This report summarizes the results of geological mapping of the Thirtymile Range, Sheet 105 C/9, and an investigation of tin skarns at the Mindy (MINFILE 105 C 054) and Ork (MINFILE 105C 038) prospects. The Thirtymile range is a tectonic melange of Upper Palaeozoic and (? )Mesozoic siliciclastic and carbonate sediments, the more competent members of which survive as sheared and brecciated disrupted units or phacoids surrounded by highly foliated ultramylonite of slaty appearance. These have been deformed by later low angle thrusting, moderate angle faulting and extensional faulting and have been intruded by Mid-Cretaceous granitic plutons. Ting-tungsten-boron-fluorine-bearing skarns have formed in the aureoles of the granites.
A granitic stock about 6 km in diameter which occurs north of the Thirtymile Range, a smaller granite at the Ork prospect further south, and the fluorine-boron mineralized skarns at the Ork and Mindy prospects were mapped in detail. Initial petrographic and analytical work shows that the Mindy skarn horizons are predominantly pure diospide-andradite or diospide-actinolite ±pyrrhotite/arsenopyrite assemblages, with complex retrograde assemblages containing magnetite, magnesium borate/fluoride and cassiterite/tin borate mineralization. The Ork showing contains abundant fluorite and axinite but little metallic mineralization. Mapping indicates that extensional faulting played an important role in control of the tin-tungsten mineralization at the Mindy prospect.
