Geochemistry of surface waters associated with an undisturbed Zn-Pb massive sulfide deposit: Water-rock reactions, solute sources and the role of trace carbonate

摘要

We report results of a detailed geochemical study of surface waters from several streams in a small catchment in northern New Brunswick, Canada. Rocks in the catchment represent metamorphosed (greenschist facies) felsic volcanic rocks, metavolcaniclastic sediments, and mafic volcanic rocks; no massive carbonate or evaporite lithologies are present, thus providing an excellent opportunity to investigate the relative influence of silicate weathering compared to trace carbonate (vein and disseminated) dissolution and the influence of volcanogenic massive sulfide (VMS) mineralization on surface water chemistry. Surface waters, catchment lithologies and stream sediments were analyzed for a full suite of major and trace elements. Surface waters are dilute (typically <60mg/L total dissolved solids), and are of dominantly Ca-HCO_3-type. Most waters have Ca/Na_(molar)>1. The major ion chemistry of the waters is consistent with binary mixing between silicate weathering and dissolution of trace calcite; Si/Ca_(molar) relationships suggest that trace calcic silicates are insignificant compared to calcite as a Ca source, and PO_4 concentrations of waters are too low for apatite to be a major source of Ca. Host rocks have highly variable Ca/Na, Mg/Na and K/Na values complicating the assignment of a silicate end-member. Geochemical modeling indicates that surface waters range from essentially 100% of the Na and Ca being derived by silicate weathering, to being dominantly controlled by trace calcite, also consistent with mass balance calculations. The Cl/Ca ratios are consistent with the host felsic and mafic metavolcanic rocks as being an important source of Cl in addition to precipitation. The host lithologies have much larger variations in Mg/Na, Ca/Na, K/Na, and Sr/Na than the stream sediments and waters and the sediments are shifted to higher Mg/Na and K/Na and lower Ca/Na than the waters. It is remarkable that the waters have such small variations in major ion ratios relative to the host rocks, indicating that the controls on solute loads of these streams is more a function of relative elemental solubility (Ca>Mg>Na), secondary mineral formation (e.g. Mg- and K-rich clays), incongruent dissolution, and water-rock reactions than end-member rock compositions. Oxidation of massive and disseminated sulfide mineralization accounts for on average 60% of the dissolved sulfate. Calculations indicate that sulfide oxidation (sulfuric acid) weathering accounts for around 20% of the cation flux in the watershed. On average 12% of the Ca and 72% of the Mg are derived from silicate weathering, although cationic silicate denudation rates are only 1.88tonnes/km~2/year compared to 4.16tonnes/km~2/year for trace carbonate dissolution. The total cation denudation rate and CO_2 consumption rate are similar to watersheds draining volcanic rocks in the Western Canadian Cordillera.