Presentation on theme: "ESC3162 Economic Geology Tectonics and ore deposits Robin Armit Asst. Lecturer - Geophysics and Tectonics Room 242 Building 28"— Presentation transcript:
www.monash.edu.au ESC3162 Economic Geology Tectonics and ore deposits Robin Armit Asst. Lecturer - Geophysics and Tectonics Room 242 Building 28 Robin.Armit@monash.edu
2 What are we going to do today? Lectures on: What is ore (context) Relationship with tectonics (info) Some examples about how tectonics influences ore deposition (cool stuff) Aim is to give you some context and an overview about tectonics of ore deposition
3 WHAT IS ORE? Ore is a naturally occurring mineral aggregate that is usable as mined or from which one or more valuable constituents may be economically recovered. All ore deposits represent an unusually rich concentration (compared to average continental crust) of particular valuable constituents.
4 CONCENTRATION (ORE FORMING) PROCESSES In order to concentrate the 'valuable constituent' we must provide a mechanism for physical and/or chemical differentiation of the valuable constituent from the remainder of the rock mass. There are lots of ways to do this, but in essence there are two basic possibilities: 1. 1.Remove everything else in the rock mass leaving a concentration of the valuable constituent, (e.g. residual deposits of aluminium (bauxite) and nickel in laterites). 2. 2.Remove the valuable constituent from the rock mass, move it to a new location and provide conditions where it can deposit within a smaller rock mass.
5 The second of these is a restatement of one of the basic philosophies of economic geology. Many mineralising systems contain three elements: 1. 1. A SOURCE region, from which the valuable constituents of the ore were originally derived. 2. 2. A TRANSPORT mechanism, by which the valuable constituents were removed from their source and moved to a new location (whether it be centimetres or kilometres away). 3. 3. A TRAP, which causes the valuable constituent to deposit from the transport medium in a particular location.
6 In many cases we tend to focus on the study of the trap (ie. the orebody itself), less so on the transport mechanism and very little on the source. This is partly because it is more difficult to study the transport mechanism and the source (because they may be transient or hidden features). In this lecture we will attempt view the entire system. Systems like this don't just spring into existence - they occur in permissive tectonic environments where 'stuff' is going on (faulting, movement of fluid, chemical and physical gradients, magmatism).
7 The mineralising system is intricately bound to the tectonic environment in which it occurs. TO UNDERSTAND THE MINERALISATION WE NEED TO UNDERSTAND THE SYSTEM (from source to trap) IN THE CONTEXT OF ITS TECTONIC ENVIRONMENT.
8 One of the most interesting aspects of economic geology is that whilst we can make an empirical judgement regarding the flavour of mineralisation that we might expect in a specific tectonic setting. There is no guarantee that significant mineralisation of that flavour will actually exist in that specific setting. Some parts of arcs have proven particularly 'fertile' with respect to porphyry-type Cu-Au deposits whereas other parts of the same arc appear to be relatively barren (we’ll have a look at this at the end of the lecture).
9 A NOTE ON ORE DEPOSIT NOMENCLATURE In the field of economic geology it is a practical necessity to simplify the range of mineral deposits by classifying them into a finite number of ‘types’ based on certain shared characteristics. Such classification schemes are usually based on empirical observation same host rock same commodity same alteration assemblages, etc.) but generally imply a common origin for deposits of the same type. This implication is reflected by the terminology of deposit classification schemes that often carry a genetic connotation, for example volcanogenic massive sulphide, epithermal gold, porphyry copper and sedimentary exhalative base metal deposits.
11 A NOTE ON ORE DEPOSIT NOMENCLATURE It is important to remember that all mineral deposit classification schemes are human impositions on a complex spectrum of natural phenomena. The boundaries between deposit types are subjective and often blurred and variation within deposit types can be large. Try to look at the system and the controls on mineralisation before lumping a particular deposit into a category.
12 That is to view the mineralising system from WITHOUT (in the context of its tectonic environment), rather than from WITHIN (in the context of particular processes occurring at the site of deposition). Apply plate tectonic concepts to modern and ancient terranes, although plate tectonic processes may have been slightly different in the ancient world (e.g., heat flow in the crust may have been greater). attempt to isolate the fundamental tectonic controls which condition certain segments of crust, in TIME and SPACE, for the formation of ore deposits. Different approach to that typically applied to economic geology
13 ORE DEPOSITS THROUGH TIME 1 Certain periods of Earth history appear to be particularly well endowed in certain varieties of ore deposit. Does this implies GLOBAL SCALE variations in the fundamental controls on mineralisation on the scale of 10s to 100s of millions of years and 100s to 1000s of kilometres? Unidirectional - e.g. a cooling earth, development of an oxygenated atmosphere Long Wavelength episodes - e.g. supercontinent cycles
15 Cycles of crustal growth Kenorland Columbia Rodinia
17 ORE DEPOSITS THROUGH TIME 2 Long wavelength variations may be superimposed by shorter wavelength phenomena that control the timing and location of mineralisation at scales of thousands to millions of years and from 100s of kilometres to the deposit scale. These “episodes” relate to specific processes which can be tied to the changing tectonic environment. Could these also be ultimately controlled by tectonics at a global scale?
18 STARTING POINT To form ore deposits you need to concentrate material that is usually dispersed in the crust (or mantle). You need: fluids or melts, heat, chemical gradients, fault networks... processes that can move material from one place to another. Put simply - Ore deposits form when STUFF HAPPENS Predominantly (but not always) at Plate Margins
19 SECOND POINT Certain segments of the Earth’s crust are particularly well endowed in certain varieties of ore deposit: e.g. porphyry Cu-Au and epithermal Au in the Andes, shale-hosted base metals in Proterozoic Australia Why should this be so: The tectonic environment imparts a fundamental control on the types of mineralising systems that might form.
20 HOWEVER... Not all magmatic arcs contain major porphyry deposits and not all Proterozoic extensional basins (or extensional basins through time) contain major shale-hosted base metal deposits. So, what is it about particular mineral belts that seems to make them more “metallogenic FERTILE” than others and... How can we recognise the quality of “FERTILITY” in terranes that do not have a known “ENDOWMENT”?
21 Lets have a look at some deposits that I have experience in. A PROTEROZOIC EXAMPLE Shale-hosted base metal deposits in north-eastern Australia. e.g., Mount Isa, HYC, Century Ongoing debate about the genesis of these deposits – – Exhalative – – Diagenetic – – Epigenetic Do we really care if we want to discover another one or identify another prospective terrane.
23 Despite the different genetic models there are some important ingredients which are required to form these deposits. – – Pre-existing basin evolution in which metal rich sedimentary & volcanic successions are deposited. – – These may act as aquifers or aquitards. – – Protracted fluid migration to scavenge metals from within the basin. – – Driving mechanism for migration of basinal brines (e.g., Topography, elevated heat flow). – – Deep seated fault architecture so that deep basinal brines can migrate to the upper parts of the basin. – – Favourable reduced basin conditions which allow sulphide reduction & metal precipitation (Chemical trap). – – Reactivation of faults to conduit fluid. Intracontinental rifts?
24 YET most intra-continental rifts do not contain SHMS- Pb-Zn-Ag mineralisation of any significance! Have we missed something? What is so special about the North Australian Craton that makes it so well endowed in this type of deposit? As it turns out the basins of the NAC did not evolve in a intra-continental rift. Rather a series of extensional basins that evolved in the over-riding plate of a major subduction zone.
26 Protracted extensional basin evolution led to the deposition of metal-rich volcanic & sedimentary successions - reworking of crust. Complicated extensional fault architecture. Protracted elevated geothermal gradients to circulate metalliferous brines within the basin - scavenge metals & increase salinity. topographic driven deformation driven dilatancy density driven fluid flow CRUSTAL PRE-CONDITIONING
28 All the major SHMS Pb-Zn- Ag deposits are hosted within basins that evolved subsequent to lithospheric extension Is there something special about this phase of the basin evolution that made it favourable for mineralisation?
29 Thermal subsidence of the lithosphere (sag-phase) allowed the basins to evolve in marine conditions for longer periods ANOXIC BASIN conditions - reduced sedimentary successions The role of asymmetric extension. Fault reactivation - driven by plate boundaries. Transient tectonism - reawakening fluid flow in the basins. POST-EXTENSIONAL BASIN EVOLUTION
30 Mineralisation events
31 Northern and eastern Australia occupied in a continental back-arc setting between ~1.8-1.6 Ga (Giles et al., 2002). Why are the deposits restricted in time and space within that interval and is there a fundamental difference between this setting & continental rifts, which are generally poorly endowed?
32 A summary of what is important for mineralisation: 1. 1. Protracted period of crustal pre-conditioning 2. 2. Anoxic basins 3. 3. Fault reactivation and transient tectonism 4. 4. Fault intersections 5. 5. Preservation If one or more of these things did not occur in this order then the likelihood of producing a world-class SHMS Pb-Zn-Ag deposits is reduced!!!!! Lets have a look at the difference between the North Australian Craton and a continental rift
33 Many of the characteristics of extensional basins of the North Australian Craton & continental rifts are the same Why don’t they all have world-class SHMS Pb-Zn-Ag mineral deposits? In summary they are not the same - despite having the same haircut Lets look at the differences at it will become clear - I hope.
34 Continental rifts crustal extension, magmatism, asthenospheric upwelling, & sedimentation occur in discrete zones. PROBLEM: Limited lateral size of hydrothermal convection cells. Volume of sediment in which to scavenge metal and transport hydrothermal fluid. High extension rates - limited time for pre-conditioning. Fault architecture relatively narrow.
35 North Australian Craton Crustal extension, magmatism, asthenospheric upwelling, & sedimentation occur over large areas of the craton. Possible large hydrothermal convection cells. Large volume of metal-rich sediment over a large area Low extension rates for protracted period - long time for crustal pre- conditioning Extensive Fault architecture
36 Passive margin or Aulocogen (failed rift) Once break-up occurs and a passive margin evolves, anoxic basins with favourable sedimentary traps may occur. However, as extension is taken up along the Mid-Ocean Ridge then the passive margin becomes quiescent Restricts fluid convection because deep basinal brines do cannot migrate to the surface.
37 North Australian Craton Tectonism during the post-rift evolution of the basins is driven by the plate boundary processes Reactivation of fault architecture is related to rapid roll-back, accretion of plateaus, ridges, seamounts, continental ribbons. Potential for faults to reactivate and allow metalliferous basin brines to migrate to the surface during sag-phase overprinted by transient tectonic events.
38 Preservation issues No point in making a world class deposit if you cannot preserve it. SHMS Pb-Zn-Ag - need to preserve the sag-phase of the basin. If you don’t erode it is to deep. If you erode to much it is reworked. Why the NAC?
39 Why a continental back-arc basin? How do you tell you are in a continental back arc setting? You can’t at the scale of the terrane? Similar feature to a continental rift. There are some differences Large areas of lithospheric extension – polycyclic. elevated geothermal gradients over larger volume of crust basin distribution wider than narrow rifts convection cell size larger area to scavenge metals fault network large Protracted basinal brine migration because of long-lived extension Protracted pre-conditioning – magmatism, fault activity, sedimentation. Post rift evolution – fault reactivation because controlled at the plate margin. Preservation
41 A TERTIARY EXAMPLE Porphyry related mineralisation in the Andes is episodic in time and space. What controls this episodicity? dip of the slab, subduction of a ridge, activation of favourable faults (shortening or extension), chemistry of melts or fluids, preservation... So, we must understand the configuration and evolution of the convergent margin
47 Summary Recent advances in plate kinematics allows detailed reconstruction of the kinematics and geodynamics of subduction systems. Subduction of crustal anomalies changes the geodynamics of the subduction zone and leads to immediate tectonic responses in the orogen. These changes affect the paragenesis of porphyry Cu-Au and Zn deposis, triggering episodes of intense metallogenic activity
48 Subduction of the Nazca Ridge The Nazca Ridge is located in the convergent margin between the Nazca plate and the South America plate The Nazca Ridge is subducting beneath Peru in an oblique angle relative to the plate motions The area of ridge subduction is characterized by a relatively flat subducting slab and the absence of arc volcanism. Nazca Ridge
49 Reconstruction of the Nazca Ridge Hampel, 2002, EPSL Nazca Ridge East Pacific Rise Tuamotu Plateau Reconstruction is based on the assumption that the Nazca Ridge formed in the East Pacific Rise during sea-floor spreadingReconstruction is based on the assumption that the Nazca Ridge formed in the East Pacific Rise during sea-floor spreading Therefore, it should have a counterpart in the Pacific Plate (Tuamotu Plateau)Therefore, it should have a counterpart in the Pacific Plate (Tuamotu Plateau)
52 Kinematic reconstruction The Reconstruction is based on calculated motions for the Nazca and S. America plates The reconstruction clearly shows the metallogenic response to ridge subduction Rosenbaum et al., 2005, EPSL 239, 18-32
54 Tectonic response to subduction of anomalously thick oceanic crust Cessation of volcanism and formation of gaps in the magmatic arc (McGeary and Nur, 1985, Tectonophysics) Appearance of adakitic volcanism (Gutscher et al., 2000, Geology) Occurrence of higher rates of tectonic erosions (Ranero & Huene, 2000, Nature; Clift et al. 2004; Tectonics)
55 Application to mineral exploration Exploration for porphyry-related ore deposits should target the mapping of anomalies within the subduction systems. Such anomalies can be mapped in both modern and ancients orogens providing that a detailed spatio-temporal analysis is conducted
56 Flat subduction
57 Additional information Earth and Planetary Science Letters paper is available from: http://www.earth.uq.edu.au/rosenbaum/EPSL_2005.pdf A movie showing the subduction of the Nazca Ridge is available from: http://www.earth.uq.edu.au/rosenbaum/movies.html
58 Where is this heading? Is there a plate to global scale control on the location of mineral deposits which is predictive? How does this scale of process translate to the province and deposit scale? Which faults, host rocks, magmas etc will be prospective? How do you recognise the appropriate phenomena in the ancient geologic record?
59 Where is this heading? Is there a plate to global scale control on the location of mineral deposits which is predictive? How does this scale of process translate to the province and deposit scale? Which faults, host rocks, magmas etc will be prospective? How do you recognise the appropriate phenomena in the ancient geologic record?