Abstract: Hot Brines in a Thin Irish Basin: How and Where are the Fluids Heated and Mineralized?

H. Lewis, G. D. Couples

The very thin (Mississippian) Midland basin of central Ireland contains lead-zinc ores precipitated from hot fluids. These deposits formed near the paleosurface at about 250°C and are generally thought to be syndepositional or early diagenetic. Did the hot fluids originate in this basin, were they transported from the deeper Munster basin to the south, and/or was the basement involved in generating hot fluids?

Assessing the size and character of the fluid system requires geometric knowledge of both basin and basement during mineralization. Reconstruction

reveals a tilted fault-block structural style with lower Paleozoic rocks (acting as basement) overlain by partially folded, partially faulted Mississippian strata. 2-D coupled heat- and fluid-flow models suggest that topographically driven long-distance flow, either within the Midland basin, or from the Munster to the Midland basin, is insignificant. But topographically driven local flow from a (small) uplift and basement fluid circulation are both feasible. Basement circulation (convection) can bring hot (260+°C) fluids to near-surface, but within-basin fluids remain cool.

Combining these fluid flow models with mass-balance calculations of the metal mass that each system can transport, we conclude that local flow can provide neither enough metals nor sufficiently hot fluids. Basin-only flow can provide adequate metals, but not the required temperature. Basement flow can provide both the metals and adequately hot waters.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994