The Timing and Location of Major Ore Deposits in an Evolving Orogen" the Geodynamic Context

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The timing and location of major ore deposits in an evolving orogen" the geodynamic context

DEREK J. BLUNDELL Department of Geology, Royal Holloway, University of London, Egham, Surrey TW20 OEX, UK (e-mail: d. blundell@gl, rhul.ac, uk)

Abstract: Although it is possible to identify the potential controls on mineralization, the problem remains to identify the critical factors. Very large mineral deposits are rare occurrences in the geological record and are likely to have resulted from the combination of an unusual set of circumstances. When attempting to understand the mineralization processes that occurred to form a major ore deposit in the geological past, especially the reasons why the deposit formed at a particular time and location within an evolving orogenic system, it is instructive to look at mineralization in modern, active subduction complexes. There it is possible to measure and quantify the rates at which both tectonic and mineralizing processes occur. In a complex subduction system, regions of extension develop. For example, subduction hinge retreat is a process that creates extension and generates heat from the upwelling of hot asthenosphere ahead of the retreating slab, producing partial melting, magmatism and associated mineralization. Seismic tomography not only images mantle as it is now, but subduction slab anomalies can be interpreted in terms of the past histo12¢ of subduction. This can be used to test tectonic plate reconstructions. Tectonic and magmatic events occur rapidly and are of short duration so that many are ephemeral and will not be preserved. Furthermore, they can be diachronous as is the case with the lithospheric slab tear clockwise around the Carpathian Arc during the Neogene. If the tectonic setting is paramount in determining the onset of the mineralization process and generation of mineralizing fluids, the fluid transport system that localizes the mineralization in space and time and concentrates the metal charge is the key to finding when and where the ore deposits occur. Fault and fracture networks in the crust provide various mechanisms for the localized expulsion of fluid in pulses of short duration. Excess surface water flow following large earthquakes in the Basin and Range region of USA offers a modern analogue to quantify fluid flow related to extensional faulting. Evidence from the Woodlark basin, east of Papua New Guinea, suggests that similar conditions pertain in the oceanic environment. Whilst there are limits to the use of regions of active tectonism as modern analogues to explain the mineralization of ancient orogenic systems, they do provide the best opportunity to understand the mechanisms of mineral processes and the controls on the location and timing of major ore deposits.

The formation of a large, world class ore deposit the common factors provides a set of input is a relatively rare event in geological history and parameters for 'hard' modelling to quantify and requires the concurrence of a particular set of predict the likely occurrences of ore deposits circumstances in space and time. These are likely prescribed by a specific set of circumstances. to operate on a range of scales from the major However, in order to do this successfully, an tectonic context on a lithosphere scale, through essential pre-requisite is to understand the geody- the crustal-scale structural context down to the namic processes involved so as to constrain the deposit and microscopic scales that determine the mechanisms incorporated into the modelling. An- mode of mineralization. There are various ap- other approach has been used recently by Goldfarb proaches that can be made to determine the key et al. (2001) to produce a global synthesis of factors conducive to the formation of large ore orogenic gold deposits. In this, they began by deposits. Ideally, a systems approach advocated by examining the characteristic features of the ore Ord et al. (1999) recognizes common crustal-scale deposits and their tectonic settings, before making fluid flow systems in ore deposit formation that a detailed synthesis of their occurrence through are amenable to modelling, regardless of the geological time from the Archaean to the Present, diversity of tectonic settings. 'Soft' modelling of within a succession of orogenic systems. This

From: BLUNDELL,D.J., NEUBAUER, E & VON QUADT, A. (eds) 2002. The Timing and Location of Major Ore Deposits in an Evolving Orogen. Geological Society, London, Special Publications, 204, 1-12. 0305-8719/02/$15.00 © The Geological Society of London 2002. Downloaded from http://sp.lyellcollection.org/ by guest on September 26, 2021

2 D. J. BLUNDELL emphasises the long term variation through time volved in an evolving orogen. Across the very of the amount of mineralization that has occurred. large area depicted in Figure 1, three major plates However, to examine the timing and location of are interacting; the dominantly continental Eur- major ore deposits within an evolving orogen, the asian plate in the NW, the India-Australia plate in best starting point is an appraisal of modern the south containing both old oceanic lithosphere orogenic systems, in which the dynamically and the Australian continent, and the oceanic changing tectonic activity and lithospheric struc- Pacific plate in the east. In between are smaller ture responsible for metallogeny can be related plates containing continental fragments and young through the use of geophysical and geochemical oceanic lithosphere that form a collage of rapidly observations. These systems can be used as changing and deforming tectonic units. Present modern analogues to substantiate those geody- day horizontal motions of structural elements have namic processes and structures that, together, been quantified from analysis of GPS data to control the timing and location of large ore deduce relative velocity vectors. Rates of uplift or deposits within an evolving orogen. subsidence are measured by various means. Earth- Regardless of deposit type or form of miner- quake hypocentre and focal mechanism determi- alization, there are certain common factors in ore nations define in three dimensions the genesis (equivalent in hydrocarbon parlance to configuration of subduction slabs in the upper source, migration pathway and trap). mantle and quantify the state of stress. Their (1) A source region within the Earth to provide configurations are confirmed by seismic tomo- the metal charge. This can range from a region of graphic images of seismic velocity anomalies in partial melting within the upper mantle (or possi- the upper mantle, along with those of various bly deeper if generated by a mantle plume) to a other features (Bijwaard et al. 1998; Spakman & region within the crust that is scavenged by hot Bijwaard 1998). Gravity, seismic and heat flow brine within some form of hydrothermal system. measurements also provide information about The size of the source region is likely to be large lithospheric structure and physical properties at in comparison with that of the ore deposit, so that depth. All these provide a snapshot of this huge a high degree of concentration of metals within subduction complex at the present time. the fluid between source and ore deposit is In addition, palaeomagnetic data combined with essential. high resolution dating provide evidence of plate (2) A fluid system that provides the mechanisms movements in the past, both from ocean floor of transport and concentration between source basalt anomalies and from rotations and relative region and ore deposit. This is controlled by the translations of continental fragments. From these evolving thermo-tectonic setting of the orogen and and other data, the rates of relative motions the rheological properties of its location within between the plates have been measured and their the lithosphere. evolution over the past 50 Ma has been tracked in (3) A localized structural/stratigraphic setting a succession of plate reconstructions at 1 Ma and chemical regime that is conducive to the intervals (Hall 1996, 2002). This evolutionary precipitation of a large quantity of metals in ore model plus measurements of currently active minerals within a deposit. Furthermore, conditions processes provide the framework for attempting to for the preservation and possible exhumation of understand the conditions required to generate the deposit are required subsequently for it to be large ore deposits. In this paper, just a few at or near surface at the present day and thus examples are presented to illustrate the value of exploitable as a resource. this approach. Taking these factors into account, modem oro- (1) Within a plate system of long-term conver- genic systems offer insights into processes that are gence a large amount of extension has occurred, ephemeral and allow rates and duration of process especially across the large region of Sundaland to be quantified. These can be used to interpret and the Banda Sea behind the Banda Arc. Many the key factors in past orogenies that have run to of the subduction zones defined by earthquake completion, where evidence of processes active activity are steeply dipping and, together with during their evolution are no longer preserved. seismic tomographic imagery, offer ample evidence of subduction hinge retreat, or 'rollback'. One effect of rollback is to introduce additional A modern orogenic system: the SE Asia- heat in front of the retreating slab, giving rise to SW Pacific region partial melting in the mantle (see Macpherson & Hall 2002). Rollback also results in the develop- The SE Asia-SW Pacific region forms an active ment of back-arc and intra-arc extensional basins subduction complex that provides excellent exam- which, as Hutchinson (1973) first pointed out, are ples of most of the geodynamic processes in- amongst the range of tectonic settings conducive Downloaded from http://sp.lyellcollection.org/ by guest on September 26, 2021

ORES IN AN EVOLVING OROGEN: GEODYNAMICS 3

Fig. L Main plate tectonic features of the SE Asia-SW Pacific region, based on Hall (2002), reproduced with his permission, showing locations of Manus basin (M), Woodlark basin (W) and Banda Arc cross-section (B) of Figure 2.

to the formation of VHMS deposits. These are is commonly linked to extensional movements on characterized by bimodal volcanics with rhyolitic low angle detachment faults at depth. Within the and basaltic compositions. same arc, epithermal deposits may be younger In areas where the subduction hinge has ad- than the porphyry deposits and have formed vanced, volcanism often ceases as the mantle through a different mechanism. Most of the major wedge is not continually being replenished by deposits are of Pliocene age, associated with mid- fresh mantle material and fewer conduits for fluid Miocene to late Pliocene magmatism. Carlile & flow to the surface are available. Magmatism Mitchell (1994) suggest that uplift and erosion instead leads to the formation of intrusions within have removed all the older mineral deposits but the crust. According to Sillitoe (1999), this is a have been insufficient to reveal those of Quatern- more likely setting for porphyry copper deposits, ary age. Macpherson & Hall (2002) suggest characterized by cylindrical stocks related to instead that the timing of magmatism and style of intrusions, associated with andesite-dominated mineralization relates to a change in plate dy- magmatism where arcs are not in extension. namics from a period of hinge advance of the Examining the tectonic settings of mineral depos- Banda arc between 25 and 15 Ma to one of hinge its in Indonesia, Carlile & Mitchell (1994) noted retreat since 10 Ma. Barley et al. (2002) relate that all known mineral deposits lie within mag- these plate reorganizations to periods of gold matic arcs and formed during or shortly after the mineralization. They note that both the largest and magmatic activity. But of 15 Cenozoic magmatic the largest number of ore deposits, associated with arcs recognized, only six were known to contain arc magmatism in unusual tectonic settings, significant mineralization. The mineralization is formed in the past 5 million years during a major dominated by porphyry Cu-Au and epithermal period of changing motions between the India- Au deposits, all founded on continental crust, the Australia and Pacific plates. former occurring on both island arc and continen- (2) The northward movement of the India- tal settings and the latter best developed in Australia plate at 60 mm a -1 relative to the Banda continental arcs. In contrast with the settings of Sea plate has resulted in the collision of the NW the porphyry deposits, epithermal mineralization Australian shelf with the Banda Arc in the region Downloaded from http://sp.lyellcollection.org/ by guest on September 26, 2021

4 D. J. BLUNDELL of Timor. The collisional history appears to have teeming with bacteria and archaea, and provides begun 8 Ma ago when an outlier of thin continen- clear evidence of current mineralization in the tal crust reached the north-dipping subduction area (Binns et al. 2002). zone and transferred from the lower (India-Aus- (3) A mantle plume has been recognized tralia) plate to the upper (Banda Sea) plate when beneath the Manus basin in the Bismark Sea subduction jumped to the south of it. Oceanic (location, Fig. 1) on the basis of the petrology, lithosphere was subducted until 2.4 Ma ago when geochemistry and He isotope ratios of active the NW Australian shelf arrived. A section of volcanics (Macpherson et al. 1998). It is con- continental crust was subducted, to around 100 km finaaed by the presence of a cylindrical-shaped depth, until relatively recently when subduction seismic tomographic low velocity anomaly in the jumped to the north of the volcanic arc and upper mantle (Spakman & Bijwaard 1998) with a changed polarity. Thus Timor and the adjacent diameter of 700 kin. Investigating the widespread section of the volcanic arc are currently moving occurrence of Mg- and Si-rich boninite magma- north as part of the India-Australia plate tism in the Izu-Bonin-Mariana forearc of the (McCaffrey 1996). To the north, the Banda Sea is Philippine Sea plate during the middle Eocene, a back-arc basin currently being overthrust at the Macpherson & Hall (2001) have proposed that this south-dipping Wetar Fault. The present cross-sec- is due to the presence of a mantle plume. They tional configuration is sketched as a cartoon in point out that the generation of boninite magma Figure 2 (Richardson & Blundell 1996), showing requires excess heat in order to melt residual the thickened continental mass below Timor, peridotite within the mantle portion of an oceanic wedged between the Australian shelf continental lithospheric plate overriding a subducted plate. mass to the south and the Banda Sea plate to the Additionally, a hydrous fluid flux from the sub- north, laterally shortened by over 50% and cur- ducted lithosphere has to be incorporated into the rently being uplifted at up to 2 mm a -1 . melt. Using a plate reconstruction from 50 Ma The Banda Arc-Australia collision has been (Hall 1996), they demonstrate that the boninite used by Brown & Spadea (1999) as a modern volcanics would have been located within a circle analogue for the evolution of the Uralides during of 1,500 km diameter. Continuing with subsequent the late Devonian, Figure 3, involving the colli- reconstructions they have been able to track the sion of the Magnitogorsk Arc with the East path of this plume to its present-day location in European continental mass. It also offers the basis the Manus basin, where VHMS style mineraliza- of an explanation for the location and timing of tion has been found (Binns & Scott 1993). Thus VHMS deposits in the southern Urals. However, the presence of a mantle plume beneath a subduc- the Uralide orogeny continued beyond arc-con- tion complex may be relatively long lived and can tinent collision to complete continent-continent interact with a range of subduction processes as a collision, granite magmatism and post-orogenic result of the ever changing configurations of extensional collapse in the early Carboniferous. microplates within the complex. The additional VHMS deposits in the southern Urals are found in thermal energy provided by the plume can thus well-preserved mounds (Zaykov 2000), whose promote substantial magmatic activity-the volume modern counterparts are linked with black smo- of boninite volcanics is comparable with other kers, such as the 'black smoker' recovered from plume-related volcanism such as the Tertiary 2 km depth on the seafloor of the SE Manus basin. igneous province of NW Britain or the Columbia This is 2.7 m long, 80 cm in diameter at the base, River basalts. The characteristic style of the middle Eocene boninite volcanism provides a Cenozoic analogue that can be recognized within the geological record, such as the oldest volcanic S unit found in the Magnitogorsk forearc in the I Australian volcanic southern Urals (Brown & Spadea 1999) or the late shelf Timor arc Archaean Abitibi Belt in Canada (Wyman 1999). (4) A key observation of the SE Asia-SW T , Pacific subduction complex is the speed at which -60 km the various tectonic processes take place and the short duration of tectonic and magmatic events. For this reason, the single most important require- ment for a better understanding of mineralization Fig. 2. Cartoon cross-section of the Banda Arc-NW is to have high precision, high accuracy dating of Australian shelf collision zone interpreted from BIRPS suites of ore minerals. Dynamic changes in plate deep seismic sections (Richardson & Blundell 1996); motions, which are dependent upon time-variable stippled area Australian lower crust. parameters, are the events most likely to control Downloaded from http://sp.lyellcollection.org/ by guest on September 26, 2021

ORES IN AN EVOLVING OROGEN: GEODYNAMICS 5

(a)

AC S F SL

vvvvv vvvvvvvvvv vvvv vvvvvvvvv (c) ...... vvvvvvvvv vvvv ...... vvvvvvvv vvvvvv ...... vvvvvvvvv vvvv ...... vvvvvvvv vvv ......

oceanic continental arc-derived I crust I[:[[[:[I crust ~ sediments

transitional ~vWvVvvvvlvVvVTvVTvV volcanic crust arc Fig. 3. Schematic model of arc-continent collision, redrawn from Brown & Spadea (1999), reproduced with permission from the Geological Society of America, based on modern analogues from the western Pacific, to explain the evolution of the southern Uralides: (a) Intra-oceanic subduction of young, hot oceanic lithosphere, with volcanic arcs of tholeiite and boninite composition during the early to mid-Devonian: SL is sea level. (b) As continental crust arrives at the subduction zone during the late Devonian, collision-related processes predominate, with calc-alkaline suites indicating a mature island arc setting. (e) In the Early Carboniferous, when collision is nearly complete, the accretionary complex (AC) and forearc (F) areas attain their final architecture; S is the suture. the timing and location of mineralization. Sillitoe boundaries from convergent to transform, or the (1997) observed that various unusual arc settings cessation of subduction resulting from arc-con- can be conducive to mineralization, such as tinent collision. He found that some of the largest subduction polarity reversals, changes of plate gold deposits in the circum-Pacific region oc- Downloaded from http://sp.lyellcollection.org/ by guest on September 26, 2021

6 D.J. BLUNDELL curred in short-lived, areally restricted events that be preserved after the orogeny has run its course resulted from partial melting of stalled subduction through to continental collision and eventual slabs conducive to the oxidation of mantle sul- collapse. Van Staal et al. (1998) illustrated this by phides and release of gold. The changes in modelling the SE Asia-SW Pacific complex palaeogeography evident in Hall's plate recon- forward into the future some 45 Ma, until the end structions over the past 50 Ma underline the of collision. Figure 4 replicates this model, which ephemeral nature of many of the subduction they used as an analogue for the Appalachian- elements present today. For example, the Woo- Caledonian orogen, pointing out that it would be dlark basin only began to develop as an oceanic impossible to reconstruct the evolution of the ridge started to rift, driven by slab pull, just over orogeny because so much of the evidence had 7 Ma ago. It is currently being subducted as fast been lost. Thus there are bound to be severe as it is growing so that no trace of it will remain limitations on interpretations of the tectonic set- after another 2-3 Ma. A region of oceanic litho- tings of mineral deposits in ancient orogenic belts sphere flooring the Molucca Sea north of Halma- unless, like the Uralides, they are exceptionally hera was subducted between 11 Ma and 2.5 Ma, well preserved. both to the east and to the west (Hall 1999), and is imaged by seismic tomography as an inverted V in cross-section (see Macpherson & Hall 2002, fig. 3). The forearc wedges of the upper plates to A modern orogenic system: the the east and west of it have now collided, with the Carpathian Arc western one overriding the other. Two arcs are now in contact with an accretionary complex Forming part of the Alpine-Carpathian system, sandwiched between. Meanwhile the inverted V of the Carpathian Arc has been evolving for the past oceanic lithosphere is sinking through the upper 80-100Ma, but is now in the late stage of mantle. Geologists in a few million years time will continental collision when plate convergence has have great difficulty in distinguishing this arc- almost come to an end. This is one of the collisional history and in recognizing the sunken circumstances when slab detachment occurs, by plate. lateral tear, particularly when continental litho- Most volcanic episodes are over within 3- sphere is incorporated into the subducting slab. 5 Ma, though they may recur subsequently in the Seismic tomographic images on radial cross-sec- same location, but under different stress conditions across the Carpathian Arc (Wortel & Spak- tions. The rates of uplift of many of the islands, man 2000) reveal the presence of a gap between such as Timor, at up to 2 mma -I ensure rapid the lithosphere and the down-going slab, apart erosion and exhumation. Thus the epithermal from the Vrancea region in the SE where the slab copper and gold deposits on East Mindanau is unbroken. The down-going slab turns to hor- associated with the Philippine Fault (probably izontal at depth, an indication of an earlier hinge formed at depths around 5 km less than 5 Ma ago) retreat and rollback. The effect of a partly torn and the VHMS deposits on Wetar (currently at slab is to increase the weight of the slab where it surface but formed in deep water 4.7 Ma ago) is attached, resulting in subsidence at surface, and could be gone in another 2-3 Ma, possibly to to unload the lithosphere where it has torn, reappear later in the sedimentary record as placer resulting in surface uplift (Fig. 5). The history of deposits. In New Guinea, gold deposits are asso- slab tear clockwise around the Carpathian Arc ciated with thrusts and lateral ramps (Pubellier & since the Oligocene can be tracked by the migra- Ego 2002). Syn-tectonic porphyry Cu was injected tion of the null point between subsidence and between 7 and 2 Ma into thrust planes and then uplift around the arc. Earthquake activity is strong cut by later thrusts. The tectonic setting changed in the Vrancea region where the slab is still again after 2 Ma becoming wrenching at the front attached. Working with numerical models, Wortel of the belt, the lateral ramps subsequently being & Spakman (2000) have shown that detachment is re-activated as normal faults. Mineralization may dependent on the strength of the down-going slab: come with one phase and be destroyed by the next it can occur if the slab strength becomes less than one. These rapidly evolving systems show how the tensional stress from slab pull. The strength essential it is to have accurate dating of events can be reduced, for example through a change and underline the over-simplifications that we from oceanic to continental lithosphere in the make when we try to understand ancient orogens. subducting slab, sufficient to initiate slab tear. Thus, although the SE Asia-SW Pacific sub- Leech (2001) has pointed out that eclogitization is duction complex may provide examples of tec- likely to be a major factor in weakening the tonic settings within an actively evolving orogen, lithosphere in these circumstances and that an much of what is observed at present is unlikely to influx of fluids into the subduction zone is Downloaded from http://sp.lyellcollection.org/ by guest on September 26, 2021

ORES IN AN EVOLVING OROGEN: GEODYNAMICS 7

/j,-~ Pacific NOW / 45 Ma HENCE I/t plate

Asian plate

Asian late

S~" i) Pacific plate /~ ~-"~'- / ,, \ . ::i:iii!ii!i!i:i:i::-... ~ ::,:':~.,,. ~ ,

7 oceanic plateaux / VI-=--I-z-. back-arc/itback-arc/intra-arc tra-ar basins ~ subduction zone -- thrust .... arc Ausra:,an 'a'e t -- - suture "-- transform • ...... ridge

Fig. 4. Forward model of plate motions of the SE Asia-SW Pacific region 45 Ma into the future when continental collision occurs and relative plate motions cease (Van Staal et al. 1998).

required in the metamorphic processes that are analysis capable of predicting the tectonic settings involved. The actual depth of detachment is of mineralization formed in the later stages of dependent upon the thermal structure and rheol- orogenesis. De Boorder et al. (1998) have shown ogy of the subducting slab-the warmer and weak- that there is a close spatial link between Late er it is, the shallower is the depth of detachment. Cenozoic deposits of Hg, Sb and Au, which have Considering the thermal consequences, a shallow a mantle origin, and areas of lithospheric slab detachment can locally thin the lithosphere and detachment beneath the Carpathian Arc picked bring hot asthenosphere closer to surface, with a out by seismic tornographic low velocity (i.e. hot) consequent thermal pulse and the likelihood of regions. They report a 2.2Ma age difference partial melting. A slow convergence velocity between the volcanic host and the mineralization, results in a shallow detachment at a later time in which may indicate the time lag between slab the orogenic process, but a relatively small tem- detachment and mineralization. Neubauer (2002) perature jump. A minimum depth of detachment finds that, within the long-term subduction com- is at 35km when very warm lithosphere is plex of the Alpine-Balkan-Carpathian-Dinaride subducted, but the depth is greater for cooler region that evolved over the past 120 Ma, two lithosphere, around 70-80km. Modelling indi- short-lived, late-stage collisional events occurred, cates that partial melting is restricted to the plate one in the Late Cretaceous, the other of Oligo- contact region where the tear has formed. The cene-Neogene age. Plate reorganization led to overlying lithosphere, which is uplifting and ex- magmatism and extensive mineralization in three tending laterally, is thus affected by a sudden flow distinct belts. Within these belts, the type of of heat from the hot lithosphere. This can cause mineralization varies significantly along strike. In anatexis and granite maginatism, with related ore the Oligocene-Neogene Serbomacedonian-Rho- deposits. A further significance of this thermo- dope belt, related to the Carpathian arc, the timing tectonic modelling is that it can be related to P- of magmatism and mineralization migrates along T-t information from metamorphic minerals and strike in keeping with the Carpathian arc, and so the isotopic and geochemical signatures of the can be linked with the process of lithospheric slab partial melts. Thus there is now a method of tear. Downloaded from http://sp.lyellcollection.org/ by guest on September 26, 2021

D. J. BLUNDELL

arrows show direction subsidence

I asthenosphere flows ir

Fig. 5. Mechanism of slab tear proposed byWortel & Spakman (2000), reproduced with permission from Science 290, 1910 (2000) fig.4. Copyright 2000 American Association for the Advancement of Science. The progress of the tear can be followed by the migration of the null point between subsidence and uplift around the arc. Slab tear enhances roll-back, inflow of hot asthenoshere into tear gap creates specific conditions for short-lived magmatism and mineralization.

Modern fault systems and fluid flow sion. Figure 6 illustrates these differences, as imaged on deep seismic reflection profiles, which Whilst the tectonic settings just discussed deter- are a consequence of the differing theological mine the location of the source region and the responses of rocks under tensile and compres- timing of the generation of magmatic and/or sional stress (Cloetingh & Banda 1992). In con- hydrothermal mineralizing fluids, the transporta- tinental regions under extension, the seismogenic tion system to bring these fluids to the ore deposit zone in which earthquakes occur is normally location is of fundamental importance to ore above a mid-crustal level of around 15-20 km but genesis. The close association of ore deposits with in regions of active thrusting during continental faults and fracture systems implies that these are collision, such as the Himalayas, earthquakes oc- the prime components of the transport system, at cur at depths nearly down to the Moho. least within the brittle upper crust. Evidence from A considerable body of research has established seismic s-wave velocity anisotropy by Crampin various possible mechanisms for fluid transport in (i994) has established the ubiquitous presence of the crust. Sibson (1996) showed that mesh struc- vertical cracks in the crystalline crust. Deeper in tures involving faults interlinked with extensional the continental crust and upper mantle, anasto- vein-fracture systems can act as major conduits mosing ductile shear zones hold the key in regions for large volume fluid flow. Fluid pressure pro- of extension, whilst crustal-scale shears, ramps vides the drive and the mechanism for opening and thrust anticlines dominate regions of compres- elements in the mesh, allowing transient increase Downloaded from http://sp.lyellcollection.org/ by guest on September 26, 2021

ORES IN AN EVOLVING OROGEN: GEODYNAMICS 9

(a) COMPRESSION Strength (MPa) network of interconnected cracks becomes highly 0 500 1000 organized and effective when fluid pressure 0 I I reaches a critical point. Based on a random polygonal network of cracks, their model shows 20 that hydraulic conductivity increases when rising fluid pressure reaches a critical value, less than 40 but close to the lithostatic pressure. At this point, deformation of the network results in the forma- 60 Y tion of straighter, longer cracks, allowing slip, Depth (km) localized stress concentrations and localized open- (b) TENSION ing of the more organized fractures. The sudden Strength (MPa) increase, locally, in hydraulic conductivity by 500 0 several orders of magnitude allows the sudden upward expulsion of fluid from the system. In- creasing fluid pressure can arise from loading due 20 to tectonic stress or from heating-the latter a 40 'pressure cooker' effect. This mechanism is particularly applicable where flow localization is im- 6O portant, as in vein-hosted mineralization. Cox epth / (kin) (1999) regards the crack networks as composed of through-going, or 'backbone' fractures linked to branching crack systems. Upstream, at depth, the Fig. 6. Cartoons based on deep seismic reflection profiles across continental lithosphere showing typical branching elements act as tributaries that feed structural configurations in relation to strength profiles: fluid into the system whereas downstream, at (a) under compression where crustal-scale thrust ramps higher levels, the branching elements act as and anticlines dominate, (b) under tension where brittle distributary structures and are the likely sites for conditions and faulting occur in the upper crust and mineral deposits. He emphasizes the transient ductile conditions with anastomosing shears pervade the nature of fluid flow, as well as its localization, as a lower crust. The Moho M can act as a detachment, result of the competition between processes creat- below which, occasionally, dipping reflections are ing crack growth and processes that close or seal observed in the upper mantle. the cracks. Continuing deformation regenerates crack growth to exceed crack closure and maintain a succession of fluid flow pulses. in hydraulic conductivity, resulting in fluid flow Of particular significance are the findings of and earthquake swarm activity. Sibson (1981, Muir-Wood & King (1993) from a study of the 1987) had earlier proposed an earthquake 'pump- amounts of water expelled at surface due to earth- ing' mechanism to explain the association of quake-related fault movements. They showed that epithermal mineralization with extensional offsets whilst normal faults in a continental setting between fault segments. Dilation in the relay zone expelled substantial quantities of water, strike-slip between fault segments as the fault ruptures faults expelled less than a tenth of the water from causes a reduction in fluid pressure so that fluid is normal faults and flow was variable, and reverse sucked into the void space. This discharges subse- faults expelled virtually no water. They quantified quently as strain is reduced during the aftershock their findings with detailed measurements of ex- sequence. Sibson et al. (1988) proposed a similar cess surface water flow from two earthquakes mechanism to explain the association of mesother- related to normal faults in the Basin and Range mal gold deposits with high-angle reverse faults at region of USA, at Hebgen Lake in 1959 and mid-crustal levels. In this case the brittle upper Borah Peak in 1983. In both cases, monitoring crust (see Fig. 6) acts as a seal to fluids trapped networks covering the complete catchment areas below the seismogenic zone until fluid pressure were operating at the time so that flow rates could increases to a point when the fault fails and be measured on a daily basis. Within a few days rupture occurs. The fault acts as a valve that opens of each earthquake the flow rate peaked and an from time to time, allowing pulses of high excess flow was recorded for nearly a year. In that pressure fluid to be released upwards. However, time the total quantity of water expelled from a this mechanism may have more to do with fault 20 km length of fault break due to a magnitude 7 inversion events than crustal-scale thrusting. earthquake was 0.5 km3. To explain this flow, Sanderson & Zhang (1999) presented numerical Muir-Wood & King (1993) proposed a model of models with coupled mechanical and hydraulic coseismic strain illustrated in Figure 7. During the behaviour to demonstrate how fluid flow through a interseismic period, which would normally last Downloaded from http://sp.lyellcollection.org/ by guest on September 26, 2021

10 D.J. BLUNDELL oo/oo (a) (b)

Fig. 7. Coseismic strain model redrawn from Muir-Wood & King (1993), reproduced with permission from the American Geophysical Union, of fluid flow related to earthquake-induced rupture of a normal fault. (a) Cracks open during the interseismic period under horizontal tensile stress. The cracks fill with fluid. (b) Cracks close at the time of an earthquake due to coseismic compressional rebound as the stress drops and strain is released. Fluid is expelled from the cracks and upwards via the fault.

100-150 years for earthquakes of this magnitude, The fault is at the northern edge of the Moresby horizontal tensile stress opens a network of verti- Seamount at the western tip of an active spreading cal cracks, allowing water into the space. When ridge. The fault has been clearly imaged on a the earthquake occurs and the fault breaks, the seismic reflection profile as a planar surface stress drops and the dilatational strain is released. dipping north at 26 °, visible to 9 km depth below The cracks close and the water is expelled. Since the ocean floor. It developed 3.5 Ma ago and now the fracture density is greatest close to the fault has a displacement of 10 km. It is related to a and the fault gouge provides a relatively high number of magnitude 6 earthquakes with normal permeability pathway towards the surface, this is fault focal mechanisms. From a careful analysis of the prime route for the expulsion of water. the reflection characteristics on the seismic sec- Although the mechanism is similar to the 'seismic tion, Floyd et al. deduced that the fault gouge is pumping' schemes of Sibson (1987) and Sibson et 33 m in thickness with high (>30%) porosity, al. (1988), it links fault-related expulsion of fluids maintained by a high (near-lithostatic) fluid pres- specifically with normal faulting and extensional sure. They suggest that high extensional stress and tectonics, together with strike-slip fault systems magmatic heat ahead of the ridge tip have created where transtension occurs. It predicts the inverse conditions for strain localization and hydrothermal effect for compressional faulting, in which cracks fluid flow. close during the interseismic periods, expelling water, but open when the earthquake causes fault rupture when water is drawn in. Of further Conclusions significance for mineralization is the relatively long residence time of fluid in basement cracks Currently active orogenic systems can provide with high aspect ratios under horizontal extension modem analogues for mineralization in ancient during interseismic periods. With a large surface orogens. Geodynamic processes can be observed area relative to volume, each crack is conducive to and measured and their rates and duration quanti- fluid-wall rock interaction and the possible fied, both at the Earth's surface and throughout the scavenging of metals. The transport of mineraliz- lithosphere and upper mantle. In consequence, it ing fluids from the crystalline basement to higher is possible with modem systems to identify a levels in the crust where they may concentrate in variety of tectonic settings conducive to the initia- an ore body by this mechanism is by a large tion of mineralization at various stages within the number of intermittent pulses of flow sustained evolution of an orogeny. Because geodynamic over a significant period of time. For many large processes, such as subduction rollback and slab hydrothermal ore deposits, the total quantity of tear, can be modelled quantitatively, not only can fluid required is between 103 and 104 km 3. At the mechanism be understood but the consequen- 0.5 km 3 per seismic event at 150 year intervals, a tial effects, such as localized heat production, time span of between 200 000 and 2 million years partial melting, metamorphism and mineralization is required. can be predicted. Furthermore, these models can A similar situation appears to hold in an be tested with observations on mineral deposits of oceanic setting. Floyd et al. (2001) report evi- Neogene age. Modern orogenic systems, such as dence of fault weakness and fluid flow from a the SE Asia-SW Pacific region, demonstrate the low-angle normal fault in the Woodlark basin east complexity, speed and short duration of many of of Papua New Guinea that is seismically active. the tectonic and magmatic processes involved and Downloaded from http://sp.lyellcollection.org/ by guest on September 26, 2021

ORES IN AN EVOLVING OROGEN: GEODYNAMICS 11 the ephemeral nature of many of their present arcs and associated gold and copper mineralization features. Rapid uplift and erosion, as well as in Indonesia. Journal of Geochemical Exploration, subduction can destroy many of the present fea- 50, 91-142. tures within a few million years. Much of the CLOETINGH, S. & BANDA, E. 1992. Europe's lithosphere evidence of geodynamic processes taking place - physical properties. In: BLUNDELL,D., FREEMAN, R. & MUELLER, S. (eds) A continent revealed: the within an evolving orogen are not preserved by European Geotraverse. Cambridge University Press, the time it has reached its end when relative plate Cambridge, 80-91. movement and deformation have ceased. Thus, CRAMPIN, S. 1994. The fracture criticality of crustal there are limitations to the use of modern systems rocks. Geophysical Journal International, 118, to interpret what happened in ancient orogens that 428-438. ran to a conclusion long ago. Cox, S.F. 1999. Deformational controls on the dynamics As important to the formation of a large ore of fluid flow in mesothermal gold systems. In: deposit as its tectonic setting is the mechanism for MCCAFFREY, K.J.W., LONERGAN, L. & WILKINSON, J.J. (eds) Fractures, Fluid Flow and Mineralization. transporting mineralizing fluids to the site of ore Geological Society, London, Special Publications, deposition. The close association observed be- 155, 123-140. tween the location of ore deposits and faults and DE BOORDER, H., SPAKMAN, W., WHITE, S.H. & fracture networks has led to the appreciation that WORTEL, M.R. 1998. Late Cenozoic mineralization, these are the main elements of the transport orogenic collapse and slab detachment in the system. Various mechanisms for generating up- European Alpine Belt. Earth and Planetary Science ward fluid flow through networks of intercon- Letters, 164, 569-575. nected fracture systems and faults have been FLOYD, J.S., MUTTER, J.C., GOODLIFFE, A.M. & TAY- proposed, supported by numerical models, but LOR, B. 2001. Evidence for fault weakness and fluid observations of fluid flow generated by modem flow within an active low-angle normal fault. Nature, 411,779-783. earthquake-related fault movements provide quan- GOLDFARB, R.J., GROVES, D.I. & GARDOLL, S. 2001. titative information. In particular, it has been Orogenic gold and geologic time: a global synth- observed that normal faults deliver much greater esis. Ore Geology Reviews, 18, 1-75. flows than strike-slip faults and that reverse faults HALL, R. 1996. Reconstructing Cenozoic SE Asia. In." produce practically no flow. This has clear impli- HALL, R. & BLUNDELL, D.J. (eds) Tectonic Evolu- cations for modelling mineralizing systems in the tion of SE Asia. Geological Society, London, upper crust and provides quantitative information Special Publications, 106, 153-184. to constrain them. HALL, R. 1999. Neogene history of collision in the Halmahera region, Indonesia. Proceedings of the Indonesian Petroleum association, 27, 487-493.

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