Reactive transport modelling of hydrothermal dolomitisation using the CSMP++GEM coupled code: Effects of temperature and geological heterogeneity

Abstract

University of Bristol -Explore Bristol Research General rightsThis document is made available in accordance with publisher policies. Please cite only the published version using the reference above. Full terms of use are available: http://www.bristol.ac.uk/pure/about/ebr-terms Reactive transport modelling of hydrothermal dolomitisation using the CSMP++GEM coupled code:Effects of temperature and geological heterogeneity AbstractReactive transport simulations using our CSMP++GEM coupled code were applied to study the major controls on replacement dolomitisation and the development of dolomite geobodies in a hydrothermal setting. A series of 2D simulations show how elevated temperature and reactive surface area increase the rate of dolomitisation, and result in a dolomite replacement front that is both sharper and inclined at a higher angle from vertical. This inclination, an effect of gravity segregation, is apparent in thick homogeneous units, but in layered systems the lithological contrast determines the shape of the dolomite front. The increase in permeability resulting from porosity generation upon replacement of calcite by dolomite has a major effect on accelerating the overall progress of dolomitisation. In contrast, the changes in fluid density due to chemical reactions and the pressure dependence of thermodynamic data have a minor influence under simulated conditions. Primary dolomite forms slowly after complete replacement of host calcite, leading to porosity decrease, and is only locally important around the source of the hydrothermal fluid.For a simple layered system, our model results are in excellent agreement * Corresponding author. Present address: ETH Zurich, Institute of Geochemistry and Petrology. E-mail address: alina.yapparova@gmail.comPreprint submitted to Chemical Geology July 6, 2017with those obtained using TOUGHREACT code. They do, however, show the advantage of unstructured triangular over structured rectangular meshes for resolving complex curved/inclined front shapes. Such meshes also offer benefits in simulating fault-controlled hydrothermal dolomitisation. Our simulations predict dolomite geobodies comparable in scale and morphology to natural examples documented at outcrops, and underline the importance of understanding the permeability structure within and around the fault zone.

Understanding controls on hydrothermal dolomitisation: insights from 3D Reactive Transport Modelling of geothermal convection

“…fronts perpendicular to the plane of the fault, where model resolution is also higher, are within a single 10 m wide cell. This is in the same order of magnitude as those predicted by 2D simulations of HTDs formed by flow along through-going faults (Yapparova et al, 2017), and also many field examples with sharp dolomitisation fronts (Grandia et al, 2003;Nurkhanuly and Dix, 2014;Shah et al, 2012). Detailed analysis of dolomitisation fronts within fault-controlled dolomite bodies in nature are remarkably understudied, however, and are likely complex; for example, Koeshidayatullah et al (2020b) show that reaction fronts can back-step in time, as progressive phases of recrystallization result in porosity reduction (overdolomitisation), restricting the extent to which subsequent fluids can flux away from the fault.…”

“…Following the early work of Wilson et al (2001) and Jones and Xiao (2005), over the past decade process-based numerical modelling of heat and fluid flow coupled with simulation of water-rock reactions using reactive transport models (RTMs) have contributed to understanding of shallow, relatively low-temperature dolomitisation (Al-Helal et al, 2012;Garcia-Fresca, 2009;Whitaker and Xiao, 2010;Xiao et al, 2013). RTM simulations of HTD have focussed on understanding controls on the nature of dolomitisation fronts, which are characteristically sharp in fault-related dolomites, and on the development of fault-associated stratabound dolostone bodies (Corbella et al, 2014;Xiao et al, 2013;Yapparova et al, 2017). These studies demonstrate the importance of the geometry of the fault, termination of the fault against a sealing layer and distribution of permeability and precursor mineralogy in the country rock, but are all predicated on a supply of hot, Mg 2+ -rich fluid at the base of the fault.…”

Understanding controls on hydrothermal dolomitisation: insights from 3D reactive transport modelling of geothermal convection

“…Simulations of structurally complex reservoirs are particularly challenging as they comprise discrete structures with properties that contrast strongly with those of the country rock and range over vastly different length scales (Matthäi et al, 2007). Variations in permeability and fault zone thickness are fundamental controls on fluid circulation and subsequent mineralisation distribution (Guillou-Frottier et al, 2020;Harcouët-Menou et al, 2009), whereas the effect on hydrothermal convection of other parameters, such as porosity, is considerably less significant (Gow et al, 2002;Kühn et al, 2006;Zhao et al, 2003). However, our models do not attempt to capture the complexities of flow within FDZs, for which the porous media formulation employed by TOUGHREACT is inappropriate.…”

Hydrodynamic Constraints on Ore Formation by Basin‐Scale Fluid Flow at Continental Margins: Modelling Zn Metallogenesis in the Devonian Selwyn Basin

“…Metal leaching, transport and precipitation in nature depends on chemical speciation and therefore also on other parameters like pH and redox conditions. However, full reactive-transport modelling rather focusses on lower-temperature and geochemically less complex systems (Yao & Demicco, 1997;Yapparova et al, 2017bYapparova et al, , 2017a and the thermodynamic databases that can cover chemical speciation for base metal transport at such high temperatures are currently still under development, e.g. investigating the roles of chloride and bisulfide complexing for metal transport (Etschmann et al, 2019;Zhong et al, 2015).…”