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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 Downloaded from http://sp.lyellcollection.org/ by guest on September 26, 2021 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 mineraliza- tion. 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 evi- dence 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
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