Chemical mass transfer during hydrothermal alteration of carbonates: controls of seafloor subsidence, sedimentation and Zn-Pb mineralization in the Irish Carboniferous

Petrographic and geochemical data were acquired to evaluate models for the formation of hydrothermal dolomite breccias that host zinc–lead orebodies in the Rathdowney Trend of the Irish orefield and assess element mobility during hydrothermal alteration of carbonates. Macroscopic and microscopic textures indicate that the breccias largely formed by a pseudobrecciation process involving dissolution and replacement of precursor Waulsortian dolomite of diagenetic origin during subseafloor hydrothermal fluid flow. Geochemical analyses show that, compared with the Waulsortian dolomite precursor, breccia “clasts” and “matrix” contain respectively higher concentrations of Al, Ti and light rare earth elements which are linearly correlated, indicative of mass loss during hydrothermal interaction. Bulk rock mass loss estimates average 33% indicating extensive dissolution of carbonate although petrographic evidence and limited clast rotation or translation rule out the development of significant open space. This supports a mechanism of alteration whereby hydrothermal fluids, migrating along grain boundaries, microcracks and solution seams, drove dissolution and replacement of the surrounding host rock with partial collapse of the transient microporosity produced. Progressive interaction led to the gradual isolation of domains (“clasts”) surrounded by replacement dolomite (“matrix”). Massive sulfide mineralization formed subsequently by stratabound replacement of the dolomite pseudobreccia by hydrothermal fluids preferentially focused within the more permeable and possibly more reactive lithology. This process involved further volume loss (30% of the replaced rock). Quantitative mass transfer analysis indicates that during hydrothermal interaction, net loss of Na, P, Mn, Sr and sometimes Cu, and net gain in Li, K, Fe as well as the ore-associated elements typically occurred. Marked fractionation of the REE is observed: Eu was added, implicating elevated temperature, acidic and reduced hydrothermal fluids; Ce was also added, possibly derived from remobilization of pre-sulfide oxide mineralization; the LREE were typically immobile or partially removed; and the HREE were preferentially mobilized and transported beyond the scale of sampling. Thus, the HREE, together with Na, P, Mn, Sr and possibly Cu, may form a distal chemical anomaly at the base of the Waulsortian Limestone Formation. The net addition of Li, K and Be in dolomite breccia matrix samples, combined with their increase in concentration due to dolomite dissolution, produced a broad anomaly of these elements extending for at least 1.5 km from the center of the Lisheen deposit. These various geochemical anomalies have potential utility in mineral exploration. A total volume loss of up to 68% at Lisheen is consistent with a marked thinning of the Waulsortian biomicrite facies above massive sulfide. The observed thickening of upper and supra-Waulsortian facies in the trough developed above this zone is interpreted to reflect infill of a seafloor depression up to ~ 90 m deep, developed in response to underlying hydrothermal dissolution. This model implies mineralization initiated during the latter stages of Waulsortian deposition in the late Courceyan/early Chadian at a depth of approximately 150 m below the seafloor.