FROM REGIONAL TO MICRO-SCALE: KEYS TO INTERPRETING THE GENESIS OF THE IRISH LOWER CARBONIFEROUS DOLOMITE AND ZN-PB MINERALIZATION

Zn-Pb mineralization hosted by Irish Lower Carboniferous carbonate rocks has physical and temporal associations with multiple dolomitization events. Four main dolomitization events are documented on the regional to micro-scale as occurring during the Dinantian Subsystem in Ireland. Three of these events can be constrained paragentically, to be prior to main sulfide mineralization.

Replacement planar dolomite formation is the earliest event and is attributed to slightly modified (elevated ⁸⁷Sr/⁸⁶Sr ratio) 50^oC to 70^oC seawater interacting with host limestones. These low temperatures are consistent with early diagenetic dolomite forming in a shallow burial environment.

The second dolomitization event produced nonplanar dolomite ranging from common open-space-filling cements to rarer replacement morphologies. This dolomite is geochemically (T_h, T_m, d¹⁸O, Sr and ⁸⁷Sr/⁸⁶Sr ratio) variable. Halogen data indicate that all the dolomites formed from evaporated seawater beyond halite precipitation with halite dissolution supplying additional limited salinity. One nonplanar dolomite generation (Pk2) can be correlated regionally and extends stratigraphically from Courceyan up to Early Asbian units. Modelling of the nonplanar dolomite (including Pk2) precipitation appears most compatible with fluid mixing.

The third dolomitization event is a modification of the early planar dolomite. The higher temperature (up to 270^oC) nonplanar dolomitizing fluids induced neomorphism in the planar dolomites during which they re-equilibrated their d¹⁸O values.

For the genesis of the Lower Carboniferous dolomite and associated sulfides, we invoke a hybrid multiple-component fluid-flow model. This model originates with low temperature seawater planar dolomitization that was a critical ground preparation event producing intercrystal porosity allowing for later hydrothermal fluid-flow. Regional and localized hydrothermal fluid mixing and flow followed planar dolomitization and produced the sulfides and associated nonplanar dolomite.

The mixed nature of the fluids required for dolomitization and mineralization precludes genesis by a single primary fluid. The requirements of the model are met only if the Lower Carboniferous mineralization took place at a younger age (c. 330 Ma) than previously proposed.