CONTROLS ON SPRING GEOCHEMICAL PROCESSES IN THE SOUTH NAHANNI RIVER BASIN, MACKENZIE MOUNTAINS, NORTHWEST TERRITORIES, CANADA

Geochemical analysis of over 160 spring waters were sampled within a 30000 km² region of the South Nahanni River basin (SNRB), Mackenzie Mountains, Northwest Territories, Canada, to assess the economic mineral potential of the remote region which is being considered for National Park designation.  The SNRB area is dominated by carbonates and encompasses two main geological regions:  1) the relatively flat eastern region which is characterized by broadly folded karstic terrain, shallowly-incised topography, and inferred faults; and 2) the high mountains of the western region with a more complex structure in addition to igneous intrusions of the Cretaceous Selwyn Plutonic Suite, and strong glacially-eroded terrain. 

 

Within the study area a broad range of springs are observed (thermal, acidic, saline, high pCO₂, and combinations thereof), and stable isotope data indicate that they are all meteoric in origin.  The springs are classified into basic geochemical types, where each group can be associated with dominant rock types and processes of water/rock interaction.  The major processes include sulfide oxidation, carbonate dissolution and contact-metamorphism near the plutons.  Typical characteristics include: 1) metaliferous, low pH and high sulfate springs associated with shales and pyritic sandstones; 2) calcium carbonate dominated geochemistry, variable total dissolved solids (TDS), and only a few detectable trace elements in karst-dominated areas; and 3) hot or warm springs with variable flow rates, low TDS, high pH, sodium bicarbonate dominated, and including some high-pCO₂ springs in association with plutons.  Importantly, pluton-associated springs have low concentrations of major ions but are relatively high in trace elements, similar to regions of known base metal mineralization. 

 

Outstanding natural features are dictated by the water chemistry.  Low pH springs form extensive red travertine mounds of iron oxides.  Carbonate dominated springs can form extremely large travertine mounds of carbonate minerals.  High temperature springs often have large microbial mats dominated by cyanobacteria, in addition to thiotrix and purple sulphur bacteria.