A zinc isotope and trace element study of the Irish Zn-Pb orefield
Creators: name: Crowther, Helen Louise
Publisher: name: Imperial College London
Year: 2023

description: This project is concerned with quantifying trace element and Zn isotope variations from source to sink in a hydrothermal ore-forming system, utilizing the carbonate hosted Irish Zn-Pb orefield as a natural laboratory. This study focuses on the Zn-Pb deposits of the Rathdowney Trend; Lisheen and Galmoy, located in the southern Irish Midlands. To date, the Irish orefield has been extensively studied and the general ore-forming processes are now well established (Hitzman and Large, 1986; Hitzman and Beaty, 1996; Wilkinson, 2003). A hydrothermal fluid carrying metals derived from deep underlying rocks up-wells along reactivated Caledonian fault structures into breccia lenses in the semi-lithified, (and in the Rathdowney area, regionally dolomitized) Courceyan limestone horizon. Upon reaching this unit, the hydrothermal fluid mixes with a surface derived brine carrying abundant sulphur. Upon mixing of the two fluids, rapid precipitation of massive sulphides ensues, replacing the host breccia lenses adjacent to hydrothermal fluid conduits and occasionally venting onto the seafloor. Sulphide lenses comprise predominantly sphalerite, galena and pyrite, along with several hydrothermal gangue phases; most notably D2 dolomite, which forms the matrix to the laterally extensive breccia lenses which, locally, host sulphide mineralization. Whilst the major ore-forming process are well established, several issues remain contentious, including; the source of Zn and other ore metals (Lower Palaeozoic basement or Old Red Sandstone), the relationship between mineralization and brecciation (are breccias formed by hydrothermal fluids, or are breccias tectonic in origin?) and the timing of mineralization. Accordingly, this study aims both to develop the novel technique of Zn isotope analysis, whilst further enhancing the understanding of some of the finer details of Irish-type mineralization. The primary objectives of this study are: 1) to determine the Zn isotope and trace element signatures of the putative metal source terrains (Lower Palaeozoic basement and Old Red Sandstone (ORS)) and the signatures which may be imparted to hydrothermal fluids interacting with those lithologies for comparison with the signatures of overlying mineralization; 2) to determine if, and to what extent, Zn isotopes are fractionated during precipitation of sphalerite from hydrothermal fluids in ore-forming environments; and 3) to determine the trace element composition of D2 breccia matrices and evaluate how trace element concentrations vary both as a function of mass change and as a function of distance from hydrothermal feeder structures. These data will indicate whether brecciation is a function of mass loss and provide geochemical vectors towards areas where sulphide lenses may be encountered. The results of this study indicate that ORS and Lower Palaeozoic basement rocks have indistinguishable 866Zn values (mean values of 0.24%o (n=3) and 0.2l%o (n = 3) ±0.04, respectively). The most easily accessible Zn fractions (extracted by a cold 6N HC1 leach) of each rock type account for ~20% of the total whole rock concentration of Zn and are characterised by isototopically heavy signatures (mean values of ORS and Lower Palaeozoic basement of 0.43%o +0.07 (n = 3) and 0.73%o ± 0.05 (n = 3), respectively). The modal S66Zn value of sphalerite in ore deposits is ~0.2%o (equivalent to whole rock isotopic compositions), indicating that the hydrothermal fluids which precipitated ore deposits extracted Zn pervasively from metal source rocks. Conversely, sub economic prospect mineralization is characterised by 866Zn values up to 0.64%o (Wilkinson et al., 2005b), potentially indicating that hydrothermal fluids from which some prospects precipitated were Zn deficient, having only leached the most easily extractable isotopically heavy Zn from the metal source terrain. Consequently, it is considered that the S66Zn value of a hydrothermal fluid may correlate inversely with the concentration of Zn carried therein. Zn isotopes are also fractionated on an orebody scale. Within the Lisheen Derryville orebody, S66Zn values vary systematically from -0.06%o ± 0.03 proximal to the hydrothermal fluid conduit, to 0.32%o ±0.03 at the periphery of the orebody, consistent with kinetic fractionation of Zn isotopes during rapid precipitation of Zn from hydrothermal fluids. Assuming an initial hydrothermal fluid S66Zn value of 0.2 l%o (equivalent to both the modal 866Zn value of deposit sphalerite and the average 866Zn value of Lower Palaeozoic basement rocks) and modelled on the 866Zn values of sphalerite measured across the Lisheen Derryville orebody, a fractionation factor has been determined; AflUid-sphaierite = -0.307%o. Immobile element concentrations (Al and Ti) determined during trace element analyses of D2 reveal that during the formation of D2, up to 45% of the original carbonate mass has been removed. Consequently, is suggested that a considerable degree of brecciation may be attributed to sagging and settling of the dolomite as underlying rocks were removed. This ongoing mass removal affected the surface topography, the resulting ‘trough’ shaped structure which developed becoming in filled by a thick pile of WLF facies, constraining the age of brecciation and mineralization to the late Courceyan/early Chadian and the depth of mineralization to &lt;150m beneath the seafloor. Within D2 dolomite, Ba and Pb are enriched proximal to hydrothermal fluid conduits, progressively returning to background Waulsortian dolomite values several hundred metres distal to those structures. Conversely, Mn and Sr are relatively depleted proximal to fluid conduits and become enriched above background values distal to those structures, indicative of remobilisation of Mn and Sr during the outwards flow of hydrothermal fluids. Ba, Pb, Mn and Sr vary coherently and systematically as a function of distance from hydrothermal fluid conduits, whilst all other ore-associated elements analysed (Fe, As, Ni, Cd, Cu, Co and Zn) display erratic distributions and are therefore considered unreliable as vectors towards hydrothermal fluid conduits. A ‘proximity factor’ calculation has been derived to provide reliable vectors towards hydrothermal fluid conduits in D2 dolomite horizons; Pb/(Mn/Al). Proximity factor values vary from &gt;1,000,000 proximal to feeder structures to &lt;100 at the periphery of D2 breccia lenses.