Understanding controls on hydrothermal dolomitisation: insights from 3D reactive transport modelling of geothermal convection
Authors: Benjakul Rungroj , Hollis Cathy , Robertson Hamish A. , Sonnenthal Eric L. , Whitaker Fiona F. 
Published: December 2020

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.