REPLACEMENT ORIGIN OF CRINKILL IRONSTONE - IMPLICATIONS FOR GENETIC MODELS OF BASE-METAL MINERALIZATION, CENTRAL IRELAND

Iron-oxide minerals occur in many Irish Carboniferous-Mississippian ba se metal sulfide deposits. A striking example of banded ironstone (iro n oxide-silica) occurs in the Crinkill deposit, central Ireland, where it is spatially associated with base metal mineralization. The iron-o xide mineralization at Crinkill and at other localities provides evide nce of the nature and timing of Zn-Pb mineralization. Previous models have suggested that the ''ironstone'' formed either as non-hydrotherma l precipitates in small closed basins, or as a direct result of hydrot hermal exhalations onto the lower Carboniferous seafloor. Petrographic evidence from Crinkill (and from Tynagh) clearly shows that the iron mineralization is later than the cementation of the host limestones an d that much of the bioclast material is in fact replaced by hematite. Cross-cutting hematite veins and veinlets are also known. There is no biotic change as would be expected in a sub-toxic hydrothermal exhalat ive environment. Given that the ironstone mineralization is spatially associated with Zn-Pb+/-Cu mineralization at Crinkill and other Irish base metal deposits and that the iron-oxide mineralization is replaciv e and epigenetic, it opens up stratigraphically larger targets with mi neralization not necessarily confined to the lower Carboniferous. As s uch, the occurrence of iron oxide-silica precipitates may prove to be a useful exploration guide. Copyright (C) 1996 Canadian Institute of M ining, Metallurgy and Petroleum.