Re-Os dating of pyrite confirms an early diagenetic onset and extended duration of mineralization in the Irish Zn-Pb ore field

Abstract

The Irish Midlands region contains one of the world's largest hydrothermal Zn-Pb ore districts, but uncertainty exists in the timing of mineralization relative to host rock ages. Consequently, genetic models for ore formation are poorly constrained and remain controversial. Here we use Re-Os geochronology to show that ore-stage pyrite from the Lisheen deposit formed at 346.6 ± 3.0 Ma, shortly after host rock deposition. Pyrite from the Silvermines deposit returns an age of 334.0 ± 6.1 Ma, indicating that at least some mineralization occurred during later burial. These age determinations show that the much younger paleomagnetic ages reported for the Irish Zn-Pb deposits reflect remagnetization during the Variscan orogeny, a process that we suggest affects paleomagnetic dating more widely. The Re-Os ages overlap with the ages of lower Carboniferous volcanic rocks in the Midlands, which are the product of magmatism that has been invoked as the driving force for hydrothermal activity. The relatively low initial Os ratios for both Lisheen (0.253 ± 0.045) and Silvermines (0.453 ± 0.006) are compatible with derivation of Os from these magmas, or from the Caledonian basement that underlies the ore deposits.

Re-Os pyrite geochronology of Zn-Pb mineralization in the giant Caixiashan deposit, NW China

“…Wilkinson and Hitzman (2014) advocate for magmatic heat, derived from the underplating of mid-crustal sills, as a driver for the regional flow associated with Irish-type deposits. This model explains the unusual high fluid temperatures documented in the Irish orefield and the low radiogenic initial Os contents of both the Lisheen (0.253 ± 0.045) and Silvermine (0.453 ± 0.006) deposits (Hnatyshin et al 2015) and may account for the unradiogenic values found in this study. The existence of 830-740-Ma magmatic events in the northern Tarim block Zhan et al 2007;Zhu et al 2008) may have provided a possible heat source to generate these hydrothermal fluids.…”

“…Accurate dating of mineralization has been a major problem in low-temperature Pb-Zn deposits (Leach et al 2010). The majority of attempts have utilised Rb-Sr dating of sphalerite or paleomagnetic techniques, but the accuracy of both techniques has recently been disputed by comparison with reliable results based on Re-Os geochronology (Selby et al 2005;Hnatyshin et al 2015; David L. Leach, personal communication, 2015). Consequently, Re-Os dating of sulphides, especially pyrite, is the preferred choice for dating Zn-Pb mineralization.…”

Re-Os systematics and age of pyrite associated with stratiform Zn-Pb mineralization in the Howards Pass district, Yukon and Northwest Territories, Canada

“…However, Red Dog is the only CD Zn-Pb deposit that has been dated successfully by Re-Os methods, due in part to the coarse-grained nature of the sulphide minerals there and to the relatively high Re contents (tens to hundreds of parts per billion) of pyrite within the vein and massive ore (Morelli et al Kirkham et al (2012) 2004). Base-metal sulphides in other types of sediment-hosted deposits have also been directly dated using Re-Os geochronology, including the Lince-Estefanía Cu deposit in northern Chile (Tristá-Aguilera et al 2006), the Kipushi Cu-Co deposit in the Democratic Republic of Congo (Schneider et al 2007), the Ruby Creek Cu-Co deposit in Alaska ), the Tuolugou Co-Au deposit in northwestern China (Feng et al 2009), the Lisheen and Silvermines Zn-Pb deposits in Ireland (Hnatyshin et al 2015), and the Caixiashan deposit in northwestern China (Li et al 2016).…”

Diatremes Act as Fluid Conduits for Zn-Pb Mineralization in the SW Irish Ore Field

“…There is some controversy surrounding the relative importance and timing of large-scale extensional basement faults that provided fluid pathways; however, there is a general consensus that the majority of Irish-type deposits were formed by the epigenetic sulfide replacement of hydrothermal breccias (e.g., Hitzman and Beaty, 1996;Anderson et al, 1998;Hitzman et al, 2002;Wilkinson et al, 2005b;Redmond, 2010). The timing of mineralization at Lisheen and Silvermines has been constrained by Hnatyshin et al (2015) to 346.6 ± 3.0 and 334.0 ± 6.1 Ma, respectively, and within a similar time frame to Stonepark.…”

“…The Irish ore field hosts a number of globally important stratabound, carbonate-hosted Zn-Pb deposits of Viséan age (346.7-330.9 Ma), which exhibit strong spatial and genetic associations with reactivated Caledonian basement faults (Mitchell, 1985;Hitzman and Beaty, 1996;Everett et al, 1999;Hitzman, 1999;Blakeman et al, 2002;Wilkinson et al, 2005a;Ashton et al, 2015;Hnatyshin et al, 2015) and are designated "Irish-type." Mineralization in the Limerick region is typically hosted in dolomitized country rock breccias termed "black matrix breccias" (Hitzman et al, 2002;Wilkinson et al, 2005b;Redmond, 2010).…”