Fluid Inclusion and Stable Isotope Studies of Carboniferous Limestone-Hosted Zn-Pb Mineralization In NW Ireland

Sulfide mineralization at the Abbeytown Zn-Pb deposit and minor mineralization at the nearby Twigspark/Trotter's Quarry (T/TQ) prospect is the only significant base metal mineralization known in NW Ireland. Metal sulfide mineralization is hosted in the Lower Carboniferous Ballyshannon Limestone and Dartry Limestone formations, west of the Proterozoic Ox Mountains metamorphic complex.

Petrographic, cathodoluminescence (CL), fluid inclusion microthermometric, and stable isotope analysis of epigenetic, void-filling carbonate cements and ore minerals indicate significantly different paragenetic histories for the two localities. Multiple sulfide events are recognized at the Abbeytown deposit, whereas a single, paragenetically late, mineralizing event is observed at T/TQ. CL microstratigraphies in dolomite can be correlated laterally and stratigraphically within each deposit, but no CL correlation between deposits is evident. Fluid inclusion analysis indicates a warmer, less saline (T_h 105 to 207° C, 4.5-15.2 eq. wt. % NaCl) fluid in Abbeytown dolomite cements compared to fluids in T/TQ dolomites (T_h 95 to 168° C, 5.4-20.0 eq. wt. % NaCl). Lateral variations in T_h are present at Abbeytown Mine, with values from the smaller, western ore body rarely exceeding 160°C. Sphalerite-hosted inclusions are cooler and more saline (T_h 109 to 168° C, 10.5-16.5 eq. wt. % NaCl) than those in associated dolomite.

Dolomite cements from Abbeytown display a trend of δ¹³C values from 3.6 to 2.1‰ and δ¹⁸O values of -5.4 to -11.9‰; those from T/TQ vary from δ¹³C values of 1.6 to 0.1‰ and δ¹⁸O values of -9.5 to -12.9‰. Calcites from the two localities display overlapping ranges of δ¹⁸O (-7.1 to -13.5‰) and δ¹³C (3.2 to 0.3‰).

The absence of CL correlation, coupled with differing fluid inclusion and stable isotope values in epigenetic carbonate cements, suggests the presence of two structurally isolated fluid flow systems that were responsible for ore mineralization in this part of NW Ireland.