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Exploration Science Linking fundamental controls on ore deposition with the exploration process Andy Barnicoat AESC Perth July 2008 Outline Mineral Systems as a framework for understanding ore bodies Fundamental controls on ore formation Exploration science Some system perspectives Using Mineral Systems’ concepts in targeting Acknowledgements All in the pmd*CRC, especially Bruce Hobbs, John Walshe, Heather Sheldon & Alison Ord for conversations ERC members, particularly Nick Archibald, Greg Hall, Jon Hronsky & Dugi Wilson for encouragement Bob Haydon & Russell Korsch for support 2 Mineral Systems ‘All geological factors that control the generation and preservation of mineral deposits…’ ‘…stressing the processes that are involved in mobilising ore components from a source, transporting and accumulating them…’ From ‘Australian Proterozoic Mineral Systems: Essential Ingredients and Mappable Criteria’ Wyborn, Heinrich & Jaques, 1994 Mineral Systems Science is a part of Earth Systems Science, which encompasses the interactions between the biosphere, atmosphere, hydrosphere and ‘lithosphere’ (read solid Earth) 3 Why Mineral Systems? Predictive capacity going beyond pattern matching Enables exploration undercover Mineral Systems form a basis on which to drive integration of research efforts and ensure a focus on exploration outcomes • a means to link geology to physical and chemical controls on ore formation • a way to link to exploration • a cross-scale view • a framework within which to place projects and individual’s work 4 Mineral Systems work flow The Why Question Why is the ore body there? 5 Questions 1. What are the geodynamic and PT histories? Inputs from: 2. What is the architecture of the system? Data Compilation 3. What are the fluid sources/reservoirs? Data Collection 4. What are the fuid flow drivers & pathways? Simulation 5. What are the metal and sulphur transport & depositional processes? The Where Question Where is the next ore body? Developed from inspiration provided by Tom Loutit by John Walshe, Alison Ord, Bruce Hobbs & Greg Hall in the AGCRC 1997-98 5 Mineral systems and deposit types Mineral systems are broad, and the concept embraces explicitly factors across all scales Differences in depositional environment will lead to distinctive accumulations controlled by variations in • lithology • structure Deposit types represent the expressions of these relatively local variations 6 An analogue for Mineral Systems VW Golf platform 7 An analogue for Mineral Systems A ‘Common platform’ underpins the obvious differences in surface expression VW Golf Skoda Octavia Audi TT VW Beetle 8 Why do ore deposits form? ...because a lot of the appropriate mineral(s) have been deposited Zinc - Century, Queensland Gold - Goldstrike, Nevada So – what controls mineral deposition? 9 Why deposition of minerals occurs Rate of deposition Velocity Gradient of in = . transport carrying medium capacity Examples: 1. Heavy mineral deposition controlled by flow rate and entrainment capacity (proportional to velocity2) 2. Magmatic deposits controlled by magma supply rate and changes in temperature, magma composition, etc. causing deposition 3. Residual deposits (e.g. bauxites) where dissolution and removal of gangue leads to ore formation 4. Hydrothermal systems, where fluid flow rate and changes in P, T or chemistry lead to deposition 10 Fundamental relationship Rate of mineral deposition = Fluid velocity •S Rate of change of solubility with P, T, C . Gradient of P, T, C Maximise… Focused fluid flow P, T & C at values where largest solubility changes occur Temperature, pressure, compositional gradients After Phillips, 1990, 1991 and Hobbs & Ord, 1997 11 Exploration science Geology Key Parameter is reflected in A. Gradient in hydraulic potential B. Porosity C. Permeability D. Solubility sensitivity to P, T, C E. Spatial gradient of P, T, C Exploration scale-dependent translation 5 Questions 1. Geodynamics 2. Architecture 3. Fluid reservoirs 4. Flow drivers & pathways 5. Deposition Terrain Selection Area Selection Drill Targeting F. Time (duration) Why? What? ‘Practical Proxies’ Where? 13 Olympic Dam Cu-U-Au --------------------------------- 29b In summary….. Sandstone-hosted Pb-Zn ------------------------------- 30a Sediment-hosted Cu ----------------------------------- 30b Sandstone U ----------------------------------------- 30c Sedimentary exhalative Zn-Pb ---------------------------- 31a Bedded barite ----------------------------------------- 31b Amount krg Hydraulic Solubility of Emerald veins ----------------------------------------31c . Spatial . = . dt • Gradient m sensitivities Gradients mineral ∫ ) S( Southeast Missouri Pb-Zn ------------------------------- 32a Appalachian Zn --------------------------------------- 32b Critical factors for forming a (giant) Kipushi Cu-Pb-Zn ------------------------------------32c ore system: Low-sulfide Au-quartz vein --------------------------------- 36a Homestake Au ------------------------------------------36b Gradient in hydraulic potential Driver Unconformity U-Au ------------------------------------37a Geodynamics; Architecture; Deposition Porosity Gold on flat faults ----------------------------------------37b Architecture; Pathway Permeability Solubility Sensitivity * Spatial gradient of P, T,the X process….. Architecture; Deposition 1 equation describing Geodynamics Maximum duration 65 USGS deposit types! *This will be zero if anfor appropriate fluid… is not present! Thanks to John Walshe the inspiration 14 Geodynamics: Solubility Sensitivity Crustal growth and gold 200 Gold Resource Moz Vol % Crustal growth 150 100 50 14 12 10 8 6 4 2 3 2 Ga 1 0.5 From Groves et al., 2005 15 Geodynamics: Solubility Sensitivity Mantle Convection Two layer convection in early Earth Periodic whole-mantle overturns Facilitated by spinel-perovskite transition (670 km) From Davies (http://rses.anu.edu.au/gfd/davies/pages/episodtect.html) 16 Geodynamics: Solubility Sensitivity Evolution of mantle convection Mantle temperature undergoes episodic temperature surges due to convective overturns Whole mantle convection Intermittent penetration by plates Internal boundary layer instability Davies, 1995 17 Geodynamics: Solubility Sensitivity Yilgarn crustal growth Compiled by Karol Czarnota & Richard Blewett 18 Geodynamics & Architecture: Porosity, Permeability and Solubility Sensitivity Great Basin facies distribution 1 Cook & Corboy, 2004 19 Geodynamics & Architecture: Porosity , Permeability and Solubility Sensitivity Great Basin facies distribution 2 Deep Structures seen in Geophysics as well as isotope data etc. 20 Geodynamics & Architecture: Porosity , Permeability and Solubility Sensitivity Great Basin geophysics Grauch et al., 2003 21 Geodynamics & Architecture: Porosity, Permeability and Solubility Sensitivity Great Basin geodynamics 118° 42° 116° Oceanic crust Transitional crust Continental crust Dense crust 40° Tertiary Magmatism Acid IntermediateMafic Carlin-type deposits After Grauch et al., 2003 22 Fluid sources: solubility sensitivity What about ore-forming fluids? Ore-forming fluids are chemically similar to other crustal fluids Controlled by reactions with (crustal) rocks Yardley, 2006 23 Fluid sources: solubility sensitivity Potential fluid sources Meteoric water Bittern Evaporites Crust Sea water Basinal fluid Magmati c fluids Metamorphic fluid Mantle Note: all paths schematic Mantle fluid 24 Fluid sources: solubility sensitivity Salinity in crustal fluids Fluids from shallow marine & continental settings Temperature °C Fluids from accretionary/ oceanic settings From Yardley & Graham, 2002 25 Fluid sources: solubility sensitivity Meteoric Fluids Sourced from the surface • Needs rainfall & sunaerial topography Wide range of chemistries • Acetate especially important around oil window (80-120°C) Size of meteoric systems may be huge Transit time for fluid ~2 Myr Don’t in general carry a lot of metal (uranium?) but can be • directly important dilution • Indirectly important – interact with rocks and other fluids and evolve to more saline solution 26 Radke et al., 2000 Fluid sources: solubility sensitivity Basinal Fluids Formation water • water present in pores and fractures immediately prior to drilling Connate water • water trapped with the sediment and subsequently unmodified. Sedimentary basin waters have a range of origins • connate, • (modified) meteoric water • recent • in the geological past 27 Fluid Sources – solubility sensitivity Basinal fluids - origins Most basinal fluids (formation waters) are of recent meteoric origin (c.f. Great Artesian Basin) Kharaka & Hanor, 2005 28 Fluid Sources – solubility sensitivity Metamorphic fluids ‘Metamorphic’ fluids have mixed origin • Devolatilisation • External fluids reacting with metamorphic rocks Devolatilisation • During heating, volatiles released • Micas, chlorite, amphiboles yield water • Carbonates react (eg to calc-silicates) and CO2 and H2O may be formed • Constant temperature leads to no fluid generation • Decrease in temperature leads to resorption of residual fluid by retrogression 29 Fluid Sources – solubility sensitivity Metamorphic fluid production Key results: • Maximum fluid flux ~10-11 m3/m2/s (very small) 15km Basalt T = 20 °C Granite magma e.g. voluminous High-Ca granite magma (M2) Sheldon, 2008 • Duration << 1 million years • Time-integrated flux is ~1000 times smaller than flux required for mineralisation (Cox, 1999) Metamorphic fluid must be FOCUSED for mineralisation 30 Geodynamics – solubility sensitivity Magmatic fluids – components CO2 facilitates formation of a fluid phase/phases • primary source of CO2? CH4-rich fluids occur in mid-ocean ridge settings • high-T re-speciation of initially CO2-rich fluids produced methane ; may co-exist with NaCl-rich fluids S levels are elevated in arc-related magmas • S isotope data shows ‘extra’ sulphur to be derived from subducted material Cl is also sourced from slab-derived fluids in arc settings • Additional (lower crustal) sources may also exist Data from Kelly & Früh-Green, 2001; de Hoog et al 2001; Kent et al., 2002 31 Geodynamics: solubility sensitivity Cl and magmas Cl-rich fluids will form due to changes in melt composition 6 4 basalt • mafic melt cooling, crystallising and differentiating can develop brine ± vapour phase insoluble in felsic magma 8 wt% H2O in melt • Cl solubility decreases dramatically from mafic to felsic melts 2 0 0 1 2 3 wt% Cl in melt Webster, 2004 32 Geodynamics: solubility sensitivity Role of mafic magmas Carriers of most of the CO2, sulphur and Cl seen in magmatic systems and their fluids Injection of mafic magmas into felsic magma chambers critical • add volatiles • add energy • add volume (magma plus separating fluid phase) Bimodal magmatic systems likely to be an important sign • Williams suite in Isa • mafic to low-Ca suite in Yilgarn • Hiltaba suite in the Gawler Hollocher; www.union.edu 33 Fluid flow drivers: hydraulic potential Fluid flow rates log fluid flux (m/yr) -8 -6 -4 -2 0 Deformation Convection Topographic/meteoric Metamorphism Compaction Intrusions There is considerable overlap in flow rates, making it difficult to predict which one will dominate. From Heather Sheldon 2008 presentation 34 Fluid sources and drivers, geodynamics: hydraulic potential, solubility sensitivity What’s really important? Two types of fluid source are likely dominate the fluid budget • meteoric • magmatic Understanding palaeogeography • brine sources • sub-aerial topography Magmas • bimodal suites • evidence of mantle input (cf Olympic Province in the Gawler) Karakoram Mountains Yellowstone 35 Deposition: porosity, solubility sensitivity Base metal transport ZnCl+ + H2S(aq) = 8 ZnS + 2H+ + ClH2S, H+ and Cl- influence zinc solubility 2H+ 3KAlSi3O8 + = KAl3Si3O10(OH)2 + 6SiO2 + 2K+ CaCO3 + H+ = Ca2+ + HCO3- total Cl m Acidity can be absorbed by reacting with feldspars, carbonates, etc. 6 4 total Zn ppm 2 100 200 300 400 500 1000 500 400 300 200 100 50 10 1 0.1 0.01 T(C) Zinc solubility at 1 kbar from sphalerite 36 Deposition: porosity, solubility sensitivity Base metal precipitation To be viable, base metal grades need to be >10% That means space is required Muscovitisation of K-feldspar reaction has a solid volume decrease of 15% Calcite dissolution shows a solid volume decrease of 100% • Best host rocks for base metals are carbonates and arkosic sandstones • Clean sandstones will not be effective • Other potentially good hosts include fractured felsic igneous rocks Image by Graham Phillips, RDR 37 Using Mineral Systems Conceptual Mineral System Understanding Geodynamic Episodes Pre-ore endowment (<2665 Ma) Lithospheric extension Au Geochemical Gradients Inversion Au (≥2660 Ma) (~2665-2655 Ma) Identification of mappable mineral system process proxies for each subdivision Practical • Crustal endowment Pre-ore prospectivity • Extensional SZ • Metasomatised mantle melts • Deep pathways D3 prospectivity • Domes • Major faults • Upper plate D4-D5 prospectivity • Redox gradients • Hydrothermal system indicator Geochemical prospectivity Au mineral camp (60x60 km area) selection Slightly modified after Karol Czarnota et al., 2008 38 Camp-scale Au Targets Large gold deposits reflect long lived mineral systems affected by multiple gold events A combination of the prospectivity maps related to various processes identifies prospectivity based on Mineral Systems 100 km From Czarnota et al., 2008 39 Summary Exploration Science is an integrated view of ore-forming processes that explicitly ranges across scale Exploration science connects • fundamental physico-chemical controls on deposition to • observable view of the Mineral Systems (the ‘practical proxies’) and • the exploration process Understanding the connections will ensure that ‘practical proxies’ are relevant to understanding ore formation 40 41