Geology and geochronometry of tin mineralization associated with the Seagull batholith, south-central Yukon Territory

CIM Bulletin, Vol. 76, No. 854, 1983

GONZALO MATO*, G. DITSON and COLIN GODWIN, The University of British Columbia, Vancouver, B.C.

The mid-Cretaceous Seagull batholith intruded Mississippian sedimentary and volcanic rocks and Jurassicf?) granitic and ultramafic rocks within the Omineca Crystalline Belt in the Yukon Territory near the British Columbia boundary. Leuco-cratic fluorine- and boron-rich granites of the batholith exhibit many textural variations from aplite to pegmatite, but are dominated by a coarse-grained seriate unit and a fine-grained porphyritic unit. Four distinctive tin vein occurrences within and near the batholith are described, the most interesting of which is the MC showing of quartz-calcite-chlorite-cassiterite veins in intensely fractured zones of sedimentary and volcanic rocks. Three other showings occur within the batholith:1) the EC showing of quartz-fluorite-cassiterite-sulphide veins with prominent muscovite-illite alteration;2)   the DU showing  of quartz-hematite-fluorite-sulphidecassiterite with considerable manganese staining and sericitic alteration; and3) the VAL showing of quartz-tourmaline-cassiterite-fluorite-arsenopyrite veins with little associated alteration. Most cassiterite occurrences are within easterly-trending, vertical structures and all occurrences are thought to represent parts of a single system. These mineralogical and physical variations are apparently related to the distance from the upper contact of the batholith. For the Seagull batholith, Rb-Sr isotopic data give a 100 ±1.4 Ma isochron age that is concordant with the K-Ar model age from biotite of 101 ±4 Ma. Initial Sr isotopic ratios for the batholith are high at 0.712, indicating an old, sialic crustal source for this pluton. This initial ratio is similar to values for granitic rocks associated with tungsten deposits in the Yukon Territory. The high initial ratio for Sr, the high SiO2 content, the high S/I index, the presence of normative corundum, leucogranitic composition and associated lithophile-tungsten mineralization demonstrate that the Seagull batholith is probably an S-type granitic body. This factor is significant to regional exploration for tin and tungsten in the Canadian Cordillera.

Keywords: Economic geology, Tin mineralization, Seagull batholith, Yukon Territory, Tungsten, Canadian Cordillera, MC showing, EC showing, DU showing, VAL showing.