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2002 Denver Annual Meeting (October 27-30, 2002)

Paper No. 14
Presentation Time: 1:30 PM-5:30 PM

RELATIONSHIP OF EVAPORITES TO BRINE MIGRATION AND CARBONATE HOSTED ZN-PB MINERALIZATION IN THE LOWER CARBONIFEROUS OF SOUTHEASTERN IRELAND


BECKER, Stephen P.1, NAGY, Zsolt R.1, JOHNSON, Aaron W.2, GREGG, Jay M.1, SHELTON, Kevin L.2, SOMERVILLE, Ian D.3 and WRIGHT, Wayne R.4, (1)Department of Geology & Geophysics, Univ of Missouri-Rolla, 125 McNutt Hall, Rolla, MO 65409, (2)Department of Geological Sciences, Univ of Missouri-Columbia, Columbia, MO 65211, (3)Department of Geology, Univ College Dublin, Belfield, Dublin, 4, Ireland, (4)Robertson Research International, Tyn-y-coed Site, Llanrhos, Llandudno, North Wales, LL30 1SA, United Kingdom, sbecker@umr.edu

Previous fluid inclusion studies of ore minerals and carbonate cements in Waulsortian (Lower Carboniferous) strata in the Rathdowney Trend Zn-Pb district of Ireland indicated the presence of fluids originating from seawater evaporated near and beyond the point of halite precipitation. The d13C and d18O values of micrite (0.4 to 1.6‰; -2.9 to -2.2‰ PDB, respectively) and planar-s dolomite (-0.4 to 3.0‰; -0.2 to -2.6‰, respectively) in peritidal Supra-Waulsortian carbonates near the Leinster Massif indicate the presence of evaporative fluids during early diagenesis. Cl/Br ratios of inclusion fluids in these carbonates also indicate extreme evaporative conditions. These rocks also contain nodules of length slow chalcedony and dolomite pseudomorphs after gypsum. Breccias, likely from dissolution of evaporites, are common in these strata. Near and within the Rathdowney Trend, planar dolomite replacing platform carbonates has d13C and d18O values of 3.3 to 4.5‰ and -6.1 to -7.1‰, respectively, indicating precipitation from slightly modified Carboniferous seawater as opposed to highly evaporated seawater. These rocks display less lithological evidence of evaporative conditions.

 

The d18O values of late diagenetic dolomite cements near the Leinster Massif range between two end-member populations: one from -6 to -10‰, and a second from -12 to -17‰. They may represent deposition from 1) multiple fluids derived from separate sources or 2) a single fluid whose isotopic composition evolved as a function of temperature and/or water-rock interaction. The lower d18O cements may represent a previously unrecognized incursion of meteoric fluids that moved through the evaporite-bearing section. If so, the presence of evaporites in the Supra-Waulsortian strata near the Leinster Massif may have provided an additional source of chloride, thus aiding in the development of suitable brine chemistries for Zn-Pb ore transport.

Session No. 152--Booth# 14

Economic Geology (Posters)
Tuesday, 29 October 2002: 1:30 PM-5:30 PM
Geological Society of America Abstracts with Programs. Vol. 34, No. 6, September 2002, p.338
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