IDENTIFICATION OF HYDROTHERMAL FLUID FLOW CONDUITS IN CARBONATES USING RARE EARTH ELEMENTS

The Galmoy deposit, Ireland, is a carbonate-hosted Zn-Pb deposit of Carboniferous age. Sulfide mineralization replaces and is enveloped by dolomite breccia located near the base of the Waulsortian Limestone Formation. The breccias comprise angular to subrounded clasts of dolomitized limestone cemented by a matrix of black dolomite of hydrothermal origin. Breccia samples were collected from an E-W transect across the CW orebody at Galmoy, orthogonal to a major NNW-SSE-trending fissure, believed to be an import conduit for hydrothermal fluids. Reference samples of Waulsortian limestone and dolomitized limestone were also collected.

Rare earth element (REE) analyses of both breccia clasts and matrix indicate substantial enrichment relative to the unmineralized reference samples. Mass balance calculations based on immobile trace elements suggest mass losses of up to ~80% in samples associated with sulfide mineralization. Therefore, REE enrichments of the order of five times unaltered Waulsortian limestone and pre-ore dolomite values might be expected. In fact, REE concentrations are typically only three times those of the reference samples, suggesting that net removal of the REE, and in particular La and the HREE, has occurred.

The zones of maximum mass and REE loss coincide with the location of the fissure, and both approach reference sample values towards the periphery of the orebody. A distinct Eu anomaly (Eu/Eu*) occurs directly above the fissure suggesting that Eu2+ remains relatively immobile during alteration. The results indicate that the REE, and in particular Eu, provide a sensitive discriminator for proximity to hydrothermal fluid conduits in carbonates.