Micro-to regional scale study of low-T brittle-ductile deformation in serpentinites from the Southern Troodos Transform Fault Zone• Deformation involves a microstructural progression towards increasingly well-foliated shear zone rocks with increasing shear strain• Fluid flow and deformation result in progressive weakening and creeping behaviour with the potential to host local coseismic slip

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Oceanic transform faults have low seismic coupling, and display far fewer and smaller earthquakes than expected from fault length-magnitude scaling relations, based on the Harvard centroid moment tensor catalog (e.g., Bird et al., 2002). Instead, up to 95% of displacement occurs aseismically, despite the faults cross-cutting the brittle mafic crust (Boettcher & Jordan, 2004). This earthquake deficit has been explained in two ways: (a) oceanic transforms experience both earthquakes and aseismic creep along the same fault segments, but at different times (e.g., Abercrombie & Ekström, 2001;Hilley et al., 2020;McGuire et al., 1996); and/or (b) oceanic transforms are segmented into "locked patches" hosting quasi-periodic earthquakes of Mw > 6.0, and microseismically active "rupture barriers" dominated by creep (e.g.

Accumulations of sulfide minerals that are enriched in Ni-Cu-Co-(As) occur as sea floor massive sulfide (SMS) deposits associated with ultramafic rock types on the sea floor and in ophiolite terranes as Outokumpu-type mineralization. In this study we focus on similar mineralization at Lakxia tou Mavrou in the Limassol Forest Complex of Cyprus, which represents the on-land exposure of an oceanic transform fault zone preserved within the Troodos ophiolite. Mineralization here consists of massive lenses of pyrrhotite associated with veins of isocubanite, chalcopyrite, Co pentlandite, and chrome spinel hosted in serpentinized mantle peridotite. We reexamine the field context of mineral occurrences and use in situ mineral chemistry, element mapping, and sulfur isotope ratios (δ34S) to constrain metal sources and provide an updated paragenetic model for Lakxia tou Mavrou. Highly variable S/Se ratios (304–108,571), a depletion in platinum group elements relative to mantle values, and an average δ34S value of –3.7 ± 2.4‰ (1σ, n = 17) in sulfide minerals support a hybrid hydrothermal and magmatic origin for the mineralization. Metals at Lakxia tou Mavrou were sourced from both the serpentinization of peridotites and from crosscutting intrusions, with later intrusions into the already serpentinized mantle lithosphere host providing a heat source to drive prolonged hydrothermal circulation. The reexamination of the field context of mineralization shows that the Ni-Cu-Co-(As) mineralization at Lakxia tou Mavrou originally formed because of the fault-guided intrusion of hot primitive magma bodies into serpentinized shallow mantle lithosphere in the active domain of an ocean-floor transform fault zone. The mineralization was subsequently partially disrupted by structures related to emplacement of the Troodos ophiolite. We show that the relationship between serpentinization, magmatism, and hydrothermal circulation at Lakxia tou Mavrou can be used to understand the formation of ultramafic-hosted SMS deposits in transform fault and other ultramaficdominated slow- and ultraslow-spreading mid-ocean ridge settings.

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