Abstract
Irish-type mineralization is commonly attributed to fault-controlled mixing of a seawater-derived, sulfur-rich fluid and basement-derived, metal-rich fluid. However, maar-diatreme volcanoes discovered in close spatial and temporal association with Zn-Pb mineralization at Stonepark in the Limerick basin (southwest Ireland) bring a new dimension to established geologic models and may increase the deposit-scale prospectivity in one of the world’s greatest Zn-Pb districts. Stonepark exhibits many incidences of dolomitic black matrix breccias with associated Zn-Pb mineralization, the latter typically occurring within 150 m of the diatremes. Highly negative δ 34 S pyrite values within country rock-dominated black matrix breccias (–12 to –34) are consistent with sulfide precipitation from bacteriogenic sulfur reduction in seawater-derived brines. However, δ 34 S values of Zn-Pb sulfides replacing black matrix breccias (–10 to 1) reflect multiple sulfur sources. Diatreme emplacement both greatly enhanced country rock fracture permeability and produced conduits that are filled with porous volcaniclastic material and extend down to basement rock types. Our δ 34 S data suggest that diatremes provide more efficient fluid pathways for basement-derived fluids. The diatremes introduce another potential sulfur source and facilitate a greater input of metal-rich basement-derived hydrothermal fluid into the system compared to other Irish-type deposits such as Navan and Lisheen, evidenced by Stonepark’s more positive modal δ34S value of –4. Irish-type deposits are traditionally thought to form in association with extensional basement faults and are considered unrelated to extensive Carboniferous magmatism. Our results indicate that a direct link exists between diatreme volcanism and Zn-Pb mineralization at Limerick, prompting a reevaluation of the traditional Irish-type ore formation model, in regions where mineralization is spatially associated with volcanic pipes.
Highlights
Our results indicate that a direct link exists between diatreme volcanism and Zn-Pb mineralization at Limerick, prompting a reevaluation of the traditional Irish-type ore formation model, in regions where mineralization is spatially associated with volcanic pipes
Diatremes, such as those preserved at Stonepark (Elliott et al, 2015), are typically infilled with porous and permeable material (e.g., White and Ross, 2011; Afanasyev et al, 2014) and are associated with fracture networks and breccia bodies related to their explosive emplacement (Sparks et al, 2006)
Grades of Zn and Pb mineralization are high within black matrix breccia horizons proximal to the diatremes
Summary
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). Field observations at Limerick (Fig. 1) suggest a close spatial and temporal association between Zn-Pb mineralization and basaltic maar-diatreme volcanism (McCusker and Reed, 2013; Elliott et al, 2015), but the existence of a direct genetic link is yet to be established. Our data indicate that diatreme emplacement strongly influenced mineral deposit formation at Stonepark Diatremes, such as those preserved at Stonepark (Elliott et al, 2015), are typically infilled with porous and permeable material (e.g., White and Ross, 2011; Afanasyev et al, 2014) and are associated with fracture networks and breccia bodies related to their explosive emplacement (Sparks et al, 2006). In the Limerick basin, diatremes form a NE-SW–orientated cluster, closely reflecting the regional Caledonian trend This suggests that ascending magmas utilized preexisting faults (cf Kurszlaukis and Barnett, 2003; Jelsma et al, 2009). Country rock black matrix breccia (type C) contains Waulsortian carbonate clasts, commonly
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