The Gays River Zn-Pb deposit is hosted in a Carboniferous carbonate bank on the southern marginal platform of the major (>8 km deep) Fundy-Magdalen basin. Fluid inclusion homogenization temperatures (130 degrees -215 degrees C) and structural control of the mineralization (determined in our previous study of the immediate mine area) suggest affinities with both Mississippi Valley-type and Irish-type deposits. New isotopic work, extending to within a 2-km radius of the mine area, significantly constrains genetic hypotheses for the deposit. Preliminary hydrogen isotope measurements of inclusion fluids, together with calculated oxygen isotope compositions, suggest that the mineralizing fluids were basinal brines (delta D = -39ppm; delta 18 O = +3.3ppm). Strontium isotope composition of ore-stage calcite ( 87 Sr/ 86 Sr = 0.7117) indicates that components of the strontium and calcium had a source considerably more radiogenic than Mississippian carbonate or evaporite ( 87 Sr/ 86 Sr = 0.7085-0.7090). These and limited lead isotope data suggest that the fluids had a strontium and metal source in Cambro-Ordovician basement rocks or their immediate clastic derivatives.Sulfur, probably derived from the voluminous Mississippian evaporites (delta 34 S = 14 + or - 1ppm) was probably carried as sulfate (in a fluid with neutral pH) to the depositional site where complete (nonbiogenic?) reduction (at [asymp] 200 degrees C) occurred. This is supported by the lack of extensive dissolution and brecciation and the close similarity of the delta 34 S (sub Sigma S) value for the mineralizing fluids and the delta 34 S value of the evaporites. The carbon budget of the ore-forming fluids was dominated by oxidized carbon from a carbonate source with delta 13 C value near 2 per mil, locally influenced by organic carbon with low delta 13 C values which is abundant in the Gays River carbonate bank.A basinal brine expulsion model is favored. The hot mineralizing fluid must have originated at a >4-km depth and interacted with elastic, carbonate, and evaporite sedimentary rocks on its way to the depositional site. The likely age for this mineralizing event is approximately 300 m.y.