Fluid inclusion study of the unexposed Kentstown Granite, Co. Meath, Ireland

The presence of the Kentstown Granite, of inferred late Caledonian age, at a depth of 662.5 m in Co. Meath, Ireland, has recently been confirmed by a Tara Mines, Ltd., mineral exploration borehole. The borehole was collared in basinal Brigantian shales and passed downwards into a thick sequence of Viséan shallow-water carbonates that form the Milverton Group and which unconformably overlie sediments of the Courceyan Meath and Liscartan Formations. A ca 11 m thick section of interbedded clastic and carbonate sediments unconformably overlies the granite. A petrographic and microthermometric study of fluid inclusions in the Kentstown Granite, crosscutting quartz (V₁) and calcite+quartz (V₂) veins in the granite and calcite+quartz (V₂) veins in the overlying sediments was performed on samples taken from the borehole. Aqueous-carbonic fluids (type 1 and type 2), found only in granite quartz, were the earliest fluids to have circulated through the granite. The estimated trapping conditions for these earliest fluids are ca 300-330°C and ca 1.6 kbar. Later low-salinity (<9 equiv. wt% NaCl) fluids of moderate (ca 150-250°C) temperature of homogenization (T_H) and of probable meteoric origin occur as secondary inclusions in granite quartz and as primary and secondary inclusions in V₁ vein quartz (type 3). Moderate-salinity (ca 10-15 equiv. wt% NaCl), moderate-T_H (ca 110-200°C) fluids (type 4) that were trapped as secondary inclusions in granite and in V₁ vein quartz are of uncertain origin. Low-T_H (ca 50-150°C), high-salinity (ca 20-27 equiv. wt% NaCl) CaCl₂-rich brines (type 5) occur as primary inclusions in V₂ vein calcite and quartz and also as secondary inclusions in V₁ vein quartz and in granite quartz. Comparisons of the type 1, type 2 and type 3 fluid inclusion populations recorded in the present study with those from other Irish Caledonian granites strongly suggest a similar fluid evolutionary history. Furthermore, the low-temperature CaCl₂-rich brines (type 5) of this study, which must be syn- to post-Lower Carboniferous in age, have correlatives both in Irish Caledonian granites and in the Lower Carboniferous base-metal deposits of the Irish midlands.