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The term hydrothermal dolomitization (HTD) has been widely applied to describe fault-controlled dolomitization. HTD can occur at any depth from a range of different fluids, even though it is often associated with the burial realm. To date, many studies have failed to identify a source of fluids for HTD, often invoking “deep-seated brines”. Consequently, there is a gap in our knowledge as to the likely source of water and magnesium for the formation of hydrothermal dolostone. The Middle Cambrian Mount Whyte Formation displays exceptionally preserved m- to km-wide non-stratabound dolostone bodies with stratabound terminations within a bioturbated mudstone facies. Dolomitization is fabric-destructive and shows a wide range of crystal sizes (40-800 µm). Texturally, dolomite crystals can be subdivided into different phases that are all post-dated by bedding-parallel stylolites. This cross-cutting relationship suggests a shallow depth of dolomitization (< 1 km). All the dolomite types exhibit a narrow range of both negative δ18Odolomite (-15.08‰ to -17.78‰) and seawater-like δ13Cdolomite values (-1.04‰ to 0.43‰) and enriched Fe and Mn concentrations. Both the temperature and calculated δ18Owater of the dolomitizing fluids display a wide range of values depending on their location within the dolostone bodies, from 130 to 235oC and -3 to +6 ‰, respectively. The δ18Owater is substantially heavier than Middle Cambrian seawater, and correspondingly, 87Sr/86Sr ratios are elevated (av. 0.7106) compared to the host limestone (av. 0.7094). These lines of evidence are suggestive of very hot, crustal-related dolomitizing fluids. The REE+Y profiles observed in both RD1 and RD2 (positive Eu anomaly, enriched LREE and depleted HREE) are typical of ultramafic-hosted fluids. Together with the presence of a subduction complex below the WCSB (>10 km), identified from lithoprobe data, this suggests that magnesium-rich fluids were sourced from these Paleoproterozoic subducted serpentinites. The liberation of magnesium was most likely through interactions between serpentinites and CO2 enriched fluids created during either metasomatism or recycling of seawater. The presence of enriched Ti and Yb concentrations and cogenetic relation between dolomite and quartz corroborates this idea. The approximate depth of the subducted serpentinites is also in good agreement with the interpreted depth of the fluid, inferred from the measured fluid temperature. To transport such deep-seated fluids to the shallow sub-surface required vertical, permeable conduits such as faults. Since dolomitization coincided with rifting, fluids could have rapidly ascended into the Mount Whyte Formation by seismic events along crustal-scale faults which are now largely obscured by thrusting. The shallow depth of dolomitization would also explain the pervasive dolomitization of mud-dominated facies, as replacement occurred whilst there was sufficient porosity and permeability retained in these rocks. The very hot, high salinity of the dolomitizing fluids and the high reactive surface area in the mudstone helped to accelerate dolomitization. In summary, this study describes an alternative model for the source of Mg for HTD, providing a solution to the problem of mass balance between the volumes of observed dolostone formed by HTD from hot, saline, so-called crustal fluids. Such a model could be widely applicable to other settings that are underlain by ancient oceanic crust.

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Dolomitization is commonly associated with crustal‐scale faults, but tectonic rejuvenation, diagenetic overprinting and a fluid and Mg mass‐imbalance often makes it difficult to determine the dolomitization mechanism. This study considers differential dolomitization of the Eocene Thebes Formation on the Hammam Faraun Fault block, Gulf of Suez, which has undergone a simple history of burial and exhumation as a result of rifting. Stratabound dolostone bodies occur selectively within remobilized sediments (debrites and turbidites) in the lower Thebes Formation and extend into the footwall of, and for up to 2 km away from, the Hammam Faraun Fault. They are offset by the north–south trending Gebel fault, which was active during the earliest phases of rifting, suggesting that dolomitization occurred between rift initiation (26 Ma) and rift climax (15 Ma). Geochemical data suggest that dolomitization occurred from evaporated (ca 1·43 concentration) seawater at less than ca 80°C. Geothermal convection is interpreted to have occurred as seawater was drawn down surface‐breaching faults into the Nubian sandstone aquifer, convected and discharged into the lower Thebes Formation via the Hammam Faraun Fault. Assuming a ca 10 Myr window for dolomitization, a horizontal velocity of ca 0·7 m year−1 into the Thebes Formation is calculated, with fluid flux and reactivity likely to have been facilitated by fracturing. Although fluids were at least marginally hydrothermal, stratabound dolostone bodies do not contain saddle dolomite and there is no evidence of hydrobrecciation. This highlights how misleading dolostone textures can be as a proxy for the genesis and spatial distribution of such bodies in the subsurface. Overall, this study provides an excellent example of how fluid flux may occur during the earliest phases of rifting, and the importance of crustal‐scale faults on fluid flow from the onset of their growth. Furthermore, this article presents a mechanism for dolomitization from seawater that has none of the inherent mass balance problems of classical, conceptual models of hydrothermal dolomitization.

Fault‐controlled dolostone bodies have been described as potential hydrocarbon‐bearing reservoirs. Numerous case studies have described the shape and size of these often non fabric selective dolostone bodies within the vicinity of crustal‐scale lineaments, usually from Palaeozoic or Mesozoic carbonate platforms, which have undergone one or more phases of burial and exhumation. There has been little attention paid, however, to fault‐strike variability in dolostone distribution or the preferential localization of these bodies on particular faults. This study focuses on dolostone bodies adjacent to the Hammam Faraun Fault (HFF), Gulf of Suez. This crustal‐scale normal fault was activated in the Late Oligocene, coincident with the onset of extension within the Suez Rift. Dolomitization in the prerift Eocene Thebes Formation occurred in the immediate footwall of the HFF forming two massive, non facies selective dolostone bodies, ca. 500 m wide. Facies‐controlled tongues of dolostone on the margins of the massive dolostone bodies extend for up to 100 m. The geochemical signature of the dolostone bodies is consistent with replacement by Miocene seawater, contemporaneous with the rift climax and localization of strain along the HFF. A conceptual model of dolomitization from seawater that circulated within the HFF during the rift climax is presented. Seawater was either directly drawn down the HFF or circulated from the hanging wall basin via a permeable aquifer towards the HFF. The lateral extent of the massive dolostone bodies was controlled by pre‐existing HFF‐parallel fracture corridors on the outer margins of the damage zone of the fault. The behaviour of these fracture corridors alternated between acting as barriers to fluid flow before rupture and acting as flow conduits during or after rupture. Multiple phases of dolomitization and recrystallization during the ca. 10 Ma period in which dolomitization occurred led to mottled petrographical textures and wide‐ranging isotopic signatures. The localization of dolomitization on the HFF is interpreted to reflect its proximity to a rift accommodation zone which facilitated vertical fluid flow due to perturbed and enhanced stresses during fault interaction. It is possible that the presence of jogs along the strike of the fault further focused fluid flux. As such, it is suggested that the massive dolostones described in this study provide a window into the earliest stages of formation of fault‐controlled hydrothermal dolostone bodies, which could have occurred in other areas and subsequently been overprinted by more complex diagenetic and structural fabrics.