Browsing by Author "Boyce, Adrian"

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    Basin scale evolution of zebra textures in fault-controlled, hydrothermal dolomite bodies: insights from the Western Canadian Sedimentary Basin
    (2023) McCormick, Cole A.; Corlett, Hilary; Clog, Matthieu; Boyce, Adrian; Tartèse, Romain; Steele-MacInnis, Matthew; Hollis, Cathy
    Structurally controlled dolomitization typically involves the interaction of high-pressure (P), high-temperature (T) fluids with the surrounding host rock. Such reactions are often accompanied by cementation and recrystallization, with the resulting hydrothermal dolomite (HTD) bodies including several ‘diagnostic’ rock textures. Zebra textures, associated with boxwork textures and dolomite breccias, are widely considered to reflect these elevated P/T conditions. Although a range of conceptual models have been proposed to explain the genesis of these rock textures, the processes that control their spatial and temporal evolution are still poorly understood. Through the detailed petrographical and geochemical analysis of HTD bodies, hosted in the Middle Cambrian strata in the Western Canadian Sedimentary Basin, this study demonstrates that a single genetic model cannot be applied to all the characteristics of these rock textures. Instead, a wide array of sedimentological, tectonic and metasomatic processes contribute to their formation; each of which is spatially and temporally variable at the basin scale. Distal to the fluid source, dolomitization is largely stratabound, comprising replacement dolomite, bedding-parallel zebra textures and rare dolomite breccias (non-stratabound, located only proximal to faults). Dolomitization is increasingly non-stratabound with proximity to the fluid source, comprising bedding-inclined zebra textures, boxwork textures and dolomite breccias that have been affected by recrystallization. Petrographical and geochemical evidence suggests that these rock textures were initiated due to dilatational fracturing, brecciation and precipitation of saddle dolomite as a cement, but significant recrystallization occurred during the later stages of dolomitization. These rock textures are closely associated with faults and carbonate-hosted ore deposits (e.g. magnesite, rare earth element and Mississippi Valley–type mineralization), thus providing invaluable information regarding fluid flux and carbonate metasomatism under elevated P/T conditions.
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    Controls on the formation of stratabound dolostone bodies, Hammam Faraun Fault block, Gulf of Suez
    (2018) Hirani, Jesal; Bastesen, Eivind; Boyce, Adrian; Corlett, Hilary; Gawthorpe, Rob; Hollis, Cathy; Cédric, John M.; Robertson, Hamish; Rotevatn, Atle; Whitaker, Fiona
    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 degrees 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 (super -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.
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    Evaluating new fault-controlled hydrothermal dolomitization models: insights from the Cambrian Dolomite, Western Canadian Sedimentary Basin
    (2020) Koeshidayatullah, Ardiansyah; Corlett, Hilary; Stacey, Jack; Swart, Peter K.; Boyce, Adrian; Robertson, Hamish; Whitaker, Fiona; Hollis, Cathy
    Fault-controlled hydrothermal dolomitization in tectonically complex basins can occur at any depth and from different fluid compositions, including ‘deep-seated’, ‘crustal’ or ‘basinal’ brines. Nevertheless, many studies have failed to identify the actual source of these fluids, resulting in a gap in our knowledge on the likely source of magnesium of hydrothermal dolomitization. With development of new concepts in hydrothermal dolomitization, the study aims in particular to test the hypothesis that dolomitizing fluids were sourced from either seawater, ultramafic carbonation or a mixture between the two by utilizing the Cambrian Mount Whyte Formation as an example. Here, the large-scale dolostone bodies are fabric-destructive with a range of crystal fabrics, including euhedral replacement (RD1) and anhedral replacement (RD2). Since dolomite is cross-cut by low amplitude stylolites, dolomitization is interpreted to have occurred shortly after deposition, at a very shallow depth (<1 km). At this time, there would have been sufficient porosity in the mudstones for extensive dolomitization to occur, and the necessary high heat flows and faulting associated with Cambrian rifting to transfer hot brines into the near surface. While the δ18Owater and 87Sr/86Sr ratios values of RD1 are comparable with Cambrian seawater, RD2 shows higher values in both parameters. Therefore, although aspects of the fluid geochemistry are consistent with dolomitization from seawater, very high fluid temperature and salinity could be suggestive of mixing with another, hydrothermal fluid. The very hot temperature, positive Eu anomaly, enriched metal concentrations, and cogenetic relation with quartz could indicate that hot brines were at least partially sourced from ultramafic rocks, potentially as a result of interaction between the underlying Proterozoic serpentinites and CO2-rich fluids. This study highlights that large-scale hydrothermal dolostone bodies can form at shallow burial depths via mixing during fluid pulses, providing a potential explanation for the mass balance problem often associated with their genesis.
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    Fault-controlled dolomitization in a rift basin
    (2017) Hollis, Cathy; Bastesen, Eivind; Boyce, Adrian; Corlett, Hilary; Gawthorpe, Rob; Hirani, Jesal; Rotevatn, Atle; Whitaker, Fiona
    There are numerous examples of fault-controlled, so-called hydrothermal dolomite (HTD), many of which host economic mineral deposits or hydrocarbons, but there remains a lack of consensus as to how they form. In particular, multiple phases of diagenetic overprinting can obscure geochemical fingerprints. Study of a Cenozoic succession with a relatively simple burial history here provides new insights into the development of differentially dolomitized beds. The Hammam Faraun fault (HFF) block within the Suez Rift, Egypt, hosts both massive and stratabound dolostone bodies. Non-fabric-selective massive dolostone is limited to the damage zone of the fault, while fabric-selective stratabound dolostone bodies penetrate nearly 2 km into the footwall. Oligo-Miocene seawater is interpreted to have been drawn down discrete faults into a deep aquifer and convected upwards along the HFF. Escape of fluids from the incipient HFF into the lower Thebes Formation led to differential, stratabound dolomitization. Once the HFF breached the surface, fluid circulation focused along the fault plane to form younger, massive dolostone bodies. This study provides a snapshot of dolomitization during the earliest phases of extension, unobscured by subsequent recrystallization and geochemical modification. Contrary to many models, stratabound dolomitization preceded non-stratabound dolomitization. Fluids were hydrothermal, but with little evidence for rapid cooling and brecciation common to many HTD bodies. These results suggest that many of the features used to interpret and predict the geometry of HTD in the subsurface form during later phases of structural deformation, perhaps overprinting less structurally complex dolomite bodies.
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    Origin and evolution of fault-controlled hydrothermal dolomitization fronts: a new insight
    (2020) Koeshidayatullah, Ardiansyah; Corlett, Hilary; Stacey, Jack; Swart, Peter K.; Boyce, Adrian; Hollis, Cathy
    Dolomitization is one of the most significant diagenetic reactions in carbonate systems, occurring where limestone (CaCO3) is replaced by dolomite (CaMg (CO3)2) under a wide range of crystallization temperatures and fluids. The processes governing its formation have been well studied, but the controls on the position of dolomitization fronts in ancient natural settings, particularly in a fault-controlled hydrothermal system (HTD), have received remarkably little attention. Hence, the origin and evolution of HTD dolomitization fronts in the stratigraphic record remain enigmatic. Here, a new set of mineralogical and geochemical data collected from different transects in a partially dolomitized Cambrian carbonate platform in western Canada are presented to address this issue. Systematic patterns of sudden decrease in the magnesium content (mol% MgCO3) and increase in porosity were observed towards the margin of the body. Furthermore, fluid temperatures are cooler and Owater values are less positive at the dolomitization front than within the core of the body. These changes coincide with a change from poorly ordered, planar-e dolomite with multiple crystal zonations at the margin, to an unzoned, well-ordered, interlocking mosaic of planar-s to nonplanar dolomite in the core of the body. These phenomena are hypothesized to reflect dynamic, self-limiting processes in the formation and evolution of HTD dolomitization fronts through (i) plummet of dolomitization potential at the head of dolomitizing fluids due to progressive consumption of magnesium and fluid cooling; and (ii) retreat of dolomitization fronts towards the fluid source during subsequent recrystallization of the dolomite body, inboard of the termination, once overdolomitization took place. This new insight illustrates how dolomitization fronts can record the oldest phase of dolomitization, instead of the youngest as is often assumed. Formation of porosity is interpreted to occur as the result of acidification-induced grain leaching during the development of dolomitization fronts. This mechanism, coupled with retrogradation of dolomitization fronts, may help to explain the apparent enhancement of porosity in proximity to dolomitization fronts.
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    Structural controls on non fabric‐selective dolomitization within rift‐related basin‐bounding normal fault systems: Insights from the Hammam Faraun Fault, Gulf of Suez, Egypt
    (2018) Hirani, Jesal; Bastesen, Eivind; Boyce, Adrian; Corlett, Hilary; Eker, Anja; Gawthorpe, Rob; Hollis, Cathy; Korneva, Irina; Rotevatn, Atle
    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.