K-Ar and Rb-Sr dating and the genesis of tungsten at the Clea tungsten skarn property, Selwyn Mountains, Yukon Territory

CIM Bulletin, Vol. 73, No. 821, 1980

C.I. GODWIN, R.L ARMSTRONG and K.M. TOMPSON, Department of Geological Sciences, The University of British Columbia, Vancouver, B.C.

Tungsten skarn deposits on the Clea property in the Selwyn Mountains, Y. T., are related genetically to a quartz monzonite stock, about 500 metres in diameter at the surface. Lower Paleozoic sedimentary rocks are metamorphosed for a distance of 3 km from the stock. Tungsten-bearing skarn mineralization within the altered sedimentary rocks is of two types: sulphide-rich pods and calc-silicate beds. The most significant scheelite concentration is in calc-silicate beds near or adjacent to the quartz monzonite stock.Discordant K-Ar and Rb-Sr isotopic dates indicate that the stock is probably of early Late Cretaceous age, 94 Ma or slightly older, and that it cooled slowly. High initial 87Sr/86Sr ratios and disequilibrium relationships between K-feldspar phenocrysts and groundmass minerals in the porphyritic phase of the granitic rock, and between different granitic rock specimens, indicate contamination of crystallizing magma by old sialic continental crust which extends westward under the Selwyn Basin.This study is significant to regional tungsten exploration. Granitic rocks associated with tungsten deposits, in this part of the Canadian Cordillera at least, appear to have specific isotope characteristics that indicate they were generated at a specific time and in a particular way.
Keywords: Age dating, Tungsten, Clea deposit, Selwyn Mountains, Yukon Territory, Skarn deposits, Geochronology.