Application of Numerical Modeling to Extension, Heat, and Fluid Flow in the Genesis of Giant Banded Iron Formation-Hosted Hematite Ore Deposits
John G. McLellan, Nicholas H. S. Oliver, 2008. "Application of Numerical Modeling to Extension, Heat, and Fluid Flow in the Genesis of Giant Banded Iron Formation-Hosted Hematite Ore Deposits", Banded Iron Formation-Related High-Grade Iron Ore, Steffen Hagemann, Carlos Alberto Rosière, Jens Gutzmer, Nicolas J. Beukes
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Finite difference modeling of fluid flow in response to topography, extensional collapse, and thermal structure has been employed to simulate processes leading to the genesis of Proterozoic iron ores, using input data appropriate to the Hamersley district of Western Australia and other iron ore districts. The geologic history and questions that provide the motivation for the modeling include the presence of a mountain range formed by pre-ore genesis convergent deformation, extensional collapse of that mountain range, and evidence at the deposits for two or more different fluid types, including a deep-seated (reduced) and a surface-derived (oxidized and 18O-depleted) fluid. In terms of fluid-flow rates, topographically driven downward fluid flow is seen to be comparable to both deformation-driven flow and also to heating and/or basal overpressures for comparable permeability structures and mountains with elevations in excess of 1 km. During extensional deformation at geologically realistic strain rates, downward flow is created by the combination of dilation produced by deformation with the inability of the fluid always to flow quickly enough to account for the dilatant volume change, producing areas of fluid under pressure, particularly across permeability interfaces. This effect is most pronounced where extensional faults cut through low-permeability basement. Upward fluid flow of heated fluids, as has been proposed to initiate genesis of these giant iron ore deposits, can be achieved at the start of extensional deformation if the deep fluid is initially overpressured, for example, due to input of fluids derived from magmas or to heating and/or devolatilization deep in the system. This initial upward flow can produce substantial temperature anomalies at relatively shallow depths, particularly in the hanging wall of dipping faults. However, with time, the extension and topography drives cooler meteoric fluids downward, which competes with and then eventually swamps the initial upflow. This scenario matches the envisaged sequence of events at the major deposits of the Hamersley district and also explains how different deposits record different degrees of preservation of the early-formed high-temperature assemblages, depending on the extent to which later surface-derived fluids have utilized the same structures as the initial upflowing fluid. Questions remaining from this modeling, and in consideration of the geochemical and stable isotope data, relate to which of the fluids (or both) was largely responsible for silica dissolution and whether both deep-seated and shallow fluids are prerequisite ingredients for genesis of this ore type.
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The spark to put together this volume on banded iron formation (BIF)-related high-grade iron ore was born in 2005 during a steamy night in Carajás where the iron research group from the Universidade Federal Minas Gerais, Vale geologists, Carlos Rosière and Steffen Hagemann, were hotly debating the hypogene alteration genesis for the high-grade, jaspilite-hosted Serra Norte iron ore deposits. A couple of caipirinhas later we decided that the time was opportune to put together a volume that captured the new and innovative research that was being conducted on BIF-related high-grade iron ores throughout the world. We had little problem convincing our South African colleagues Jens Gutzmer and Nic Beukes to join the effort and decided that the 2008 biannual Society of Economic Geologists' (SEG) meeting in South Africa would be the perfect place to present this project through a combined field trip and workshop near Sishen.
The enthusiastic support that we received from the research community, SEG, and industry to put this volume together was generated by the significant increase in exploration activity, and with it the need for more detailed information on what exactly controls the location of high-grade iron orebodies, and renewed research interest around the world in models for the genesis of BIF-related high-grade iron ore, and particularly the relative importance of hypogene and supergene processes in formation of high-grade ore.
This volume concentrates on new research on the characteristics and metallogenesis of BIF-related high-grade iron ores. It contains a state of the art series of papers on established and new iron ore districts and deposits, the different components of the BIF iron mineral system, and how to best explore for this ore type. Although the emphasis of many of the contributions to this volume is on the hypogene aspect of high-grade iron ore formation, it is important to note that most BIF-related iron ore districts have a very pronounced supergene overprint due to deep lateritic weathering. The transformation of many hypogene iron orebodies of reasonable grade and size to the giant deposits exploited today can be related to this geologically recent supergene overprint; most of the past and still much of the present mining of high-grade iron ore relates to soft ore interpreted in most cases to be the direct result of supergene processes. Also mentioned here should be the recent resurgence of a syngenetic model that advocates the formation of chert-free BIF