Fluid evolution in base‐metal sulphide mineral deposits in the metamorphic basement rocks of southwest Scotland and Northern Ireland

Abstract

The Dalradian and Ordovician-Silurian metamorphic basement rocks of southwest Scotland and Northern Ireland host a number of base-metal sulphide-bearing vein deposits associated with kilometre-scale fracture systems. Fluid inclusion microthermometric analysis reveals two distinct fluid types are present at more than half of these deposits. The first is an H 2 O-CO 2 -salt fluid, which was probably derived from devolatilization reactions during Caledonian metamorphism. This stage of mineralization in Dalradian rocks was associated with base-metal deposition and occurred at temperatures between 220 and 360 C and pressures of between 1.6 and 1.9 kbar. Caledonian mineralization in Ordovician-Silurian metamorphic rocks occurred at temperatures between 300 and 360 C and pressures between 0.6 and 1.9 kbar. A later, probably Carboniferous, stage of mineralization was associated with base-metal sulphide deposition and involved a low to moderate temperature (T h 70 to 240 C), low to moderate salinity (0 to 20 wt% NaCl eq.), H 2 O-salt fluid. The presence of both fluids at many of the deposits shows that the fractures hosting the deposits acted as long-term controls for fluid migration and the location of Caledonian metalliferous fluids as well as Carboniferous metalliferous fluids.

Atypical Gold Mineralization in an Orogenic Setting--The Bohaun Deposit, Western Irish Caledonides

“…This is indicated by the proximity of: i) a significant late Palaeozoic (Dinantian) carbonate basin (<10 km east: Graham et al, 1989) and; ii) fluid inclusion salinities more typical of basinal brines associated with Carboniferous and later mineralization in the British Isles (e.g. Ireland: Samson and Russell, 1987;Banks and Russell, 1992;Banks et al, 2002;O'Reilly et al, 1997;Wilkinson, 2010;Devon: Shepherd et al, 2005;Scotland: Samson and Banks, 1988;Curtis et al, 1993;Baron and Parnell, 2005). These two contrasting scenarios are discussed below.…”

“…In the neighbouring Zn-Pb province of the Irish Midlands there is clear evidence for regional flow of Lower Carboniferous ore forming fluids within the metasedimentary basement, to depths of >3-5 km (Everett et al, 1999a). Furthermore, Caledonian structures in the British Isles are suggested to represent important fluid pathways for Carboniferous metalliferous fluids Baron and Parnell, 2005). In Connermara, metalliferous fluids potentially exploited both pre-existing Caledonian basement structures, reactivated in response to regional extension and contemporaneous Variscan structures.…”

Mobility of gold during metamorphism of the Dalradian in Scotland

“…Multiple fluid flow events have affected the Dalradian metamorphic rocks including pre metamorphic spilitisation (Graham, 1976;Skelton et al, 2010), metasomatism during prograde metamorphism (Ague, 1997;Masters and Ague, 2005;Masters et al, 2000), syn metamorphic carbonation (Skelton et al, 1995;Pitcairn et al, 2010b;Arghe et al, 2011) and post-metamorphic quartz-carbonate-sulphide veining (Anderson et al, 2004;Baron and Parnell, 2005;Craw and Chamberlain, 1996). Spilitisation and carbonation events are observed in the SW Highlands.…”

Reconnaissance-Scale Prospectivity Analysis for Gold Mineralisation in the Southern Uplands-Down-Longford Terrane, Northern Ireland

“…Quartz-galena-sphalerite veins with a similar orientation to the goldbearing veins at Clontibret occur in the area surrounding the deposit, and are thought to be part of the same vein swarm (Steed and Morris 1997;. A potential association between base-metal mineralisation and gold deposits is further supported by the work of Baron and Parnell (2005). They identify two stages of mineralisation in many of the base metal occurrences in south-west Scotland and Northern Ireland, the first related to Caledonian metamorphism and the second of probable Carboniferous age.…”

“…Carboniferous base-metal bearing fluids (Baron and Parnell 2005). In addition it is possible that the later basemetal bearing fluids remobilised Caledonian gold, as has been demonstrated in the Dalradian of Northern…”