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

Abstract

Abstract. The dominant paradigm for petrogenesis of high-temperature fault-controlled dolomite, widely known as “hydrothermal dolomite” (HTD), invokes upwelling of hot fluid along faulted and fractured conduits from a deep over-pressured aquifer. However, this model has several inherent ambiguities with respect to fluid sources and their dolomitisation potential, as well as mechanisms for delivering enough of these reactive fluids to form substantial volumes of dolomite. Here, we use generic 2D and 3D reactive transport simulations of a single transmissive fault system to evaluate an alternative conceptual model whereby dolomitisation is driven by seawater being drawn down into the subsurface and heated. We examine the evolution of fluid chemistry and distribution of diagenetic alteration, including predictions of the rate, distribution, and temperature of HTD formation, and consider the possible contribution of this process to the Mg-budget of the World’s oceans. The simulations suggest that it is possible for convection of seawater along the fault damage zone to form massive dolomite bodies that extend hundreds of meters vertically and along the fault within a timescale of a few tens of kyr, with no significant alteration of the country rock. Dolomitisation occurs as a gradient reaction by replacement of host limestones and minor dolomite cementation, and results in discharge of Mg2+-poor, Ca2+-rich fluids to the sea floor. Fluids sourced from the basement contribute to the transport of heat that is key for overcoming kinetic limitations to dolomitisation, but the entrained seawater provides the Mg2+ to drive the reaction. Dolomite fronts are sharper on the “up-flow” margin where Mg2+-rich fluids first reach the threshold temperature for dolomitisation, and the “down-flow” dolomite front tends to be broader as the fluid is depleted in Mg2+ by prior dolomitisation. The model demonstrates spatial contrasts in the temperature of dolomitisation and the relative contribution of seawater and basement-derived fluids which are also commonly observed in natural fault-controlled dolomites. In the past, such variations have been interpreted in terms of major shifts in the system driving dolomitisation. Our simulations demonstrate that such changes may also be a product of emergent behaviour within a relatively stable system, with areas that are dolomitised more slowly recording the effect of changes in fluid flow, heat and solute transport that occur in response to diagenetic permeability modification. Overall, our models robustly demonstrate that high-temperature fault-controlled dolomite bodies can form from mixed convection and act as a sink for Mg in the circulating seawaters. In addition, comparison of our 3D simulations with simplifications to 2D, indicate that 2D models misrepresent critical aspects of the system. This has important implications for modelling of systems ranging from geothermal resources and mineralisation to carbonate diagenesis, including hydrothermal karstification and ore genesis as well as dolomitisation.

Dolomitisation of carbonate platform margins by fault‐controlled geothermal convection: Insights from coupling stratigraphic and reactive transport models

“…Thus, dolomite precipitation is specified as a kinetic process using the rate law of Arvidson and Mackenzie (1999), and all other reactions are considered to be thermodynamically controlled. The use of this rate law is consistent with previous RTM simulations of dolomitisation (Al-Helal et al, 2012;Benjakul et al, 2020;Gabellone et al, 2016;Jones & Xiao, 2006;Whitaker & Xiao, 2010;Wilson et al, 2001;Yapparova et al, 2017).…”

“…It is also possible that platforms in tectonically active settings, such as modelled here, will have deep-seated faults that crosscut the platform away from the platform margins. These can play an important role in vertical transfer of heat and fluids over many hundreds of metres to kilometres, with influx of surface-derived diagenetic fluids (Benjakul et al, 2020;Hirani et al, 2018;Stacey et al, 2021) or discharge of fluids from depth (Bons et al, 2014;Frazer et al, 2014). In contrast, open-mode tensile bank-marginal fractures that are intrinsic in carbonate platforms with cemented margins (Frost & Kerans, 2010) tend to pinch out at depth (Nolting et al, 2018).…”

Regional fault-controlled shallow dolomitization of the Middle Cambrian Cathedral Formation by hydrothermal fluids fluxed through a basal clastic aquifer

“…The interpretation that faults controlled the dolomitization of the Cathedral Formation is supported by 3-D reactive transport modeling (Benjakul et al, 2020), which shows that fluid convection can be driven by the mixing of cool seawater descending through faults and hot, ascending, basement-derived brines. The model results show that the resulting dolomite bodies can extend away from the fault for hundreds of meters, which is equivalent in scale to the observed lateral and vertical extent of the dolomitized zone at Whirlpool Point.…”

“…Compared to replacement dolomite, saddle dolomite cement exhibits an inverse temperature relationship, as faultproximal phases precipitated at greater mean temperatures (TΔ 47 212 °C) than did the faultdistal equivalent (198 °C). The most likely explanation for the variable temperatures of each phase is that there were localized differences in temperature due to the convection of fluids along the fault plane (as shown by Benjakul et al, 2020) and during replacement dolomitization. The permeability of the Cathedral Formation may have also been a factor, with fluids potentially able to migrate farther from the fault during replacement dolomitization (up to ∼6.5 km) than at the time of saddle dolomite cementation (up to ∼18 m).…”

“…One of the key uncertainties with the HTD model is the ultimate source of dolomitizing fluids, which are often interpreted to be "evolved crustal fluids" based on highly saline, high temperature fluid inclusions within dolomite crystals (e.g., Wendte et al, 1998;Lonnee and Al-Aasm, 2000;Nelson et al, 2002;Al-Aasm, 2003;Morrow, 2014). However, recent work (Gomez-Rivas et al, 2014;Hollis et al, 2017;Rustichelli et al, 2017;Hirani et al, 2018a;Hirani et al, 2018b, Benjakul et al, 2020 has shown that HTD can occur from the convection of seawater along fault planes, facilitated by basal clastic aquifers (e.g., Martín-Martín et al, 2015;Lukoczki et al, 2019), indicating that fault-controlled dolomitization can occur from fluids that are heated at shallow depths. As such, this study will evaluate recently recognized components of the HTD model on the pervasively dolomitized carbonates of the Middle Cambrian (509-497 Ma; Miaolingian Epoch) Cathedral Formation and determine whether this example of HTD is more complex than previously thought.…”