Origin, characteristics and distribution of fault-related and fracture-related dolomitization: Insights from Mississippian carbonates, Isle of Man

Visean limestones on the Isle of Man host numerous examples of fault-controlled and fracture-controlled dolomitization, which have been investigated to determine their macro-scale to micro-scale characteristics, geofluid origin, timing and relation to basin evolution. Geobodies composed of fabric destructive, ferroan, non-planar dolomite range from several centimetres to >300 m wide and tens to hundreds of metres long parallel to faults and/or fractures; they have sharply defined margins, cross-cut stratigraphy and locally finger out along beds or bed boundaries for tens of metres. Larger geobodies accompany NNE-SSW extensional faults with substantial breccia zones. One of these bodies hosts a sphalerite-rich breccia deposit cemented by dolomite. Saddle dolomite lines or fills vugs and fractures within dolomite geobodies, and is a minor late diagenetic phase in undolomitized limestones. Replacive dolomite has low matrix porosity owing to non-planar texture and associated cementation, and there is no evidence for subsequent leaching. Three dolomite stages are discriminated by texture, cathodoluminescence petrography and electron microscopy. Disseminated 'Dolomite 1' is substantially replaced and may be residual early diagenetic dolomite. Pervasive 'Dolomite 2' and 'Dolomite 3' have overlapping carbon-oxygen-strontium isotopic and fluid-inclusion characteristics that indicate precipitation from allochthonous, high-temperature (98 to 223 degrees C) and high-salinity (15 to 24 wt% NaCl eq.) brines. These variably equilibrated with host limestones and mixed with resident pore fluids. Overlying mudrocks formed a seal for ascending fluids. Integration of data from the mineral deposit suggests that fault-fracture systems tapped different deep-seated fluid reservoirs at different temperatures, and implies fluid interactions with both metamorphic basement and sedimentary cover in large-scale circulation systems. This phenomenon probably took place during Mesozoic rifting, although an earlier event at the end of the Early Carboniferous cannot be discounted. In either case, a transient heat flow anomaly, previously unrecognized in the Irish Sea region, may be required to account for the hottest fluids.