeo-Reservoirs Olympic Dam IOCG deposit Cu-U-Au-(Ag-REE-Fe) Source: G Heinson, Univ of Adelaide, pers comm
Palaeo- Reservoirs 10 km Gruyere deposit Orogenic gold MT modelling by Jessica Spratt Slide courtesy of Gold Road Resources Ltd, Minerals Research Institute of Western Australia, Geological Survey of Western Australia
Mineral Systems: Petrophysics What do mineral system components actually look like? Fluid source regions Fluid flow conduits (pathways) Fluid reservoirs All are expected to be regions where there is fluid-rock interaction Petrophysical databases organised by lithology What are the petrophysical consequences of the alteration? Detectable physical property contrasts? Develop a predictive capability petrophysics first not last?
Mineral Systems: Petrophysics Towards a conceptual framework to understand geological controls (lithology+) on physical properties prediction! Recognise end-member behaviour of the petrophysical properties Need to treat different types of petrophysical data in different ways Increasing likelihood of prediction from chemistry/mineralogy Bulk (Overall Mineralogy) Density Increasing likelihood of correlation with rock type Seismic Paramagnetism Velocity Decreasing influence of the dominant mineral components Increasing likelihood of representative sampling Grain (Volume, size and shape of minor grains) Texture (Geometric relationships between minor grains) Increasing influence of poro-perm and pore contents
Petrophysics & Alteration Lithology and seismic properties) Ultra-mafic, mafic, intermediate, felsic Increasing likelihood of prediction from chemistry/mineralogy Bulk Dens Velo Velocity (m/s) 9000 Olivine Garnet Velocity (m/s) 9000 Olivine Grain Garnet Texture 8000 8000 7000 6000 5000 4000 Amphibole Pyroxene Plag Fsp Quartz Alkali Fsp Mica 3000 2.0 2.5 3.0 3.5 4.0 Density (g/cm 3 ) 7000 6000 5000 4000 Plag Fsp Quartz Alkali Fsp Amphibole Mica Pyroxene Rock-forming Minerals Felsic Intermediate Mafic Ultramafic 3000 2.0 2.5 3.0 3.5 4.0 Density (g/cm 3 )
Petrophysics & Alteration Alteration and seismic properties Olivine or Opyx Serp group minerals + mg (Serpentinisation) K Feldspar Muscovite or sericite Increasing likelihood of prediction from chemistry/mineralogy Bulk Dens Velo Velocity (m/s) 9000 8000 Olivine Orthopyroxene Velocity (m/s) 9000 8000 Olivine Texture 7000 6000 5000 4000 3000 2.0 Antigorite Lizardite/ Chrysolite Magnetite Minerals Serpentinisation 0-20% 20-40% 40-60% 60-80% 80-100% 2.5 3.0 3.5 4.0 Density (g/cm 3 ) 7000 6000 5000 4000 Plag Fsp Quartz Alkali Fsp Amphibole Mica Pyroxene Rock-forming Minerals Felsic Intermediate Mafic Ultramafic 3000 2.0 2.5 3.0 3.5 4.0 Density (g/cm 3 )
Petrophysics & Alteration Seismically transparent zones Stuart Shelf, South Australia Alteration by mineralising fluids? Probable they are due to alteration of mafic components of the country rock Approx Depth (km) 0 Wirrda Well Olympic Dam Titan Vulcan Gravity 6 12 18 South Zones of reduced reflectivity In seismic section North Source; Wise, et al., 2015
Min Systems & Geophysics A role for deep-penetrating geophysical methods Passive seismic methods, MT Need a better understanding of the new mineral system targets Palaeo-reservoirs and camp-scale targets? Need for research in petrophysics More than just a constraint for modelling Need to think beyond variation with lithology and include alteration Collect these data early and with good geological context Exploring for mineral system components under cover Need to develop a predictive capability
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