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Abstract

The Giant Mascot Ultramafite is on the southeastern flank of the Coast Crystall ine Belt 100 miles east of Vancouver, B.C. It is roughly elliptical (1 x 2 mi.), crudely zoned ranging from peridotite through pyroxenite and horn- blende-pyroxenite to marginal hornblendite. It is enclosed in dioritic rocks of the Spuzzum Intrusions. Twenty-eight pipe-like orebodies within the ultramafite have been mined for their massive Ni-Cu ores. Studies on the 3050 level cross-cut show that silicates and sulfides in pyroxenites differ in composition from those in perldotites. Furthermore, pent- landite from orebodies differs in composition from accessory pentlandite in ultrabasic rocks. K-Ar dating yields minimum ages for orebodies and ultramafite ranging from 104 m.y. to 119 m.y, Hornblendite dykes, the youngest rocks in the Eiine are dated at 95 m.y. K-Ar ages of Spuzzum Diorite near the mine and several miles to the south are 89 m.y, Tonalite, a border phase of the Spuzzum Intrusions, yields a hornblende age of 85 m.y. and a biotite age of 79 m.y., consistent with earlier investigations. Temperatures calculated from 15 coexisting clino- and orthopyroxene pairs from the 3050 level cross-cut average 990°C for equilibration of these silicates. Distribution coefficients (mean K[sub D] = 0.738) for these silicates suggest a magmatic origin. The Climax and Chinaman orebodies on the 3050 level cross-cut are steeply plunging pipe-like, bodies with higher grade sections concentrated in the trough or footwall. They are found at peridotite-pyroxenite contacts and appear spatially related to norite. It is concluded that the Giant Mascot Ultramafite originated by cliapiric re-emplacement of crudely stratiform crystal mushes and sulfide melts from a differentiating sub-volcanic magma chamber, possibly an early phase of Spuzzum magmatic activity. This material was subsequently engulfed by rising Spuzzum dioritic magmas which superimposed a hornblendite rim on the ultramafite.