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Atypical gold mineralization in an orogenic setting : the Bohaun Deposit, western Irish Caledonides

By Paul A. Lusty, Jon Naden, John J. Bouch, John A. McKervey and James A. McFarlane

Abstract

The Bohaun gold deposit in County Galway, western Ireland is located in the Caledonian orogenic belt, proximal to a major accretionary boundary. The mineralization is hosted in a normal extensional fault, representing a zone of repeated extension and vein infilling of open cavities, within clastic sedimentary rocks, which have only reached the late diagenetic zone of low-grade metamorphism. \ud \ud The mineralization consists of undeformed veins, stockworks and breccias. Quartz dominates all the veins and commonly displays multiple growth stages and a variety of classic epithermal textures, including comb, plumose and banded, indicative of low confining pressures. The veins have a simple mineralogy comprising quartz–sericite–chlorite with minor sulfides, hematite, rare visible gold, with a high silver content (up to 41 wt %), and late barite and dolomite. \ud \ud Vein quartz is associated with two fluids compositions: Type I with moderate to high salinity (8.0–23.6 wt % NaCl equiv) and Type II with low to moderate salinity (0.0–7.8 wt % NaCl equiv). CO2 contents in both fluid types are <3.5 wt percent. The majority of fluid inclusion homogenization temperatures range from 126 to 257oC and geologically constrained depth estimates (≤5 km) indicate maximum fluid inclusion trapping temperatures between 175 and 245°C.\ud \ud The geodynamic setting of Bohaun, which comprises a deformed continental margin of an allochthonous terrane, is typical of orogenic gold deposits. However, an orogenic association for the Bohaun mineralization is discounted on the basis of fluid inclusion evidence, atypical vein textures, gold grain composition and undeformed veins. While vein textures locally resemble those found in epithermal quartz–adularia mineralization, there is no obvious magmatic association and alteration is restricted to the veins and immediate host rocks. \ud \ud The origin of the gold at Bohaun remains enigmatic. However, we favor an association with widespread Carboniferous or later mineralizing fluids and a model which involves transport and deposition of gold from a basinal brine type fluid, in a similar fashion to red-bed Au–Pd mineralization. Bohaun provides further evidence that low temperature brines may be capable of generating economic grade gold mineralization, independent of other fluids.\u

Topics: Earth Sciences
Publisher: Society of Economic Geologists
Year: 2011
DOI identifier: 10.2113/econgeo.106.3.359
OAI identifier: oai:nora.nerc.ac.uk:14361

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Citations

  1. 1999a, Fracture-controlled fluid flow in the Lower Palaeozoic basement rocks of Ireland: implications for the genesis of Irish-type Zn–Pb deposits,
  2. 1999a, Short-lived continental magmatic arc at Connemara, Ireland: implications for the age of the Grampian orogeny: Geology,
  3. 1999b, Geochronological constraints on the magmatic, metamorphic and thermal evolution of the Connemara Caledonides,
  4. (1979). A dynamic stratigraphy of the British Isles, London: Allen and Unwin,
  5. (1997). A fluid inclusion and stable isotope study of 200 Ma of fluid evolution in the Galway Granite, Connemara, Ireland: Contributions to Mineralogy and Petrology,
  6. (2010). A Review of Fluid Inclusion Constraints on Mineralization in the Irish Ore Field and Implications for the Genesis of Sediment-Hosted Zn-Pb Deposits: Economic Geology, doi
  7. (1990). Application of quartz textures to the classification of gold deposits using north Queensland examples,
  8. (1987). Basement controls on sediment-hosted PbZn deposits: a lead isotope study of Carboniferous mineralization in Central Ireland: Economic Geology, doi
  9. (1990). Basement geology and tectonic evolution of Ireland as deduced from Pb isotopes:
  10. (1996). Basement structural controls on Carboniferoushosted base metal mineral deposits in Ireland, doi
  11. (2009). Caledonian strike-slip terrane accretion in W. Ireland: insights from very low-grade metamorphism (illite–chlorite crystallinity and b0 paramter): Geological Magazine,
  12. (1998). Carboniferous dykes as monitors of postCaledonian fluid events in West Connacht, doi
  13. (2006). Characterisation of The Bohaun Mountain Gold System, County Galway, Ireland, unpublished MSc. thesis,
  14. (1989). Chemistry and Rb–Sr age of the Corvock Granite,
  15. (1994). Chemistry of low-temperature hydrothermal gold, platinum and palladium ( uranium) mineralization at Coronation Hill,
  16. (1993). Collodial gold and silica in mesothermal vein systems: Geology, doi
  17. (2002). Constraints on the Origins of Fluids Forming Irish Zn-Pb-Ba Deposits: Evidence from the Composition of Fluid Inclusions: Economic Geology,
  18. (2005). Distribution, Character, and genesis of gold deposits in metamorphic terranes,
  19. (1978). Downward-excavating hydrothermal cells and Irish type ore deposits: Importance of an underlying thick Caledonian prism:
  20. (1986). Dual origins of lode gold deposits in the Canadian Cordillera: Geology,
  21. (1988). Epithermal base-metal vein mineralization in the Southern Uplands of Scotland: Nature and origin of the fluids: Mineralium Deposita,
  22. (2005). Fluid evolution in base-metal sulphide mineral deposits in the metamorphic basement rocks of southwest Scotland and Northern Ireland:
  23. (1993). Fluid inclusion and stable isotope study of fault-related mineralization
  24. (1984). Fluid Inclusions:
  25. (1992). Fluid mixing during ore deposition at the Tynagh base-metal deposit,
  26. (1996). Fractal analysis of a mineralised vein deposit: Curraghinalt gold deposit, County Tyrone: Mineralium Deposita,
  27. (1987). Genesis of the Silvermines Zinc-Lead-Barite Deposit, Ireland: Fluid Inclusion and Stable Isotope Evidence: Economic Geology,
  28. (1994). Geochemical evolution of an Ordovician island arc,
  29. (1991). Geological and geochemical controls on the silver content (fineness) of gold in gold-silver deposits: Ore Geology Reviews,
  30. (2005). Geological characteristics of epithermal precious and base metal deposits,
  31. (1997). Geology, mineralogy and genesis of gold mineralization at CalliacharUrlar Burn,
  32. (2003). Gold deposits in metamorphic belts: overview of current understanding, outstanding problems, future research, and exploration significance: Economic Geology,
  33. (1992). Gold mineralisation in the Silurian and Ordovician of south Mayo,
  34. (1986). Gold mineralization in Ordovician greywackes at Clontibret, Ireland,
  35. (2007). Gold nuggets: supergene or hypogene?: doi
  36. (1999). Gold remobilization by low-temperature brines: evidence from the Curraghinalt gold deposit, Northern Ireland: Economic Geology v. doi
  37. (2006). Gold-palladium mineralization at Bleida Far West, Bou Azzer-El Graara Inlier, Anti-Atlas, Morocco: Mineralium Deposita,
  38. (1994). Interpretation of microthermometric data for H2O-NaCl fluid inclusions,
  39. (2005). Intracratonic crustal seawater circulation and the genesis of subseafloor zinc-lead mineralization in the Irish orefield: Geology, doi
  40. (2006). Metal sources of the Navan carbonate-hosted base metal deposit, Ireland: Nd and Sr isotope evidence for deep hydrothermal circulation, Mineralium Deposita,
  41. (1999). Movement and mineralization in the Tyndrum Fault Zone, Scotland and its regional significance:
  42. (2011). Numerical model to determine the composition of H2O–NaCl–CaCl2 fluid inclusions based on microthermometric and microanalytical data:
  43. (1998). Orogenic gold deposits: A proposed classification in the context of their crustal distribution and relationship to other gold deposit types: Ore Geology Reviews,
  44. (2003). Package FLUIDS 1, Computer programs for analysis of fluid inclusion data and for modelling bulk fluid properties: Chemical Geology,
  45. (1999). Patterns of very low-grade metamorphism in metapelitic rocks,
  46. (2005). Permo-Triassic unconformity-related Au-Pd mineralization,
  47. (2000). Phanerozoic Orogenic Lode Gold Deposits,
  48. (1993). Phase-relations in the Fe-Zn-S system to 5 kbars and temperatures between 325°C and 150°C: Economic Geology,
  49. (2009). Polyphase lowgrade metamorphism of the Ingleton Group,
  50. (1996). Pressure fluctuations, phase separation, and gold precipitation during seismic fracture propagation: Geology, doi
  51. principal ore fluid,
  52. (1993). Quartz vein morphology and implications for formation depth and classification of Archaean gold-vein deposits: Ore Geology Reviews,
  53. (2000). Regional fluid flow and gold mineralization in the Dalradian of the Sperrin Mountains, Northern Ireland: Economic Geology,
  54. (1989). Sulphur isotopic investigation of Lower Carboniferous vein deposits of the British Isles: Mineralium Deposita,
  55. (2003). Surface-derived fluids in basement rocks: inferences from palaeohydrothermal-systems:
  56. (1988). Synthetic fluid inclusions .4. Solubility relations in the system NaCl-KCl-H2O under vapor-saturated conditions:
  57. (1988). Synthetic fluid inclusions .8. Vapor-saturated halite solubility in part of the system NaCl-CaCl2-H2O, with application to fluid inclusions from oceanic hydrothermal systems:
  58. (2000). The application of microchemical analysis of alluvial gold grain to the understanding of complex local and regional gold mineralization: a case study in the Irish and Scottish Caledonides: Economic Geology,
  59. (2000). The Caledonian orogeny redefined:
  60. (1994). The Dalradian Supergroup in Scotland, Shetland and Ireland,
  61. (1989). The Geology of South Mayo,
  62. (1992). The geology of the Cononish gold-silver deposit, Grampian Highlands of Scotland,
  63. (1996). The Gold Metallogeny of Northwest Northern Ireland: Volume One, Geological Survey of Northern Ireland
  64. (1985). The importance of CO2 on the freezing point measurements of fluid inclusions: Evidence from active geothermal systems and implications of epithermal ore deposition: Economic Geology, doi
  65. (1996). The Irish Zn-Pb-(Ba) ore field,
  66. (1992). The Lack gold deposit,
  67. (1986). The Lisglassan-Tullybuck deposit, County Monaghan: SbAs-Au vein mineralization in Lower Palaeozoic greywackes,
  68. (2002). The origin and significance of the Delaney Dome Formation,
  69. (1986). The stratigraphic and structural setting of Irish mineral deposits,
  70. (1990). The system NaCl-CaCl2-H2O: 1 the ice liquidus at 1 atm total pressure:
  71. (1988). The Upper Palaeozoic geology of the Porcupine, Erris and Donegal Basins, offshore Ireland:
  72. (2003). U-Pb zircon geochronology of the Galway Granite, Connemara, Ireland: implications for the timing of late Caledonian tectonic and magmatic events and for correlations with Acadian plutonism in New England: Atlantic Geology,
  73. (1982). Use of the lattice parameter b0 of dioctahedral illite/muscovite, for the characterisation of P/T gradients of incipient metamorphism:
  74. (2010). Young orogenic gold mineralisation inactive collisional mountains, Taiwan: Mineralium Deposita,

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