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

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 δ 34 S 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.

The Carbonate-Hosted Tullacondra Cu-Ag Deposit, Mallow, Ireland

“…The Waulsortian Limestone is considered to have been deposited at greater water depths (~200 m) than the Ballysteen Formation [15], between the photic and aphotic zones [16]. Associated with this extensional period of the Lower Carboniferous, mafic intrusions and volcaniclastic rocks are common, mainly in the southwest Irish Midlands in the Limerick area [5,[18][19][20].…”

“…However, a recent Re-Os dating has shown that vein-hosted Cu deposits can be slightly older than the Irish Zn-Pb deposits [45]. Furthermore, the spatial and temporal relationship between the mineralization and volcanic rocks from southern Irish Midlands suggests a genetic relation between magmatism and Zn-Pb/Cu-Ag mineralization [5,20]. The magmatic activity has been considered a possible source of heat and water for the Zn-Pb and Cu-Ag mineralizing fluids [5,6,19,20,46].…”

“…Formation are underlain by red beds and hosted in marine organic-rich sediments from the Lower Carboniferous [5,6,28]. Magmatic activity as a possible source of metals and heat is considered for Irish Cu deposits and sedimentary-hosted stratiform Cu deposits elsewhere [5,6,19,20,46,59]. In this sense, Tullacondra shares similar controls with both Zn-Pb Irish-type deposits and sedimentary-hosted Cu deposits.…”

Mineral textures, mineral chemistry and S isotopes of sulphides from the Tianbaoshan Pb–Zn–Cu deposit in the Sichuan–Yunnan–Guizhou triangle: implications for mineralization process

“…T, pH and fO 2 ) of a single hydrothermal fluid could cause sulphide precipitation, mostly sulphide mineralization is related to mixing of at least two fluids with contrasting components (e.g. sulphur-rich, metal-depleted brine and metal-rich, sulphur-poor hydrothermal fluid) (Ashton et al, 1998;Blakeman et al 2002;Wilkinson et al 2005;Bouabdellah et al 2008Bouabdellah et al , 2012Barrie et al 2009;Gagnevin et al 2014;Saintilan et al 2015;Elliott et al 2019). Compared to conventional bulk sulphur isotopic analysis, microscale variations in sulphur isotopes obtained by in situ analysis of sulphide minerals reveal more details of mineralizing processes (Deloule et al 1986;Layne et al 1991;McKibben & Eldridge, 1995;Peevler et al 2003;Ferrini et al 2010).…”

“…pyrite, sphalerite and chalcopyrite) can constrain the mechanisms of metal precipitation, particularly regarding Pb–Zn mineralization hosted by carbonate rocks. It has been suggested that the main sulphur sources for carbonate-hosted Pb–Zn deposits generally include evaporite-derived sulphur (heavy δ 34 S values), previously deposited bacteriogenic sulphur (light δ 34 S values) and magma-derived sulphur (intermediate δ 34 S values) (Perona et al ., 2018; Elliott et al 2019). Although a change in physicochemical conditions (e.g.…”