Mineral Systems 5 Questions Overview 1
Key Parameter Mineral System Exploration is reflected in scale-dependent translation A. Gradient in hydraulic potential B. Permeability C. Solubility sensitivity to P, T, C D. Spatial gradient of P, T, C E. Time (duration) 5 Questions 1. Geodynamics 2. Architecture 3. Fluid reservoirs 4. Flow drivers & pathways 5. Deposition Terrain Selection Area Selection Drill Targeting Slide after: A. Barnicoat 2
Mineral Systems Defined as: all geological factors that control the generation and preservation of mineral deposits (Wyborn et al 1994) 3
Mineral Systems versus Petroleum Systems Petroleum Systems introduced the concept of sourcetransport-trap Source only of petroleum required Change of phase in source region Buoyancy the major driver Trapping is purely mechanical or hydrodynamic Mineral Systems differ Source of various ore metals and a range of fluids Wide range of flow drivers Change of phase on deposition Trapping (of fluid) highly undesirable Slide after: A. Barnicoat 4
Mineral Systems Workflow The Why Question Why is the ore body there? 5 Questions 1. Geodynamics 2. Architecture 3. Fluid reservoirs 4. Flow drivers & pathways 5. Deposition Inputs from: Data Compilation Data Collection Modelling Simulation The Where Question Where is the next ore body? 5
Classic deposit types Groves et al 2005 6
Classic vs. Systems approach provide a calculable framework test hypotheses of ore formation in a particular location generate new hypotheses increase confidence cut exploration risk and the time before a discovery is made. THE SYSTEMS APPROACH TO MINERAL EXPLORATION GEOLOGICAL INVERSION THE CLASSICAL APPROACH TO MINERAL EXPLORATION DATA SETS DATA MINING CONSTRUCTION OF 3D AND 4D IMAGES INTERROGATION OF IMAGES Classical Approach DRILL HERE DATA SETS COLLECT DIFFERENT OR MORE DATA DATA MINING INTERROGATE THE DATA SETS DIFFERENTLY CONSTRUCTION OF 3D AND 4D IMAGES COMPUTATIONAL MODELLING Coupled mechanics-thermal transport-fluid flowreaction/transport/diffusion chemistry Classical Approach DRILL HERE New paradigm approach B. Hobbs GEOLOGICAL INVERSION 7
4 Mineral Systems? Magmatic PGE, Ni etc. in intrusions, etc, Diamonds Magmatic-Hydrothermal VHMS, Lode /slate belt gold, Porphyry & epithermal, IOCG, Intrusion-related Au, W, Sn, Witwatersrand, Cobar (?), Carlin (?) Basinal Sedex, MVT, Irish type, sedimentary Cu, Unconformity U All this is detail in the depositional environment Surficial Supergene Au, Ni, etc, bauxite 8
Key Parameter Mineral System Exploration is reflected in scale-dependent translation A. Gradient in hydraulic potential B. Permeability C. Solubility sensitivity to P, T, C D. Spatial gradient of P, T, C E. Time (duration) 5 Questions 1. Geodynamics 2. Architecture 3. Fluid reservoirs 4. Flow drivers & pathways 5. Deposition Terrain Selection Area Selection Drill Targeting Slide after: A. Barnicoat 9
Deposition Rate of deposition = Velocity of transport medium. Gradient in carrying capacity Examples: Heavy mineral deposition controlled by flow rate and entrainment capacity (proportional to velocity 2 ) Magmatic deposits controlled by magma supply rate and changes in temperature, magma composition, etc. causing deposition Residual deposits (e.g. bauxites) where dissolution and removal of gangue leads to ore formation Hydrothermal systems, where fluid flow rate and changes in P, T or chemistry lead to deposition 10
5 Processes basic relationship Key parameters: Gradient in hydraulic potential Permeability Solubility sensitivity to P, T, C Spatial gradient of P, T, C Time (duration) A κρg. c c e e ce P. T p cr. dt T p r c = + + µ r 11
3 sets of geological inputs 1) Palaeogeography: feeds into most of the critical factors Describes distribution of (emergent) topography and hydrocarbon generation potential, both potential sources of hydraulic gradient. Controls distribution of facies and diagenesis that control permeability distribution in sedimentary sequences. Describes potential source regions for meteoric fluids (emergence again) and brines in marginal marine areas: key control on solubility sensitivity. Allows identification of stable areas where P & t gradients could have been stable for long periods. Barnicoat 2008 12
3 sets of geological inputs 2) Magmatism: also plays a major role in many critical factors Source of fluids and temperature distributions that may create hydraulic gradients. Driver for fracture generation and hence a control on permeability. Creates spatial gradients in temperature (and potentially chemistry too). Act as a fluid source the nature of which will depend on the magma s origin: key control on solubility sensitivity. Repetitive magmatism will lead to long-lived hydrothermal systems. Barnicoat 2008 13
3 sets of geological inputs 3) (Structural) Architecture Controls the distribution of dilation sites that play an important role in developing hydraulic gradients. Defines most of the high-permeability domains in the crust Helps to define pressure gradients, and plays a role in facilitating fluid mixing. Repeated failure on structures (including reactivation of deeper features) allows prolonged fluid movement an/or multiple deposition/mixing/etc. events. Barnicoat 2008 14
Example Cyprus VHMS Deposits For over 2000 years, Cu has been mined in Cyprus Hosted in mafic rocks of the Troodos ophiolite Answer the Five Questions about these mineral systems 15
Cyprus - Introduction Late Cretaceous (92Ma) Troodos ophiolite with younger cover Massive sulphide deposits mined for pyrite (for H 2 SO 4 ) Cu & Zn Now only supergene Cu active Fe-Mn umbers mined for pigment (active) Lavas Dykes Plutonics Umbers VMS deposits From Pritchard & Maliotis, 1998 16
Cyprus - Skouriotissa Mine Phoenix Pit supergene Cu enrichment Heap leach and SX-EW plant Primary massive sulphides mined by Romans (smelting slag far right) 17
Geodynamics What is the P, T history? What geochronological data exists? What metamorphic and alteration assemblages exist? Timing? 18
Geodynamic History 1 Bonninitic affinities of lavas in the ophiolite implies a back-arc origin Ophiolite overlain in the west by rocks containing calcalkaline volcanic debris Troodos microplate Modern-day analogue of setting is the Lau basin Emplaced ophiolites in Syria & Turkey From Robertson et al., 1991 Africa 19
Geodynamic History 2 Sheeted dykes form much of the Troodos Mountains extreme extension with magmatism Trace of dykes Lavas ponded against a fault scarp: syn-extensional magmatism Lava flow Fault trace 20
Architecture How big is the system? Does the system involve the entire crust or just a sedimentary basin within the crust? What is the stratigraphy? Strength contrasts, permeabilities at time of mineralisation and chemistry (reduced or oxidised) of the rock units? What is the structural geology? What is the chemistry of igneous intrusions? 21
Architecture 1 Classic (definitive?) ophiolite succession Complete sequence: Mantle succession Plutonics Sheeted dykes Lavas Umbers Cherts Carbonate cover Massive Sulphide deposits Stockwork mineralisation 22
Architecture 2 VMS deposits (and associated umbers) located At the top of the succession (dominantly) Within the lavas (in places) Lavas Dykes Plutonics Umbers VMS deposits 23
Architecture 3 Fossil evidence indicates formation on sea floor Left: tube worm preserved in pyrite; Kambia Right: gastropods preserved in pyrite; Kinousa & Memi Syn-magmatic ore Formation demonstrated by common burial by lavas Little et al., 1999 24
Fluid reservoirs What was the chemical nature of the fluid or fluids responsible for mineralisation? What was the Eh and ph of the fluids? What was the salinity? What rocks did these fluids come into contact with to define their isotopic and chemical signatures? 25
Fluid Reservoirs 1 Fluid sources available are seawater and magmas δd Seawater Troodos country rocks Troodos stockworks Stable isotope data Fluids in equilibrium with country rocks trend between magmatic & seawater fields Fluids in equilibrium with stockworks plot with seawater Fresh magmatic rocks Primary magmatic waters δ 18 O Oxygen isotope profile through the ophiolite reveals changing temperatures of equilibration by using calculated water-rock fractionation factors High-T alteration Low-T alteration 26
Fluid Reservoirs 2 87 Sr/ 86 Sr shows large spread between seawater and magmatic values in lavas Cretaceous seawater ~ 0.7074 Faults do not show strong seawater influence Seawater recharge of system was general and not localised down faults Troodos fresh glass ~0.7035 27
Fluid flow drivers and pathways Fluid flow in porous rocks requires a gradient in hydraulic potential (!!): Topographic relief Changes of fluid pressure created by deformation induced dilation, compaction is a subset of this process Convection induced by thermal or chemical buoyancy Changes in fluid pressure generated by chemical reactions Pressure gradients generated by high-pressure fluids being Released from intrusions 28
Fluid Flow Drivers & Pathways 1 Whole rock d 18 O isotope patterns link reaction zone at base of dykes with orebodies d 18 O pattern defines upflow zone over epidosites driven by heat from underlying magma chamber Figures from Schiffman & Smith, 1988 29
Fluid Flow Drivers & Pathways 2 Epidosites (epidote-quartz± chlorite) in basal sheeted dykes Bands of epidosite parallel dyke margins regardless of joints or other dykes Richardson et al., 1987 30
Fluid Flow Drivers & Pathways 3 Scanning electron micrograph of epidosite (combined back scatter and cathodoluminescence) Note infilled porosity, created by alteration reaction forming denser mineral phases than protolith Alteration reaction thus leads to increased porosity and permeability, further focussing fluid flow Chlorite Epidote Pore infilled with quartz. Note varying image brightness outlining in white a euhedral and hence open-space-filling zone. 31
Deposition What was the process involved in precipitation of the mineral assemblage in the ore deposit? Fluid-rock reaction? Fluid mixing? Boiling? Pressure drop? Migration down a temperature gradient? 32
Deposition 1 Pb isotopes indicate the metal in ore deposits sourced from local oceanic crust 207 Pb/ 204 Pb 15.7 15.6 Troodos sediments Sulphides Indian Ocean MORB 15.5 18 18.5 19 19.5 206 Pb/ 204 Pb Seawater Epidosites Troodos glasses Zn ppm 100 80 60 40 20 20 40 60 80 100 120 Cu ppm Unmineralised lavas & dykes Chl-qtz-epidote rock Epidosites Epidosites and the slightly less altered chl-ep-qtz rocks have lost ~90% Cu and 50% Zn compared to background lavas & dykes Metals sourced from deep portion of dyke complex 33
Deposition 2 Total S data shows that S has been redistributed Total S ppm δ 34 S d 34 S values reveal the S above deep plutonics to be a mixture of Magmatic and seawater S Epidosites have low levels of S Epidosites Primary ranges Alt, 1994 34
Deposition 3 Umbers blanket top of lavas, infilling half-graben topography Originate by suspension fall-out of oxidised very fine grained sulphide formed at black smoker vents Umber-altered lava contact 35
Overall Model for Systems Distributed of fluid downflow Focused zone of fluid upflow Source of water and some sulphur Source of metals and some sulphur From Bickle & Teagle, 1992 Source of heat to drive fluid flow 36
Implications for Exploration If we didn t know about Cyprus-type VHMS deposits: Proximity to magmatic heat sources essential Architecture with faults to focus fluid upflow Marine environment needed to provide fluids and sulphur Appropriate metal source needed close to magmatic heat source 37
References Wyborn, L. A. I., Heinrich, C.A., and Jaques, A.L. (1994). "Australian Proterozoic Mineral Systems: Essential Ingredients and Mappable Criteia." Australasian Institute of Mining and Metallurgy Publication Series 5/94: 109-115. Barnicoat, A. C. (2008). The Mineral Systems approach of the pmd*crc. New Perspectives: The foundations and future of Australian exploration. Abstracts for the June 2008 pmd*crc Conference., Perth, Geoscience Australia Record 2008/09. Groves, D. I., Condie, K.C., Goldfarb, R.J., Hronsky, J.M.A., and Vielreicher, R.M. (2005). "100th Anniversary Special Paper: Secular Changes in Global Tectonic Processes and Their Influence on the Temporal Distribution of Gold-Bearing Mineral Deposits." Economic Geology 100: 203 224. 39