Gold Mineralization at the Cantung W-Cu skarn: Our Present Level of UnderstandingThursday, November 21, 2019 - 10:40am to 11:00am Theatre One
Located within the Canadian Cordillera in the Northwest Territories and approximately 400 km to the northeast of Whitehorse, Yukon Territory is the gold-bearing Cantung W-Cu skarn. It is genetically associated with a group of Mid-Cretaceous felsic plutonic suites known as the Tombstone-Tungsten suite, which intruded into the ancient North American continental margin resulting in the formation of a range of types of magmatic-hydrothermal systems. At Cantung, a peraluminous biotite monzogranite intruded into Neoproterozoic to Paleozoic passive margin sedimentary rocks belonging to the Selwynn Basin.
Gold was identified in samples from this study within both sheeted quartz veins and hydrous (Actinolite-Biotite-Pyrrhotite) skarn. Previous researchers have extensively the studied fluid inclusion assemblages from these quartz veins and the results show generally two types of inclusions: H2O-CO2-NaCl±CH4 inclusions along with aqueous brine inclusions. Early aqueous-carbonic inclusions followed by mild-moderate salinity aqueous brine inclusions is often observed within some of the world’s most well-known intrusion-related gold deposits.
Previous research on fluid inclusion assemblages from the hydrous skarn are also extensive, however interpreting the results are slightly more problematic. Early interpretations on the temporal relationship between each of the different skarn assemblages suggested they formed as part of a single mineralizing event. The dominant fluid inclusion assemblages found in most skarn minerals were non boiling mild-moderate salinity brines, but H2O-CO2-NaCl±CH4 fluid inclusion assemblages were also identified, with those from hydrous skarn assemblages having the highest concentrations of CH4.
Gold within both the sheeted quartz veins and the hydrous skarn both occur along with an assemblage containing native bismuth, bismuth tellurides and selenides, as well as various sulfosalts. The textures exhibited by this assemblage suggests they precipitated from hydrothermal fluids as low-temperature polymetallic melts, often remaining molten until long after the growth of silicates had ceased. The capacity of these Bi-Te-Se-S melts to precipitate directly from hydrothermal fluids and their potential to sequester gold from fluids which are undersaturated with respect to gold has been reproduced using thermodynamic calculations as well as experimentally. The data collected by other researchers in combination with our new data has helped produce an updated model of the timing of multiple mineralizing events and their potential relationships to the nearby intrusives. The results of U-Pb geochronology suggest that formation of the W-Au mineralized sheeted quartz veins preceded the formation of all skarn facies. They also suggest that the various skarn assemblages were not part of a single hydrothermal event, but rather each comprised a distinct mineralizing event with the gold-rich hydrous skarn being the youngest. These results highlight just how much more work is required to fully comprehend all the aspects of the magmatic-hydrothermal history at Cantung.