THE COMPOSITION OF BASE METAL MINERALIZING AND NON-MINERALIZING FLUIDS, PINE POINT, NORTHWEST TERRITORIES, CANADA

The Pine Point region is a classic metallogenic orefield that produced over 58 million short tons of Zn-Pb ore from approximately 40 base metal mineralized deposits hosted by Middle Devonian carbonates. The ore deposits are localized in paleokarstic features found in the epigenetic Presqu’ile dolomite that preferentially replaces some of the upper barrier limestones. Although economic mineralization was concentrated at Pine Point, there are base metal mineralized occurrences over a much wider area. This study forms part of the collaborative Targeted Geoscience Initiative that is attempting to assess the potential for undiscovered carbonate-hosted lead-zinc orebodies in southern Northwest Territories and northern Alberta. The primary aims of this part of the study is to characterize the chemistry of mineralizing brines responsible for base metal mineralization and hydrothermal carbonate phases in the North Western Territories and Alberta using fluid inclusion studies.

The main ore stage sulfides include galena, sphalerite, marcasite and pyrite. Fluid inclusion data from ore stage sphalerites and saddle dolomites indicate the mineralizing fluids are low temperature (<100°C), saline brines (25-35wt% CaCl2 + NaCl equivalent), with high Ca:Na ratios (20:1). Texturally late stage calcite, native sulfur and bitumen are also commonly observed in the deposits. Calcite is formed from a similar temperature, but lower salinity NaCl-CaCl2 fluid (15wt%).

Halogen, major and trace cation data were analysed from sphalerite, saddle dolomite and late calcite from the Pine Point deposit and nearby Westmin property. The halogen compositions of the mineralizing fluids indicate that the bulk of the fluids originated from highly evaporated seawater. The Na, K and Li compositions of the fluids have been modified on the flow path, possibly by interaction with feldspar-bearing sequences. The Ca, Sr and Ba compositions of the fluids are significantly higher than the concentrations expected for evaporated seawater. Ca has been added to the fluids either by dolomitization or by the dissolution of gypsum on the flow path.