Abstract Times of metal-rich brine discharge into ancient ocean basins, associated with the formation of sedimentary-exhalative (sedex) Zn-Pb-Ba ore deposits, coincided with short-duration positive excursions (“spikes”) in the global marine Sr isotope record. While these spikes are unexplained by conventional oceanic models, chronostratigraphic correlations, combined with mass balance evidence and oceanographic modeling, suggest that the flux of radiogenic Sr from sedex brines during ore formation is sufficient to explain these previously enigmatic 87 Sr/ 86 Sr spikes. We review existing 87 Sr/ 86 Sr data and present new data as verification of these global 87 Sr/ 86 Sr spikes and their correlations with the formation of giant sedex ore deposits. Major events include an 1 ×10 −4 (~0.7078–~0.7079) excursion contemporaneous with formation of the Rammelsberg deposit at ~389 Ma; spikes on the order of 1 to 3 × 10 −4 , coeval with formation of the Meggen deposit at ~381 Ma, several ore deposits in the Macmillan Pass district at ~379 to 375 Ma, and the Silvermines deposits at ~352 Ma; and two >6 × 10 −4 spikes coincident with formation of the giant Navan deposit at ~346 Ma and Red Dog deposits at ~337 Ma. Moreover, the timing of peak 87 Sr/ 86 Sr spikes correlates with global δ 13 C and δ 18 O spikes, deposition of metal-rich black shales and ironstones, metal-induced malformation (teratology) of marine organisms, and mass extinctions. The relationships among these features were poorly understood, but our new model explains how the flux of key biolimiting nutrients and metals contained in sedex brines, demonstrably equivalent to or exceeding that of the total modern riverine flux to the ocean, spurred ocean eutrophication, which, ultimately, through a series of positive feedback mechanisms, may have triggered global chemical and biological events. If, as we hypothesize, sedex hydrothermal systems are recorded in the global marine isotopic, geologic, and biological records, our findings define a new approach to the study of and exploration for sedex deposits. We demonstrate that fluid inclusion solute chemistry and isotopic and stratigraphic studies of sedex deposits, coupled with chronostratigraphic correlation and high-resolution 87 Sr/ 86 Sr isotope chemostratigraphy, can be used to answer long-standing questions about geologic processes responsible for formation of these extraordinary deposits. This approach provides evidence for the age, duration, and fluxes of fluids and metals vented into the ocean by these giant hydrothermal systems. Accordingly, the marine 87 Sr/ 86 Sr curve constitutes a global exploration tool that could be applied to assess the mineral potential of sedimentary basins. To illustrate the potential of this tool to identify favorable stratigraphic ages and basins with potential for undiscovered giant sedex deposits, we highlight several spikes, on par with those characteristic of the Red Dog and Navan deposits, which have not been correlated with known metal deposits. Given these strong temporal correlations, mass balance estimates, and results of ocean chemistry modeling, our study suggests that further work is warranted to determine the extent to which periodic venting of hydrothermal basinal brines into the ocean has influenced the evolution of marine chemistry. Ultimately, these global signatures can be applied to the study of and exploration for sedex deposits.

Abstract The Irish Midlands host one of the world’s major zinc orefields. The Irish zinc deposits occur in a transgressive sequence of Lower Carboniferous marine carbonate rocks lying above a wedge of Upper Devonian continental red beds. The deposits have enough shared characteristics, as well as differences from other carbonate-hosted zinc-lead deposits worldwide, to have been given the sobriquet “Irish-type.” The Irish deposits share the following features: (1) They occur preferentially in the stratigraphically lowest, non-argillaceous carbonate unit. (2) They occur along, or immediately adjacent to, normal faults which formed conduits for ascending hydrothermal fluids. (3) Sphalerite and galena are the principal sulfides. Iron sulfides occur in variable amounts; some deposits are dominated by iron sulfides while others contain very minor amounts. Barite is present in all the deposits, ranging from a dominant phase to a minor constituent. Many deposits contain minor tennantite, chalcopyrite, and/or Pb-Cu-Ag-As sulfosalt minerals. (4) They are stratabound and many display large-scale stratiform morphologies. (5) They display complex sulfide textures ranging from replacement of host rock by fine-grained, anhedral and colloform sulfides to infill of solution cavities by fine-grained, colloform and medium- to coarse-grained crystalline sulfides. Layered sulfide textures, other than colloform banding, are restricted to geopetal cavity fillings. (6) They formed from the mixing of metal-bearing, moderately saline, slightly acidic, relatively sulfur-poor fluids with relatively sulfur-rich fluids that appear to have been derived from Carboniferous seawater. The Irish orefield is regionally zoned. Copper and silver are most common in deposits located within the southern portion of the country. Pre-mineralization dolomitization is also largely restricted to southern deposits. The age of mineralization is known with certainty only for the Navan deposit which formed several million years after deposition of its host sediments; geologic relationships suggest that the other Irish deposits formed at approximately the same time as the Navan deposit. This period is marked in the Irish Midlands by the establishment of a complex facies mosaic consisting of fault-controlled carbonate basins and high-standing platforms indicating an extensional tectonic environment. Extension was relatively modest and was related to continental collision (the Hercynian Orogeny) occurring to the south of Ireland. The apparent contemporaneity of mineralization and tectonism, together with the regional zoning of metals and dolomitization, suggests that the Hercynian Orogeny was a fundamental driving force for mineralization in the Irish orefield. Topography-driven flow related to the uplift of Hercynian highlands to the south of Ireland could have produced a hydraulic head that drove formation waters northward through the confined Upper Devonian red bed aquifer. Along the flow path these formation waters increased in temperature due to burial and they leached metals. Fluids were focused into the area of present-day Ireland by a high-standing basement block to the east and by the northward thinning of the red bed aquifer. The Irish zinc deposits formed where normal faults tapped the confined red bed aquifer and focused flow of hydrothermal solutions upwards into the Lower Carboniferous carbonate sequence. This focusing allowed the development of discrete thermal anomalies capable of initiating thermal convection cells which mixed formation water from within the Carboniferous sequence with seawater from the overlying ocean. Fluid inclusion studies indicate that the hydrothermal fluid had temperatures of between 150 and 240°C and salinities of between 10 and 23 weight percent NaCl equivalent when it reached the sites of sulfide precipitation. Limited fluid inclusion data suggests that the water pulled into the system from above was significantly cooler (<120°C) and less saline (<10 weight percent NaCl equivalent). Sulfide precipitation occurred as the metal-rich, sulfur-poor, mildly acidic hydrothermal fluids reacted with carbonate sediments causing an increase in fluid pH. Sulfur isotope studies indicate that sulfide precipitation was increased due to the mixing of hydrothermal fluids with the cooler, sulfate-rich water. The host rocks and mineral textures of the Irish deposits are similar to many Mississippi Valley-type deposits. They differ, however, in having a metal suite which includes more copper, silver, and iron than most MVTs and in containing extensive zones of truly massive, often highly iron sulfide-rich, sulfide. These differences are probably the result of higher hydrothermal fluid temperatures which allowed higher metal contents in the fluids and increased reactivity.

Abstract In the first decade of the 21 st century, surface exploration drilling around the Boliden Tara mine at Navan, Ireland, aimed at ~1-km-deep targets, was becoming ineffective. During 2010, the extensive geologic knowledge of the existing Navan orebody was leveraged in an Experts Meeting to promote near-mine discovery. Two ideas, of many, were of relevance to this paper: (1) undiscovered mineralized fault-related zones were predicted south of the orebody, and (2) seismic surveys could locate subsurface faults. By late 2012, seven 2D seismic lines (totaling 101 km) had been acquired, processed, and initially interpreted. Pre-stack time migration images were used for interpretation, augmented by diamond drill core data where available. The seismic imaging proved a “game changer” in terms of subsurface visualization and a priority target was identified 2 km south of the mine on the footwall crest of a large south-dipping basin-margin fault. The first hole intersected 34 m of mineralized rock with 14% Zn + Pb, but at greater depth than anticipated. Follow-up drilling was initially successful but proved to be challenging. The first hole intersected a deep structurally complex section of the newly discovered zone that required more drilling to establish its location and attitude. Further drilling, utilizing extensive navigational deflection technology, outlined a mineralized zone similar in nature to the Navan 5 Lens at depths of 1 to 2 km. Inferred resources through 2016 were estimated at 10.2 Mt grading 8.5% Zn and 1.8% Pb. Underground exploration development of this zone commenced in April 2017, and will allow accurate delineation of this significant discovery.