Timing of Interplay between Hydrothermal and Surface Fluids in the Navan Zn + Pb Orebody, Ireland: Evidence from Metal Distribution Trends, Mineral Textures, and  34S Analyses

Abstract

Models of genesis for the Navan orebody are of two distinct types. An early hypothesis that mesothermal (though nonmagmatic) deposition of ore began when a supernatant seawater brine still had access to the host sediments (during the early to mid-Mississippian), has been challenged by recent suggestions favoring a later (mid-to late Mississippian to Pennsylvanian) mineralization derived from cooler fluids traversing either the Mississippian/Pennsylvanian basin or the underlying basement from the south. These models, characterized here as Irish-type and Mississippi Valley-type respectively, are interrogated as to their particular predictions with regard to metal distributions and sulfur isotope patterns associated with various fault geometries. The basal 5 lens of the Navan Zn + Pb deposit contains ~70 percent of the known tonnage of the ~90 Mt orebody and, thus, is the focus of this examination. Lead distribution patterns suggest that migration of metalbearing fluids was principally directed up early to mid-Mississippian, near vertical north-northeast, northeast, and east-northeast minor normal faults. These faults predate or are coeval with the major extensional, partly listric, east-northeast faults which now control the general disposition of the deposit. Only where these major east-northeast faults cross putative deep-seated northeast (Caledonoid) and northwest structures are they associated with lead enrichments. A systematic δ 34 S survey in the 5 lens across five minor north-northeast-through to east-northeast-trending faults associated with distinct lead enrichments, and one east-northeast-trending, partly listric, major extensional fault adjacent to that trend, revealed positive δ 34 S values (1-18‰) for galena, sphalerite, and marcasite sampled within 3 m of all the faults on the profile. Sulfides with positive δ 34 S values associated with the deep-seated, metal-bearing fluid generating the Navan deposit have been highlighted by previous workers (Anderson et al., 1998). The evidence reported here strongly suggests that the metal-bearing fluids rose through all the fractures. Conversely, negative δ 34 S values (-1 to-26‰) were returned in galena and sphalerite sampled 3 m or more from these faults. These negative values indicate that locally derived bacteriogenic sulfide, reduced from seawater sulfate, dominated away from these faults. Pyrite δ 34 S values suggest a background level of-29 ± 3.0 per mil across the profile. However, pyrite δ 34 S values as low as-34 ± 2.7 per mil were recorded in one sample collected from within 1 m of a fault. Thus, fluids containing highly fractionated, bacteriogenic sulfide also gravitated into these faults on at least one occasion. There is also evidence suggesting that the metal-bearing solutions periodically displaced the locally derived bacteriogenic sulfide-bearing fluid in and near the faults. Mineral sulfide petrography is used to contextualize the sampling and to give a qualitative indication of the degree of chemical disequilibrium of the system. Mineral textu...

Zn, Fe and S isotope fractionation in a large hydrothermal system

“…The Navan ore deposit (Fig. 1) is the largest of these Irish-type base metal 89 deposits, a deposit class commonly considered to be part of a larger group of carbonate-90 hosted base metal deposits that has affinities to both Mississippi Valley-type (MVT) (Leach 91 and Sangster, 1993) and SEDEX deposits (Blakeman et al, 2002). In almost continuous 92 production since 1977, and currently the largest zinc producer in Europe, the ore already 93 extracted at Navan plus reserves together amount to more than 105 million tonnes at an 94 average grade of ~8.3 % zinc and ~2.1% lead (Ashton et al, 1986(Ashton et al, , 2003 The Lower Carboniferous succession (Philcox, 1984;Strogen et al, 1996) has been 108 ascribed to a northerly marine transgression.…”

Minor Elements in Layered Sphalerite as a Record of Fluid Origin, Mixing, and Crystallization in the Navan Zn-Pb Ore Deposit, Ireland

“…In almost continuous production since 1977 and currently the largest zinc producer in Europe, the ore extracted at Navan plus reserves together amount to about 105 million tonnes (Mt) at an average grade of ~8.3% zinc and ~2.1% lead (Ashton et al, 1986(Ashton et al, , 2003. A substantial amount of research has been carried out on the Navan deposit, and its genesis is now reasonably well understood (Ashton et al, 1986(Ashton et al, , 2003Anderson et al, 1998;Fallick et al, 2001;Blakeman et al, 2002). As in other Irish-type deposits, it is widely accepted that mixing between deep-seated, basement-derived, metal-bearing fluids and shallow, cool surface brines carrying bacteriogenic sulfur (the latter being the dominant reservoir of sulfur in Navan; Fallick et al, 2001) was critical to ore deposition.…”

“…A number of processes have been proposed to account for the chemical variability of sphalerite, including both external (e.g., change in composition of the mineralizing fluid) and internal (e.g., differential kinetic absorption at the mineral-fluid interface) processes (Di Benedetto et al, 2005, and references therein). Isotopic ratios, especially of sulfur, are of particular benefit for assessing putative sources in the genesis of hydrothermal deposits, and have extensively been applied to the Irish ore field (e.g., Anderson et al, 1998;Blakeman et al, 2002;Wilkinson et al, 2005a;Barrie et al, 2009) and to some MVT deposits (e.g., Kuhlemann et al, 2001;Peevler et al, 2003;Pfaff et al, 2011). Therefore, combining chemistry and isotopes measured on layered sphalerite may help to assess whether the chemical composition of sulfides records fluid origin the Navan ore deposit.…”

Quantification of mixing processes in ore-forming hydrothermal systems by combination of stable isotope and fluid inclusion analyses

“…Stable isotope investigations of sulfur and carbon are important tools for deciphering the processes involved in the formation of hydrothermal mineralizations and can provide critical information about (1) the temperature of formation, (2) the physico-chemical conditions of the mineralization processes, and (3) the origin of the elements in solution (e.g., Rye and Ohmoto, 1974;Ohmoto and Rye, 1979;Hoefs, 1987;Rye, 1993;Ohmoto and Goldhaber, 1997;Huston, 1999;Heinrich et al, 2000;Simmons et al, 2000;Blakeman et al, 2002). In particular, sulfide-sulfate isotopic relationships are sensitive indicators for non-equilibrium assemblages resulting from fluid mixing.…”