|
Irish Zn/Pb Deposits: Structure and Fluid Flow |
1 Department of Geology & Geophysics, Yale University, New Haven CT06520, USA
2 T. H. Huxley School of Environment, Earth Sciences & Engineering, Royal School of Mines, Imperial College of Science, Technology and Medicine, London SW7 2BP, UK
A fluid inclusion study has been carried out to test whether circulation of fluids within fractured very low grade metasedimentary basement rocks was an important process in the genesis of the carbonate-hosted base metal deposits of Ireland. In the Silvermines district, Silurian greywackes and mudrocks are host to several different cross-cutting vein types which are locally abundant in east-east-northeast and northwest-north-northwest trending, high angle, brittle fault zones. Structural analysis indicates that the vein orientations are consistent with formation under a northeast-southwest dextral trans-tensional regime, controlled locally by reactivation of pre-existing faults. Three vein types have been identified: (1) early hematitic calcite-quartz ± pyrite; (2) quartz-calcite ± sphalerite, galena, chalcopyrite, pyrite, barite; (3) ankerite-ferroan dolomite-quartz ± sphalerite, pyrite. Vein types 2 and 3 are associated with weak to locally intense sericite-chlorite-carbonate alteration.
Primary fluid inclusion data from vein Types 2 and 3 show homogenization temperature-salinity characteristics (123–238°C, 9.7–20.6 wt% NaCl equivalent) overlapping significantly with the deposit data. In addition, primary inclusions hosted by quartz and sphalerite commonly contain CO2 as indicated by the formation of gas clathrate and, rarely, solid CO2 on cooling. Bulk fluid inclusion analyses on quartz vein samples show that the fluid composition is comparable with experimental data on fluids equilibrated with greywackes at temperatures of 200–350°C. Alkali geothermometry gives temperatures (158–219°C) not significantly different to homogenization temperatures suggesting that the veins formed under low fluid pressures.
New inclusion data from Navan brown sphalerite confirm the importance of a relatively high temperature, moderate salinity (187–220°C, 15–18 wt% NaCl equivalent) mineralizing fluid as observed at Tynagh, Silvermines, Lisheen and in the Lower Palaeozoic basement. In these inclusions, gas clathrate has been identified. This is the first reported occurrence of this phase in fluid inclusions from the Irish base metal deposits.
These data provide convincing evidence for the regional flow of high temperature fluids, similar to the Lower Carboniferous ore-forming fluids, within the Lower Palaeozoic metasedimentary basement. Systematic regional variations in homogenization temperature-salinity characteristics imply variable infiltration of local, low temperature, surface-derived fluids, consistent with a density driven convection model. In combination with existing isotopic data and thermal constraints, the new data strongly suggest that deep circulation of fluids was a fundamental process in the genesis of the Irish deposits. Therefore, purely topographically driven fluid flow models solely utilizing the Old Red Sandstone as a regional aquifer are inappropriate for the Irish orefield.
This article has been cited by other articles:
 |
|
 |
|
  Zs. R. Nagy, J. M. Gregg, K. L. Shelton, S. P. Becker, I. D. Somerville, and A. W. Johnson Early dolomitization and fluid migration through the Lower Carboniferous carbonate platform in the SE Irish Midlands: implications for reservoir attributes Geological Society, London, Special Publications, 2004; 235: 367 - 392. [Abstract] [PDF]
|
|
