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The giant mascot ultramafite and its related ores McLeod, James Albert

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.

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