Becker, Stephen P.¹, Zsolt R Nagy², Jay M Gregg², Aaron W Johnson³, Kevin L Shelton⁴, Ian D Somerville⁵
(1) Virginia Tech, Blacksburg, VA
(2) University of Missouri - Rolla, Rolla, MO
(3) University of Virginia's College at Wise, Wise, VA
(4) University of Missouri - Columbia, Columbia, MO
(5) University College - Dublin, Dublin, Ireland

ABSTRACT: A Local Source for Hypersaline Mineralizing Fluids in Southeast Ireland: Implications for Deep Circulation Along Extensional Faults in a Tectonically Complex Region

Source and timing of fluids associated with sulfide mineralization in Waulsortian (Courceyan to Chadian) strata of the tectonically complex southern Irish Midlands remains problematic. Previous studies of ore and gangue carbonates in the Rathdowney Trend Zn-Pb district indicate mineralizing fluids originated from seawater evaporated near and beyond the point of halite precipitation. Regional dolomitization patterns in these and overlying strata are consistent with refluxing hypersaline brines. In this study Supra-Waulsortian strata were examined from the margin of the Leinster Massif westward to the Rathdowney Trend. Individual specimens of late diagenetic cements were examined using cathodoluminescence, fluid inclusion microthermometry, halogen geochemistry, and carbon, oxygen and strontium isotope geochemistry. Cl/Br ratios of inclusion fluids in rocks indicate evaporated seawater as a likely fluid source. The presence of length-slow chalcedony and quartz and dolomite pseudomorphs, after gypsum and halite, in Supra-Waulsortian platform and ramp carbonates in this region indicate evaporitic conditions during sedimentation. This also suggests a source for the evaporated seawater fluid involved in mineralization and dolomitization. Fluid inclusion analysis and isotope data indicate multiple fluid sources and mixing of fluids. Covariation of ⁸⁷Sr/⁸⁶Sr with salinity of fluid inclusions indicates a radiogenic, highly saline brine endmember that existed in Supra-Waulsortian strata in both the Rathdowney Trend and adjacent to the Leinster Massif. If this fluid contributed to ore fluids, then mineralization is constrained to be post-Chadian. These data also are consistent with local thermal convection, with basement interaction of mineralizing fluids, and do not support a gravity driven cross-basin flow model.

AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.