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

Abstract

Viséan 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°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.

The control of basin evolution on patterns of sedimentation and diagenesis: an example from the Mississippian Great Orme, North Wales

“…This is consistent with the proposed fluid source for other copper deposits in northern England, at Alderley Edge in Cheshire and Ecton in Derbyshire, where mineralization is hosted by major faults and adjacent permeable horizons within the Carboniferous and Triassic hostrock (Holmes et al 1983). Fluid circulation and mineralization during the Mesozoic has been proposed by Hendry et al (2014) for the Isle of Man and north East Irish Sea Basin, whereby the extensional tectonic regime provided favourable structural orientations for fluid flow. Rifting was probably accompanied by elevated heat flow, and thick evaporitic strata deposited in the Visean and/or the PermoTriassic supplied hypersaline brines that also transported magnesium (e.g.…”

“…Hendry et al (2014) discussed burial history in the Manx region, citing 800 ± 300 m of post-Carboniferous exhumation on the Isle of Man and eastern margin of the East Irish Sea Basin, based upon data published by Green et al (2012). However, the northern margin of the North Wales Platform remained a relative topographic high throughout the Carboniferous (Williams & Eaton 1993) and burial depths for the uppermost Dinantian succession did not reach more than 1-1.5 km (Al-Fadel 1983).…”

“…Stable isotope data for wholerock limestone, replacive dolomite and pore-filling calcite and dolomite cements within the Pier Dolomite and Great Orme Limestone Formation, Great Orme. Visean primary marine calcite from Bruckschen et al (1999) and Hendry et al (2014). permeability bed during dolomitization. Owing to the replacive nature of D0-D2 it is not possible to determine the relationship with SC1-SC3 and earlier phases of cementation.…”

“…Complementary studies focusing on the mineralization of northern England and Wales were carried out by Pattrick & Russell (1989) and Ixer & Vaughan (1993). Similarly, Wilkinson (2003Wilkinson ( , 2010, Shelton et al (2011) and Hendry et al (2014) have detailed the paragenesis of age-equivalent successions on the Isle of Man and in southern Ireland (Johnson et al 2009). …”

87Sr/86Sr and trace element mapping of geosphere-hydrosphere-biosphere interactions: A case study in Ireland

“…This value reflects Visean seawater composition which is inferred from the analysis of well-preserved marine fossils (0.7075-0.7082) (Burke et al, 1982;McArthur et al, 2001). Similarly, Visean limestones on the Isle of Man yielded 87 Sr/ 86 Sr of 0.7079 (n=2) (Hendry et al, 2015). The mean of all limestoneassociated biosphere values in this study, 0.7089 ± 0.011 (n=11), is higher than these published whole rock values, indicating an additional more radiogenic source.…”