A Review of Fluid Inclusion Constraints on Mineralization in the Irish Ore Field and Implications for the Genesis of Sediment-Hosted Zn-Pb Deposits

Abstract

higher salinity than is typical of the Waulsortian-hosted deposits further south (Tynagh, Silvermines, 22Lisheen and Galmoy). Subeconomic prospects tend to display a narrower range in salinity, mostly at 23 the lower end of the range observed in the ore deposits. In some prospects, and on the margins of 24 some ore deposits, evidence for dilution is observed, interpreted to reflect mixing between 25 hydrothermal fluids and unmodified seawater. This process is inferred to be unfavorable for 26 mineralization. 27Homogenization temperatures, a reasonable proxy for true trapping temperatures in the orefield, 61The coincidence of structurally-controlled, high temperature reaction zones, brine-producing 62 footwalls, and hangingwall traps, with bacterial blooms above upwelling plumes of hydrothermal 63 fluids, can be interpreted as a self-organizing system that locally converged on ore-forming 64 conditions. Understanding the first-order structural control of the ore systems will therefore be 65 critical for predicting new deposits. 66The Irish orefield presents arguably the best database available on the thermal and chemical

Metastable Freezing: A New Method for the Estimation of Salinity in Aqueous Fluid Inclusions

“…In the hydrothermal fluids studied here, as in many crustal fluids (but excluding magmatic brines), there is a general increase in CaCl2 content with increasing salinity (Yardley, 2005;Wilkinson, 2010). However, there is no discernible shift to unexpectedly high degrees of supercooling in more saline inclusions, including those known to contain unusually high CaCl2, so that any such effect does not appear to be significant.…”

“…The samples used all derive from the Irish Zn--Pb orefield (Wilkinson, 2010(Wilkinson, , 2014 and include five host mineral phases (calcite, dolomite, barite, sphalerite and quartz) from the ore deposits and regional carbonate cements. All inclusions studied were aqueous, two--phase, liquid--vapor inclusions, 1.5--130 µm in maximum dimension (median = 8.4 µm), with a small proportion of vapor at room temperature (~5% by volume).…”

“…The full dataset was also subdivided into results obtained from the three different heating--freezing stage models in order to check there was no bias derived from this source. Finally, forty measurements of Tmf for the same test inclusion were used to evaluate the effect of cooling rate and were compared with a predicted Tmf based on the results from the full study of natural inclusions.Final ice melting temperatures ranged from --0.3 to --36.0°C (median = --12.4°C), with results below --21.2°C derived from very saline inclusions with elevated CaCl2 content (Wilkinson, 2010). This wide range reflects the diversity of fluid types in the Irish ore field and also the spread in salinities observed in the ore forming stages that has been attributed to fluid mixing (Samson and Russell, 1987;Wilkinson et al, 2005).…”

“…Multiple solid phase melting temperatures can also be combined to interpret compositions and fluid evolution processes in ternary compositional space, such as NaCl--KCl--H2O or H2O--FeCl2--NaCl in porphyry systems (e.g. Bloom, 1981;Lecumberri Sanchez et al, 2015); or NaCl--CaCl2--H2O in IOCG (Xu, 2000) and carbonate--replacement SEDEX deposits (Wilkinson, 2010).The determination of fluid salinity is particularly important because of the conservative nature of halogens, and particularly chloride, in crustal fluids. As a result, salinity (chlorinity) is one of the most useful general parameter for identifying fluid mixing processes, and allows quantification of mixing proportions.…”

“…Multiple solid phase melting temperatures can also be combined to interpret compositions and fluid evolution processes in ternary compositional space, such as NaCl--KCl--H2O or H2O--FeCl2--NaCl in porphyry systems (e.g. Bloom, 1981;Lecumberri Sanchez et al, 2015); or NaCl--CaCl2--H2O in IOCG (Xu, 2000) and carbonate--replacement SEDEX deposits (Wilkinson, 2010).…”