FLUID CHARACTERIZATION AT PINE POINT: A MICROTHERMOMETRIC STUDY

Carbonate-hosted deposits are important worldwide producers of base metals. Exploration models for Mississippi Valley-type deposits link large-scale fluid flow, dolomitization processes and ore formation and are based on work carried out in the Pine Point ore district. This study focuses on constraining the composition of the ore mineralizing fluids at Pine Point through a detailed petrographic and microthermometric study.

The Pine Point ore district is located in the Northwest Territories of Canada. The strata-bound ore mineralization is hosted by a Middle Devonian barrier reef complex. The main ore minerals are sphalerite and galena and they are associated with pre-, syn- and post-ore dolomitization. The relationship between the dolomitization, ore mineralization and late-stage calcite is complex and poorly understood.

Microthermometry was performed on sphalerite, ore-stage saddle dolomite and late-stage calcite. Eutectic melting temperatures are c. –60°C, suggesting that the fluids are a complex mixture of Ca and Na salts. Hydrohalite melting temperatures from saddle dolomite, range from –56.5 to –31.0˚C, and indicate a weight fraction NaCl (NaCl/(NaCl + CaCl2) of <0.1 to 0.4. Hydrohalite melting temperatures from calcite range from –31.3 to –20.0˚C, giving a weight fraction of 0.4 to 0.9. Ice melting temperatures from sphalerite range from –17.7 to –11.3˚C and from saddle dolomites from –28.8 to –15.5˚C. Calcite-hosted inclusions have ice-melting temperatures from –9.0 to –4.6˚C. All the fluid inclusions homogenize to the liquid phase between 52 and 156˚C. Data from sphalerites and ore-related saddle dolomites indicate that the ore mineralizing fluid had an average temperature of 73 °C. Late-stage calcite-hosted inclusions have homogenization temperatures averaging 94˚C.

In summary, two fluids have been identified from this microthermometric study. The Type 1 fluid, found in sphalerite and dolomite, is a calcic brine with a range of salinities (15 to 28 wt% equiv. CaCl2-NaCl), has an average temperature of 73˚C, and was directly related to the ore mineralization at Pine Point. The Type 2 post-ore fluid, found in calcite, is a slightly higher temperature (avg. 94˚C), less saline (avg. 9 wt% NaCl-CaCl2) and a more Na-rich fluid.