Origin and evolution of fault-controlled hydrothermal dolomitization fronts: A new insight

Koeshidayatullah, Ardiansyah; Corlett, Hilary; Stacey, Jack; Swart, Peter K.; Boyce, Adrian J.; Hollis, Cathy

doi:10.1016/j.epsl.2020.116291

Cited by 55 publications

(84 citation statements)

References 36 publications

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“…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. An obvious benefit of flow simulations such as these is that they can provide information on the controls on dolomite body termination during the earliest phases of fluid flux, before porosity-permeability feedback becomes an important control on the extent of fluid migration and reaction.…”

Section: Characterisation Of Dolomite Frontssupporting

confidence: 62%

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

Benjakul¹,

Hollis²,

Robertson³

et al. 2020

Preprint

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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.

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Section: Characterisation Of Dolomite Frontssupporting

confidence: 62%

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

Benjakul¹,

Hollis²,

Robertson³

et al. 2020

Preprint

Self Cite

View full text Add to dashboard Cite

show abstract

“…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.…”

Section: Model Limitations and New Insights From Simulations Of Dolommentioning

confidence: 99%

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

et al. 2020

Self Cite

View full text Add to dashboard Cite

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 the 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 metres vertically and along the fault within a timescale of a few tens of thousands of years, with no significant alteration of the country rock. Dolomitisation occurs as a gradient reaction by replacement of host limestones and minor dolomite cementation, and it results in the 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.

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“…During the emplacement of igneous intrusions, the dolomite phases were formed from the hydrothermal fluids originated from the magma source (Figure 4(b)). The high-temperature origin of these dolomite phases is confirmed by the sharp color contrast and nonplanar crystal habit [62] (Figures 3(c), 4, 5(a), and 5(b)). In addition, the presence of hightemperature minerals (i.e., ankerite and diopside) may have resulted from the interaction between hydrothermal fluids with the host limestone, which is also confirmed by various studies where diopside mineral is usually found in skarn deposits [59] (Figures 8(a) and 8(b)).…”

Section: Discussionmentioning

confidence: 65%

Diagenetic Modifications and Reservoir Heterogeneity Associated with Magmatic Intrusions in the Devonian Khyber Limestone, Peshawar Basin, NW Pakistan

et al. 2021

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In the present study, an attempt has been made to establish the relationship between diagenetic alterations resulting from magmatic intrusions and their impact on the reservoir properties of the Devonian Khyber Limestone (NW Pakistan). Field observations, petrographic studies, mineralogical analyses, porosity-permeability data, and computed tomography were used to better understand the diagenetic history and petrophysical property evolution. Numerous dolerite intrusions are present in the studied carbonate successions, where the host limestone was altered to dolomite and marble, and fractures and faults developed due to the upwelling of the magmatic/hydrothermal fluids along pathways. Petrographic studies show an early phase of coarse crystalline saddle dolomite (Dol. I), which resulted from Mg-rich hydrothermal fluids originated from the dolerite dykes. Coarse crystalline marble formed due to contact metamorphism at the time of dolerite emplacement. The second phase of dolomitisation (Dol. II) postdates the igneous intrusions and was followed by dedolomitisation, dissolution, and cementation by meteoric calcite. Stable isotope studies likewise confirm two distinct dolomite phases. Dol. I exhibits more depleted δ18O (-15.8 to -9.1‰ V-PDB) and nondepleted δ13C (-2.05 to +1.85‰ V-PDB), whereas Dol. II shows a relatively narrow range of depleted δ18O (-13.9 to -13.8‰) signatures and nondepleted δ13C (+1.58 to +1.89‰ V-PDB). Dolomitic marble shows a marked depletion in δ18O and δ13C (-13.7 to -8.5‰ and -2.3 to 1.95‰, respectively). The initial phase of dolomitisation (Dol. I) did not alter porosity (5.4-6.6%) and permeability (0.0-0.1 mD) with respect to the unaltered limestone (5.6-6.9%; 0.1-0.2 mD). Contact metamorphism resulted in a decrease in porosity and permeability (3.3-4.7%; 0.1 mD). In contrast, an increase in porosity and permeability in Dol. II (7.7-10.5%; 0.8-2.5 mD) and dolomitic marble (6.6-14.7%; 8.2-13.3 mD) is linked to intercrystalline porosity and retainment of fracture porosity in dolomitic marble. Late-stage dissolution and dedolomitization also positively affected the reservoir properties of the studied successions. In conclusion, the aforementioned results reveal the impact of various diagenetic processes resulting from magmatic emplacement and their consequent reservoir heterogeneity.

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Origin and evolution of fault-controlled hydrothermal dolomitization fronts: A new insight

Cited by 55 publications

References 36 publications

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

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

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

Diagenetic Modifications and Reservoir Heterogeneity Associated with Magmatic Intrusions in the Devonian Khyber Limestone, Peshawar Basin, NW Pakistan

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