GEOLOGICAL JOURNAL Geol. J. 40: 3–21 (2005) Published online 13 December 2004 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/gj.973 Fluid evolution in base-metal sulphide mineral deposits in the metamorphic basement rocks of southwest Scotland and Northern Ireland MARTIN BARON* and JOHN PARNELL Department of Geology and Petroleum Geology, University of Aberdeen, AB24 3UE, UK 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 tempera- tures 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 mode- rate 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. Copyright # 2004 John Wiley & Sons, Ltd. Received 15 January 2003; revised version received 8 August 2003; accepted 1 September 2003 KEY WORDS base-metal sulphide mineralization; fluid inclusions 1. INTRODUCTION The Ordovician–Silurian and Dalradian metamorphic basement rocks of southwest Scotland and Northern Ireland host a number of base-metal sulphide-bearing vein deposits associated with major fractures systems (Figure 1). Mineralization is thought to be in part Caledonian and in part Carboniferous in age (Pattrick and Russell 1989; Treagus et al. 1999). The later Carboniferous stage of mineralization, which is thought to be coeval with the eco- nomically important base-metal sulphide deposits in central Ireland, formed in response to regional extension resulting in the reactivation of pre-existing Caledonian faults (Pattrick and Russell 1989). This study presents petro- graphic and fluid inclusion microthermometric data of samples from a number of the abandoned Pb–Zn–Cu mine sites in the Ordovician–Silurian and Dalradian metamorphic basement rocks of southwest Scotland and Northern Ireland. These results document the pressures, temperatures and compositions of the mineralizing solutions res- ponsible for base-metal sulphide mineralization. Copyright # 2004 John Wiley & Sons, Ltd. *Correspondence to: M. Baron, Department of Geology and Petroleum Geology, Meston Building, University of Aberdeen, King’s College, Aberdeen, AB24 3UE, UK. E-mail: m.baron@abdn.ac.uk