Skarn mineral chemistry and implications for high-grade tungsten mineralization in the Canadian CordilleraThursday, November 21, 2019 - 11:20am to 11:40am Theatre One
The Canadian Cordillera is host to numerous ore deposits related with major magmatic events that occurred from Early Jurassic to Eocene-Oligocene. Among these are the Cantung, Mactung and Lened tungsten-bearing skarn deposits, which are spatially associated with the mid-Cretaceous Tungsten plutonic suite of the Tombstone-Tungsten magmatic belt.
The composition of skarn minerals can be used to distinguish between different skarn types, environment of formation, and the redox conditions that prevailed during the skarn formation. For example, the effect of oxygen fugacity during high temperature skarn mineralization produces hedenbergite (Fe2+-rich pyroxene) and grossular (Ca-rich garnet) in reduced environments, and diopside (Mg-rich pyroxene) and andradite (Fe3+-rich garnet) in oxidized environments. The abundance of sulfide minerals and their sulfidation state also indicate the redox conditions of the skarns; large amounts of pyrrhotite are correlated with reduced skarns, whereas large amounts of pyrite indicate a relatively oxidized skarn. Micropobe analysis undertaken in this study reveal that most of the skarn minerals at Cantung, Mactung and Lened have compositions indicative of reduced skarns: garnet is grossular-rich with a minor andradite component, and pyroxene is hedenbergite-rich with minor diopside solid solution. The widespread occurrence of pyrrhotite in the three deposits supports a reduced environment during skarn formation. However, nearly pure diopsidic pyroxene also occurs in one of the pyroxene facies of Lened, which is indicative of an oxidized environment. In this specific facies, diopside is associated with Ba-rich feldspar (celsian, ~36wt% Ba) and Ba-F-rich biotite (Ba 11-16 wt%, F ~3wt %). Considering these observations, we suggest that local oxidized conditions may have prevailed at Lened, possibly related to an influx of LILE-rich fluid in the system.
In general, oxidized tungsten skarns are relatively smaller deposits than the reduced skarns. Based on current resource estimates, Cantung (3.8Mt at 0.97%WO3) and Mactung (33Mt at 0.88%WO3) are larger and richer deposits than Lened (0.9Mt at 1.0% WO3), even though they are all associated with the same magmatic event and roughly the same type of host rock. While continued exploration may further refine and modify the deposit metal grades, the mineralogical evidence for oxidized conditions could explain why Lened is less rich. A better constraint of the extent of these oxidized skarns and how they fit in the evolution of the Lened deposit is required to determine their impact on the size and fertility of Lened.