A high-temperature hydrothermal origin for black dolomite matrix breccias in the Irish Zn-Pb orefield

Abstract

We make the first report of fluid inclusion data from a black dolomite matrix breccia from the Irish orefield which confirm a relatively high-temperature hydrothermal origin. Breccia matrix dolomite formed from NaCl-dominated fluids with temperatures primarily in the range 150 2208C and salinities of 13 20 wt.% NaCl equivalent, comparable with inferred ore-stage fluids at the Silvermines, Tynagh, Lisheen and Navan deposits. The spread in salinity is considered to be due to mixing of a moderate salinity (~12 wt.%), higher temperature fluid, probably derived from the Lower Palaeozoic metasedimentary basement, with a low temperature brine. Non-stoichiometric, black dolomites are therefore considered to be a product of relatively high-temperature hydrothermal processes involving the principal orefluid in Ireland, and thus their occurrence is believed to be a prime exploration indicator.Earlier, coarse white dolomite, texturally similar to regional dolomite cements commonly observed in the Irish Midlands, precipitated from fluids in the temperature range 140 2008C and salinities of 10.3 13.5 wt.% NaCl equivalent. This is comparable with the higher temperature, moderate salinity end-member inferred for the black dolomite. These data call into question a uniquely low-temperature burial origin for coarse white dolomite cements in Ireland. Paragenetically late low-Mg calcites were formed as a result of an influx of relatively low salinity fluids.

Base Metal Sulfide Mineralization in Lower Carboniferous Strata, Northwest Ireland

“…1). Wilkinson and Earls (2000), Wilkinson (2003), Wright et al (2004), Wilkinson et al (2005), and Johnson et al (2009) all provide similar fluid inclusion and halogen geochemical evidence suggesting the presence of as many as three seawater-derived fluids during epigenetic dolomitization and mineralization in the Irish Midlands.…”

“…Although the Th values have not been corrected for pressure effects, the temperature correction is thought to be small (<12°C according to Wilkinson and Earls, 2000). This small temperature variation does not significantly affect the calculated results or conclusions drawn from them.…”

“…Individual homogenization temperatures (Th), Eutectic (first observed melting) temperatures (Te), last ice-melting temperatures (Tm), and calculated salinities for fluid inclusions measured in this study are tabulated in Persellin (2009 minimum constraints on actual fluid temperatures. Based on previously published and their own work, Wilkinson and Earls (2000) estimated maximum pressure in the Irish ore fields during mineralization at approximately 230 bars, which indicates a pressure correction of only ~12°C. Figure 9 displays Th values plotted against wt.% NaCl equiv.…”

Constraints on the Origins of Fluids Forming Irish Zn-Pb-Ba Deposits: Evidence from the Composition of Fluid Inclusions

“…Constraints on the temperature and salinity of the ore fluids have been obtained from fluid inclusion studies of the Zn-Pb-Ba deposits, principally Silvermines (Samson and Russell, 1987), Tynagh (Banks and Russell, 1992), and from Ballinasloe, close to Tynagh (Wilkinson and Earls, 2000). Data from studies of other deposits, Lisheen Thompson et al, 1992;Eyre et al, 1996), Navan (Rizzi et al, 1993), Tatestown and Moyvoughly (Probert, 1983), and Abbeytown (Hitzman, 1986), are less extensive.…”

Hydrothermal recrystallization of the Lower Ordovician dolomite and its significance to reservoir in northern Tarim Basin

“…Accompanying with the magmatic-volcanic activities were the hydrothermal activities. Alteration by hydrothermal is a key factor for carbonate to develop into good reservoir bed, such as dissolution of carbonate [1][2][3][4][5][6][7], hydrothermal dolomitization of limestone [5][6][7][8][9][10][11], recrystallization of dolomite [5,10,12] and so on. Reservoirs associated with hydrothermal dolomite and hydrothermal alteration of dolomite were found in many basins, such as the West Canada Basin, the Michigan Basin, and the basins in the Middle East [7,9,[13][14][15].…”