ORIGIN OF HIGH-GRADE QUARTZ-SCHEELITE VEINS IN THE CANTUNG MINE, NORTHWEST TERRITORIES, CANADA

The Cantung mine, NWT, Canada is a large (>4.2 Mt), high-grade (>1.6 wt. % WO₃) skarn developed in Lower Cambrian limestones intruded by a Late Cretaceous monzogranite stock.  Quartz veins crosscutting skarn in the open-pit orebody are scheelite-rich, containing up to 3.7 wt. % WO₃.  In 2003, newly developed portions of the underground E-zone skarn orebody revealed numerous small aplite dikes.  Where these dikes crosscut skarn ore, they appear to enhance W ore grades.  In some cases, vertical aplite dikes extend upward to become quartz veins, similar to those in the overlying open pit.  A fluid inclusion study was undertaken to document the P-T-X conditions of vein formation and elucidate the relationship between high-grade quartz-scheelite veins and aplite dikes.

Analysis of >500 fluid inclusions in quartz and scheelite revealed two main fluid types:  a H₂O-CO₂-CH₄-NaCl fluid, found as primary and pseudosecondary inclusions and thought to represent the ore fluid; and a H₂O-NaCl-CaCl₂ brine, found as pseudosecondary and secondary inclusions.  The CO₂-bearing fluids have XCO₂ values of 0.05 to 0.25, XCH₄ values up to 0.05, and total densities ranging from 0.75 to 1.02 g/cm³.  They have T_h values of 220-400^oC and their salinities range from 0.2-8.2 wt. % equiv. NaCl.  The brine inclusions have T_h values of 200-400 ^oC, salinities of 1.7-10.4 wt. % equiv. NaCl, and vary in density from 0.60 to 0.95 g/cm³.  Quartz-scheelite veins likely formed at temperatures of ~400 to 500^oC and pressures of ~2 to 3 kbar, in agreement with recent estimates from F-OH apatite-biotite thermometry of the skarn ore.

Scheelite-rich quartz veins from the open pit orebody contain fluids similar to those in aplite-related quartz veins from the underlying E-zone orebody, but have higher salinities (avg., 4.3 vs. 2.2 wt. % equiv. NaCl) and higher XCO₂+CH₄ values (avg., 0.15 vs. 0.09) than fluids contained in barren, skarn-related quartz veins from the E-zone orebody.  They are also higher salinity than fluids in apatite and quartz in skarn ores.  These differences are interpreted to indicate that the scheelite-rich quartz veins from the open pit orebody are not simply a result of leakage of skarn ore fluids along fractures, but instead represent a later magmatic-hydrothermal event, arguably related to aplite emplacement.