Metal sources of the Navan carbonate-hosted base metal deposit, Ireland: Nd and Sr isotope evidence for deep hydrothermal convection

• Published in: Mineralium deposita Vol. 41; no. 8; pp. 803 - 819
• Main Authors: Walshaw, R. D., Menuge, J. F., Tyrrell, S.
• Format: Journal Article
• Language: English
• Published: Heidelberg Springer Nature B.V 01-12-2006
• Subjects: Brines; Calcite; Carbonate rocks; Carboniferous; Chemical analysis; Convection; Devonian; Dolomite; Fluid flow; Geology; Isotopes; Leaching; Limestone; Metals; Mineralization; Ordovician; Paleozoic; Precambrian; Ratios; Sandstone; Seawater; Sedimentary rocks; Sulfides; Water analysis
• ISSN: 0026-4598; 1432-1866
• DOI: 10.1007/s00126-006-0100-8

Nd and Sr isotope analyses are presented for gangue mineral samples from the giant carbonate-hosted Navan Zn-Pb deposit, Ireland, and for rocks from which Navan metals may have been derived. Analysis of gangue minerals spanning the Navan paragenetic sequence reveals systematic evolution in the composition of the mineralising fluid. Early fluid represented by replacive dolomite exhibits the lowest initial ^sup 87^Sr/^sup 86^Sr ratio (0.7083-0.7086), closest to that of the host limestone and to Lower Carboniferous seawater, and the highest ^sup 143^Nd/^sup 144^Nd ratio (0.51161-0.51176). Later generations of dolomite, barite and calcite, which encompass sulphide precipitation, have higher initial ^sup 87^Sr/^sup 86^Sr ratios (maximum 0.7105) and lower initial ^sup 143^Nd/^sup 144^Nd ratios (minimum 0.51157). All samples have initial Nd isotope ratios that are too low to have been acquired only from the host limestone. Drill core samples of presumed Ordovician volcanic and sedimentary rocks from beneath the Navan orebody have ^sup 143^Nd/^sup 144^Nd and ^sup 87^Sr/^sup 86^Sr ratios at the time of mineralisation of 0.51184-0.51217 and 0.7086-0.7138, respectively. The data are interpreted to indicate mixing of sulphide-rich, limestone-buffered brine, with a metal-bearing hydrothermal fluid, which had passed through sub-Carboniferous rocks, consistent with published fluid inclusion and S isotope data. The ^sup 143^Nd/^sup 144^Nd ratio of this basement-derived fluid is too low to have been imparted by flow through the Devonian Old Red Sandstone, as required in models of regional fluid flow in response to Hercynian uplift. Irrespective of whether such regional fluid flow occurred, the hydrothermal Nd must have been derived from sub-Devonian rocks. These conclusions broadly support the hydrothermal convection cell model in which brines, ultimately of surface origin, penetrated to a depth of several kilometres, leaching metals from the rocks through which they passed. The data also support increasing depth of penetration of convection cells with time. Metals were subsequently precipitated in carbonate rocks at sites of mixing with cooler, sulphide-rich fluids. However, comparison of the Navan hydrothermal gangue Nd-Sr isotope data with data from Lower Palaeozoic rocks strongly suggests that the latter cannot alone account for the "basement" signature. As the Navan deposit lies immediately north of the Iapetus Suture, this suggests that the Laurentian margin includes Precambrian basement.[PUBLICATION ABSTRACT]