Volcanological and environmental controls on the Snowdon mineralization, North Wales, UK: A failed volcanogenic massive sulfide system in the Avalon Zone of the British Caledonides

Abstract

14The Snowdon caldera of North Wales is host to base metal sulfide-bearing veins and stockworks, 15 mineralized breccias, disseminated sulfides, and localized zones of semi-massive to massive sulfide, 16 with subordinate magnetite-rich veins. The late Ordovician host volcanic sequence accumulated in a 17 shallow marine, back-arc environment in the Welsh Basin, which forms part of the Avalon Zone of 18 the British and Irish Caledonides. New field evidence, sulfur isotopes, and U-Pb dating indicate that 19 the Snowdon mineralization is genetically and temporally related to Late Ordovician magmatism and 20 caldera formation. It is interpreted to represent volcanogenic pipe-style sulfide mineralization, 21 resulting from focused hydrothermal fluids moving along caldera-related faults and simultaneous 22 dispersal of fluids through the volcaniclastic pile. Sulfur isotope data suggest that, whilst a limited 23 contribution of magmatic S cannot be ruled out, thermochemical reduction of contemporaneous 24Ordovician seawater sulfate was the dominant mechanism for sulfide production in the Snowdon 25 system, resulting in a mean value of about 12‰ in both the host volcanic strata and the mineralized 26 veins. Despite the tectonic setting being prospective for VMS deposits, strata-bound sulfide 27 accumulations are absent in the caldera. This is attributed to the shallow water depths, which 28 promoted boiling and the formation of sub-seafloor vein-type mineralization. Furthermore, the 29 tectonic instability of the caldera and the high energy, shallow marine environment would have 30 limited preservation of any seafloor deposits. The new U-Pb dates for the base (454.26 ± 0.35 Ma) 31 and top (454.42 ± 0.45 Ma) of the host volcanic rocks, indicate that the Snowdon magmatic activity 32 was short lived, which is likely to have limited the duration and areal extent of the ore-forming system. 33 2 The absence of massive sulfide mineralization is consistent with the general paucity of economic 34 VMS deposits in the Avalon Zone. Despite the highly prospective geological setting this study further 35 illustrates the importance of volcanic facies mapping and associated paleo-environmental 36 interpretations in VMS exploration. 37

Interplay of southern, western, and northern sources during deposition of North Wales Carboniferous sandstones, determined from heavy minerals, mineral chemistry, and detrital zircon ages

“…In fact, clast analysis on the Brownstones Formation suggests that it has a relatively proximal origin from the erosion of Ordovician and Silurian (and possibly Precambrian Mona Complex) igneous, sedimentary, and metamorphic rocks most likely sourced from North Wales (Allen, 1974), and on that evidence would proba- Thorpe et al, 1984;Tucker & Pharaoh, 1991); the Stanner Hanter Complex (ca. 702-711 Ma: Patchett, Gale, Goodwin, & Humm, 1980;Schofield, Millar, Wilby, & Evans, 2010); the Snowdon Volcanic Group of North Wales (454 ± 0.4 Ma for the base and top: Lusty, Lacinska, Millar, Barrie, & Boyce, 2017); and the Skomer Volcanic Group of southwest Wales of mid Llandovery age (ca. 440 Ma) (Ziegler, McKerrow, Burne, & Baker, 1969).…”

“…Closer to the study area, volcanism in North Wales may also have contributed to the Early Palaeozoic zircons. For example, the Snowdon Volcanic Group of North Wales (454 ± 0.4 Ma for the base and top: Lusty et al, 2017).…”

The shallow marine VMS copper deposit of Yushui, Eastern Guangdong, South China: Evidence from geology, geochronology and geochemistry

“…However, the various ocean drilling programs (Deep Sea Drilling Program, DSDP; Ocean Drilling Program, ODP; Integrated Ocean Drilling Program, IODP) have observed hydrothermal activities in shallow marine settings (< 1000 m depth), with sul de accumulation in the early-stage mid-ocean ridge opening (e.g., East African Rift, Red Sea-Atlantis II Deep, Lake Baikal) (Bonatti et al 1972;Crane et al 1991;Tiercelin et al 1993) or in the landward part of the ridge (e.g., northern Iceland, Salton Sea) (Fricke et al 1989;Olafsson 1989;Botz et al 1999;Canet et al 2005;Camprubí et al 2007). Therefore, such shallow marine settings associated with early-stage basin expansion have the potential to produce metal sul de deposits ( In shallow marine environments, vein-type and stockwork (but usually not stratiform massive) orebodies can be formed, which is different from typical VMS systems (Hannington et al 1997;Hou et al 2003;Lusty et al 2017). Although uid boiling could cause metal loss, it could also contribute to metal precipitation at/near the sea oor in shallow-water hydrothermal systems (Hannington 2021;Falkenberg et al 2021;Schaarschmidt et al 2021).…”