Brines in the Carboniferous Sydney Coalfield, Atlantic Canada

Formation waters within Upper Carboniferous sandstones in the sub-sea Princ e and Phalen coal mines, Nova Scotia, originated as residual evaporative fl uids, probably during the precipitation of Windsor Group (Lower Carbonifero us) salts which underlie the coal measures. Salinity varies from 7800 to 17 6,000 mg/l: and the waters are Na-Ca-Cl brines enriched in Ca. Sr and Br an d depleted in Na, K, Mg and SO4 relative to the seawater evaporation curve. Br:CI and Na:Cl ratios suggest that the brine composition corresponds to a n evaporation ratio of as much as 30, The brines lie close to the meteoric line on H/O isotopic plots but with a compositional range of delta O-18 fro m -4.18 to -6.99 and of deltaD from -42.4 to -23.5, distant from modern met eoric or ocean water. Mine water composition contrasts with that of nearby salt-spring brines, which are inferred to have originated through dissoluti on of Windsor Group evaporites by modern meteoric waters. However, a contri bution to the mine waters from halite dissolution and from Br in organic ma tter cannot be ruled out. Present concentrations of several elements in the brines can be explained by water-rock interaction. The original Windsor br ines probably moved up into the overlying coal-measure sandstones along fau lts, prior to the Late Triassic. The high salinity and irregular salinity d istribution in the Phalen sandstones suggests that the brines have undergon e only modest dilution and are virtually immobile. In contrast, Prince wate rs show a progressive increase in salinity with depth and are inferred to h ave mixed with surface waters. Basinal brines from which these modern forma tion fluids were derived may have been important agents in base-metal and B a mineralisation from the mid-Carboniferous onwards, as saline fluid inclus ions are common in Zn-Pb sulphide deposits in the region. (C) 2000 Elsevier Science Ltd. All rights reserved.