![]() Open spaces are partially to completely filled by saddle dolomite and less frequently by calcite cement. Trenton and Black River reservoirs in the southern Michigan Basin are composed of fault-associated, vertical dolomite bodies that are highly fractured and brecciated. However, fluid histories of these structures are only partially understood. 1999), the gray horizontal bar shows the approximate range of burial temperatures in the northeast Dublin Basin, and the red and blue horizontal bars show the possible temperature ranges for host limestone and host dolomite if they had formed or recrystallized in ambient seawater.ĭolomitized fault–fracture structures in the Trenton and Black River formations (TBR) are the type example for “hydrothermal” petroleum reservoirs world-wide. Moderatetemperature, low-to moderate-salinity fluids captured in zoned quartz of Loughshinny Formation veins at the Coppermine Fault are also strikingly similar to those from complex-CL quartz veins, sphalerite, and saddle dolomite in zinc-sulfide-mineralized Carboniferous breccia on the Isle of (Mii et al. 2002 Wilkinson and Earls 2000 Wilkinson 2003Wilkinson, 2010, and those in dolomite cements in Carboniferous strata throughout the Irish Midlands (Fluid Type 3 of Johnson et al. 2011), those from ore fluids associated with Zn-Pb sulfide mineralization in Carboniferous-hosted Irish ore systems (Banks et al. These resemble the hot fluids recorded in fault-related non-CL quartz veins on the Isle of Man (~125 km to the NE Shelton et al. NaCl), with highest temperatures from non-CL vein quartz in the Holmpatrick Formation. Requires both regionally extensive and localized fluid flow. These data suggest that dolomitization and mineralization of Mississippian rocks in the Midlands Typeġ fluids are enriched in chloride relative to bromide, reflecting a component of salinity derived via dissolution of halite The halogen geochemistry of included fluids indicates genesis from evaporated seawater. Similar fluids have been reported in ore-associated minerals and postmineralization dolomite (Type 1) and in ore-stage carbonatesĪnd sphalerite (Types 2 and 3). Waulsortian carbonates and 3) a higher temperature, lower salinity fluid found in Waulsortian and supra-Waulsortian rocks. Higher salinity fluid found in saddle dolomite 2) a low-temperature, lower salinity fluid limited to sub-Waulsortian and This study provides a regional framework within which studies of ore-related dolomite and dolomite cements may be placed.įluid inclusion data indicate the presence of three distinct fluids following early dolomitization: 1) a ubiquitous low-temperature,
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