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Origin of Fault-Fracture-Related Dolomitisation, Mississippian Limestones, Isle of Man, UK

Hendry, James; Gregg, Jay M.; Shelton, Kevin L.; Somerville, Ian; Crowley, Steve

Mississippian platform limestones in the SE of the Isle of Man contain numerous examples of fault- and fracture-related ferroan dolomite. Dolomite geobodies are several cm to > 300m wide, have sharp margins, and display both stratabound and crosscutting geometries. Larger examples are bounded by NNE-SSW faults with associated fault breccias, one of which contains a ZnS mineral deposit. Smaller dolomitisation haloes surround NW-SE fractures. Dolomitisation extends furthest from faults and fractures in grainy limestones. Resultant replacive dolomites have low porosity owing to nonplanar textures and saddle dolomite cement. Residual intercrystalline pores are filled by clay and pyrite or Fe-oxide. Vugs lined by saddle dolomite are present locally in dolomitised fault breccias.

In CL, replacive dolomite has planar to corroded, dull-moderate red cores (DOL 1) with very dark overgrowths (DOL 2). Saddle dolomite cement (DOL 3) is non-luminescent except for crystal terminations into void space, which have thin, bright to dull zones. DOL 2 and DOL 3 are locally dedolomitised to Fe-oxide and bright-dark CL-banded calcite. DOL 1 - DOL 2 have mottled textures in BSEM, but DOL 3 shows complex compositional zonation that varies between individual geobodies.

DOL 1 could not be isolated for geochemical analysis. DOL 2 and 3 encompass a wide range in δ18O (-5 to -16 ‰ VPDB) and 87Sr/86Sr (0.70864-0.71048), with elevated but diverse Th (98-223°C) and salinity (15-24 wt% NaCl eq.) values. In contrast, δ13C values are largely rock-buffered (-2 to +3‰ VPDB). Analyses of sphalerite and quartz from the dolomitised mineral deposit provide further geochemical constraints. Combined results are best explained by variable interaction between evolved marine waters in the host limestones and warmer fluids derived from underlying Lower Palaeozoic metasedimentary basement rocks. The basement strata also contain quartz vein-hosted Zn-Pb sulphide mineralisation with accompanying saddle dolomite cement.

Fluid flow and dolomitisation most likely took place during Variscan (or possibly later) reactivation of early Carboniferous normal faults. Individual fault-fracture systems tapped distinct basement fluid reservoirs at different temperatures, only one of which supplied significant Zn2+ and SiO2. Mudrocks overlying platform limestones acted as vertical seals and promoted lateral fluid migration away from faults, resulting in extensive dolomitised bodies.


AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013