Formation mechanisms of cockade texture in breccias and their relation to similar hydrothermal mineral textures

Tosun, Levent 2024. Formation mechanisms of cockade texture in breccias and their relation to similar hydrothermal mineral textures. PhD Thesis, Cardiff University. Item availability restricted.

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Abstract

Cockade texture refers to a fault-infill breccia texture, where individual core clasts are surrounded by concentric layers of cement and/or fine-grained sediment infill, with the central clasts remaining separated from one another. Cockade textures are commonly found in fault zones, where fracturing and subsequent fluid flow create open spaces for their development. Understanding their formation mechanisms is essential for improving interpretations of fault-controlled hydrothermal systems and their associated mineralisation processes. Cockade textures have been widely used to infer sequential geological events, including mineralisation processes in vein-type deposits and, more recently, seismic cycles in paleoearthquake studies. However, their use in interpreting temporal sequences of events depends on the assumed formation mechanisms. Models such as suspension in fluid and rotation-accretion propose that cockade textures develop under dynamic fluid conditions driven by episodic deformation. In contrast, the force of crystallisation model suggests that cockade textures can form in static fluid environments without requiring seismic activity, implying that they do not necessarily record deformation-driven geological events. Additionally, partial metasomatic alteration, involving recrystallisation and replacement, plays a key role in modifying their final structure, further complicating their interpretation in sequential geological event reconstructions. These uncertainties highlight the need for a comprehensive evaluation of cockade formation mechanisms before using them to infer deformation history or mineralisation processes. This study investigates the formation of cockade textures in cement-dominated breccias, addressing two primary objectives: (1) to examine how well existing theories explain why the central fragments of cockades remain separated, considering the assumptions and identification criteria of proposed mechanisms, and (2) to assess the reliability of cockade textures as proxies for interpreting temporal sequences of events, such as syntectonic mineral precipitation in vein-type deposits and seismic cycles in paleoearthquake studies. The research incorporates findings from two contrasting geological settings: Oxwich in Gower (South Wales), a carbonate-dominated system, and the Gicik low-sulphidation epithermal Au deposit in Ankara (North-Central Anatolia, Turkey), a silica-dominated system. These sites provide a valuable framework for evaluating the interplay of different formation mechanisms under varying lithological and fluid conditions, offering a more comprehensive understanding of cockade texture development. To test the validity of these formation models, this study integrates field observations, microscopic analyses, fractal dimension analysis, fluid inclusion microthermometry, and stable isotope geochemistry. Field and petrographic evidence does not support the suspension in fluid mechanism, as the conditions required for sustained clast movement in rapidly ascending hydrothermal fluids were absent at both localities. The rotation-accretion model is partially supported by local truncations on the rims, suggesting episodic movement; however, no conclusive evidence indicates that the central fragments underwent significant rotation or shearing. In Oxwich, geopetal structures within central fragments confirm that they remained in their original orientations, while in Gicik, no evidence suggests relative rotation between the clasts. The role of partial metasomatic alteration is evident at both sites, with recrystallisation and replacement textures observed in carbonate-dominated Oxwich veins and silica-dominated Gicik veins. These findings indicate that central fragments, which remained unrotated, were progressively enclosed by successive rim layers. In Oxwich, the well-defined overgrowth faces of these rims suggest that fracture opening and rim precipitation occurred gradually, with mineral growth sustained by a solution film consistently present between rim crystals. This implies that the force of crystallisation mechanism was capable of separating the central fragments without requiring repeated hydrofracturing associated with seismicity following initial faulting. At Gicik, an alternative explanation for the separation of clasts, which can result in cockade textures in breccias within low-sulphidation epithermal systems, is the infiltration and subsequent precipitation of highly viscous silica gel-like hydrothermal solutions into a fracture system. Quantitative fractal dimension analysis reveals a progressive decrease in complexity from the central fragments to the outermost rims, supporting the hypothesis that cockade textures developed through open-space precipitation. In Oxwich, fluid inclusion microthermometry and stable isotope analyses provide further constraints on the palaeo-fluid conditions responsible for cockade formation. The homogenisation temperatures (155°C–285°C) from carbonate rims are consistent with previously documented cockade textures, while stable carbon and oxygen isotope values indicate a marine carbonate source with no evidence of magmatic fluid mixing. However, pervasive recrystallisation likely altered the isotopic composition, suggesting that the measured geochemistry reflects post-depositional processes rather than primary fluid signatures. Additionally, the similarity between cockade textures and oolites, where both central fragments and ooids remain separated by cement growth into open space, further supports the force of crystallisation mechanism as a viable explanation for fragment separation without requiring seismicity. These findings underscore the complexity of cockade texture formation, emphasizing that no single mechanism can fully explain their development. While suspension in fluid and rotation-accretion models are widely accepted and often used as frameworks for interpretation, strictly adhering to their underlying assumptions may lead to overgeneralised or misleading conclusions. Cockade textures can form through various interacting processes, including force of crystallisation, open-space precipitation, and secondary alterations such as recrystallisation and replacement, all of which may contribute to their final structure. Recognizing the interplay of these mechanisms is essential for accurate geological interpretations. Future research should focus on quantifying the relative contributions of these mechanisms under varying geological conditions to refine the use of cockade textures as indicators of deformation history and hydrothermal activity.

Item Type:	Thesis (PhD)
Date Type:	Completion
Status:	Unpublished
Schools:	Schools > Earth and Environmental Sciences
Date of First Compliant Deposit:	28 March 2025
Last Modified:	28 Mar 2025 14:17
URI:	https://orca.cardiff.ac.uk/id/eprint/177237

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