Hydrothermal fluid flow in Irish Zn-Pb deposits: a Zn-Cu-S isotope and clumped isotope approach

 

PhD Candidate: Aileen Doran

Supervisor: Dr Julian Menuge

Funded by: Science Foundation Ireland through iCRAG

Abstract:

Irish-type ore deposits, such as the giant Navan deposit, have produced globally significant quantities of zinc (Ashton et al. 2015). These deposits formed by carbonate replacement of Lower Carboniferous limestones (a marine transgressive sequence) where metal-bearing fluids, equilibrated with Lower Palaeozoic basement, ascended on normal faults to mix with sinking hypersaline brines carrying bacteriogenically reduced dissolved sulphide (Wilkinson et al. 2005).

With the closure of Lisheen mine in 2015, there is now a need to find new Irish-type Zn-Pb prospects in Ireland. Only the giant Navan deposit remains operational. This project is primarily concerned with modelling fluid flow and metal deposition through space and time and to determine how these techniques could be deployed in geochemical exploration to vector towards Irish-type orebodies.

Due to their complexity, it has proved difficult to model in detail the fluid flow within and around Irish-type deposits.  Some progress has been made using metal ratios and stable isotope compositions, e.g. Blakeman et al. (2002). For this project, Zn-Cu-S isotope and clumped O-C isotope analysis will be utilized, to aid in fluid flow modelling. The initial focus will be on the recently closed Lisheen deposit (Co. Tipperary) and the Stonepark prospects (Co. Limerick). Other deposits may be incorporated into the study where suitable material and deposit understanding is available.

Clumped O-C isotope analysis of carbonates will be used to separate the temperature and fluid δ13C and δ18O signatures of hydrothermal fluids. This will allow fluid flow in orebodies and their respective haloes to be constrained. Zn-Cu-S isotope analysis of sphalerite will be designed to seek isotope fractionation trends due to prior sphalerite and possible copper sulphide crystallization. Copper isotope analysis of sulphides may be used as a means of monitoring prior chalcopyrite crystallization from a hydrothermal fluid. Recently, it has been shown that Zn isotope fractionation may occur during sphalerite crystallization, resulting in possible geochemical haloes around orebodies.This effect may be used as a geochemical exploration tool in the future.

References:

Ashton, J.H., Blakeman, R., Geraghty, J., Beach, A., Coller, D., Philcox, M., Boyce, A., Wilkinson, J.J., 2015. The Giant Navan Carbonate-Hosted Zn-Pb Deposit – A Review. In: Current Perspectives on Zinc Deposits. Irish Association for Economic Geology, Dublin, p. 85-122

Blakeman, E. J., Ashton, J.H., Boyce, A. J., Fallick, A. E., Russell, M. J. (2002) Timing of Interplay between Hydrothermal and Surface Fluids in the Navan Zn + Pb Orebody, Ireland: Evidence from Metal Distributiojn Trends, Mineral Textures, and delta S-34 Analysis. Economic Geology, 97, pp. 73 – 91

Wilkinson, J. J., Eyre, S . L. and Boyce, A. J. (2005) Ore-Forming Processes in Irish-Type Carbonate-joster Zn-Pb Deposits: Evidence from Mineralogy, Chemistry and Isotopic Composition of Sulfides at the Lisheen Mine. Economic Geology, 100, pp. 63 - 86