Yellowknife Geoscience Forum

Magmatic-hydrothermal systems are associated with a wide range of geodynamic settings that can be related to critical economic resources. Tungsten skarns deposits are formed by the magmatic-hydrothermal processes involving the interaction of metal-rich fluids issued from a highly differentiated pluton with the surrounding calcareous country rocks. In tungsten skarn deposits, the magmatic-hydrothermal transition is directly followed by tourmalinization, often visible on the margins of scheelite-mineralized quartz veins. Tourmaline, which is the most common boron-bearing mineral, is ubiquitous in hydrothermal ore deposits and is useful as a tracer of magmatic-hydrothermal processes. However, whether boron plays an active role in the fertility of magmatic systems remains unclear. The magmatic-hydrothermal processes that control the timing of tourmaline crystallization and its relationship to scheelite mineralization are still to be defined.

In this study, the properties of the most primitive fluids identified in the Lened W skarn deposit, Northwest Territories, Canada, have been used to i) model the compositional evolution of fluids during fractional crystallization of the source magma, and ii) determine its impact on both the magmatic and hydrothermal processes in this system. Lened is one of three skarn deposits that comprise the W belt of the Canadian Cordillera, and it is the most boron-rich deposit of the system. The Lened deposit comprises tungsten-rich (scheelite) calc-silicate skarn and quartz veins, as well as emerald prospects, which are spatially related to a multiphase two-mica granite pluton. Tungsten mineralization is hosted both by the limestone facies and the pluton. The most primitive fluid identified in quartz at Lened is characterized by a dominant carbonic phase and a low-salinity aqueous phase with sassolite (H₃BO₃) daughter crystals, suggesting the system CO₂-H₂O-NaCl-H₃BO₃. Microthermometry, Raman spectrometry, and LA-ICP-MS results are used to refine the fluid salinity and solute composition.

Boron (0.5 wt. % B in this study) increases the H₂O solubility of a melt, and decreases melt viscosity, allowing residual magmas to crystallize at temperatures down to 600°C. Decreasing the viscosity of the melt as well as its solidus temperature (i) facilitates crystal-melt separation and therefore fractionation, enhancing extreme rare-element enrichment in the residual melts, and (ii) enhances the generation, extraction, and migration of the melt from its source region.

From our observations and previous studies, this study models the composition of fluids associated with a boron-rich melt during fractional crystallization and aims to constrain tungsten behavior as the melt crystallizes.