Kevin L. Shelton Zoned saddle dolomite crystals Research
 

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Recognizing ore-related hydrothermal fluids outside of mineralized areas: A regional study of fluid inclusion microthermometry-halogen geochemistry of dolomites in the Irish Zn-Pb ore field

A.W. Johnsona,*, K.L. Sheltona, J.M. Greggb, I.D. Somervillec, W.R. Wrightc

a Department of Geological Sciences, University of Missouri, Columbia, MO 65211, USA
b Department of Geology and Geophysics, University of Missouri, Rolla, MO 65401, USA
c Department of Geology, University College Dublin, Belfield, Dublin 4, Ireland

Abstract

Previous studies of the dolomitization history of Viséan rocks of the Irish Midlands and Dublin Basin have focused on ore fluids in and near Zn-Pb ore zones. To provide a context within which to place studies of individual ore deposits/ore districts, we report results of a regional-scale study involving carbonate petrology, cathodoluminescence microscopy, fluid inclusion microthermometry and halogen geochemistry.

Fluid inclusion analyses indicate the presence of at least three chemically and thermally distinct fluids in dolomite cements: (1) a lower temperature, higher salinity fluid (Th = 72 to 136° C, 18.4-35.9 eq. wt. % NaCl); (2) a lower temperature, lower salinity fluid (Th = 103 to 136° C, 0.2 to 12.6 eq. wt. % NaCl); (3) a higher temperature, lower salinity fluid (Th = 169 to 271° C, 1.6 to 16.8 eq. wt. % NaCl). Fluid (1) occurs ubiquitously within the ore-hosting Waulsortian carbonate and in sub- and supra-Waulsortian units throughout the Irish Midlands, most commonly in white to pink saddle dolomite cement. Fluid (2) is also widespread geographically, but is found only in sub-Waulsortian and Waulsortian carbonates. Fluid (3) appears to be more restricted geographically and is found in the Waulsortian and supra-Waulsortian. Similar low temperature, higher salinity fluids (similar to fluid 1) have been reported from Zn-Pb deposits at Tynagh, Lisheen, Silvermines, and post-mineralization pink dolomite in the Rathdowney Trend, whereas warmer, less saline fluids (similar to fluids 2 and 3) have been reported in ore-stage carbonates and sphalerite at Tynagh, Lisheen, Silvermines, Gortdrum, Garrycam and Derrykearn.

The halogen geochemistry of all included fluids is consistent with modification of Lower Carboniferous seawater. Cl:Br ratios range from 92 to 354 and lie along the seawater evaporation/dilution trend, indicating fluid genesis from seawater evaporated beyond the point of halite precipitation. The lower temperature, higher salinity fluid (1) is enriched in chloride relative to bromide and the associated Cl:Br ratios are distinctly lower than those of less saline fluids (2 and 3). Enrichment in chloride relative to bromide reflects a component of salinity derived via dissolution of halite or from dehydration of seawater at elevated temperatures.

The data indicate the presence of multiple fluids during dolomitization and base-metal mineralization in the Irish Midlands. Lower salinity fluids (2 and 3) are similar to fluids identified in Zn-Pb ore deposits. The high salinity fluid (1) likely represents regional fluid flow that existed prior to and during the emplacement of Zn-Pb ores. Recognition of the distribution of ore-related fluids outside of mineralized zones has the potential to be a large-scale exploration tool and may be a guide to ore when coupled with other geochemical tools that identify conditions favorable to precipitating ore.

 

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Geological Column of Irish Midlands

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