The role of basement circulated fluids in the origin of sediment-hosted zinc-lead-barium mineralization in Ireland.

摘要

Ireland represents the ideal place to study the genesis of sediment-hosted ore deposits because the base metal deposits there exhibit little post mineralization deformation and they have not been metamorphosed. Furthermore, the timing of mineralization and its relationship to the tectonic evolution of the sedimentary basin containing the ore deposits is well constrained. To help determine the size and regional continuity of the mineralizing system in Ireland, the lead and sulfur isotope systematics of Mississippi Valley Type mineralization around the Kildare inlier in southeastern Ireland were compared to those of the syndiagenetic mineralization in central Ireland. These isotope studies show that the different styles of mineralization in Ireland accessed the same or similar metal and sulfur sources, suggesting a direct genetic link between all the styles. These results also indicate that the mineralizing system in Ireland effected over 20,000 km;The most important conclusion of the research presented in this thesis is that mixing of shallow and deep circulated fluids, that have had different fluid/rock interaction histories, on a regional scale (effecting more than 100,000 km;Chemical (fluid inclusion) and isotopic (sulfur, lead, strontium, oxygen and hydrogen) data from the base metal deposits in Ireland can be used to identify both shallow and deep circulating fluids in the mineralizing system there. The shallow fluid was a local basinal fluid, evaporated seawater and/or diagenetically modified seawater, rich in sulfur and poor in metals. The deep fluid, on the other hand, has unequivocally been shown to have circulated in and exchanged with the Caledonian basement graywackes. The deep fluid was metal-rich, and the ultimate origin of this fluid is unknown because it has isotopically equilibrated with the rocks in the basement. Using these fluid end members, an adiabatic fluid mixing model was constructed that explains the carbon and oxygen isotope systematics of host rock and vein carbonates from the base metal deposits. The temperatures and salinities of the mineralizing fluid predicted by the adiabatic fluid mixing model agree well with those measured in ore and gangue mineral fluid inclusions.