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Geology of the Topley intrusives in the Endako area British Columbia Bright, Edward Gordon

Abstract

Granitic rocks of the Topley intrusives underlying 100 square miles, centered around Endako, north central British Columbia, form part of a composite body which lies in a northwest trending belt of granitic plutons extending from Babine Lake to Quesnel, a distance of 180 miles. Rocks of the Topley complex in the Endako area have intruded volcanic rocks of the Takla Group (Upper Triassic to Jurassic) and are unconformably overlain by flat-lying Tertiary volcanic flows. These intrusions are probably of Lower Jurassic to Lower Cretaceous age. The granitic rocks, ranging from diorite to alaskite form six large separately emplaced and sharply bounded intrusive bodies. The development of the Topley complex can be divided chronologically, from oldest to youngest, into five major stages: (I) the Simon Bay (diorite) complex; (II) the Endako quartz monzonite which is the host rock of the Endako molybdenum deposit, and the Franco is granite; (III) the Glennanan complex; (IV) the Casey quartz monzonite-alaskite; and (V) the Stellako quartz monzonite. The Glennanan complex, which is the best exposed and most intensely studied intrusive, consists of two large asymmetrically zoned bodies and a small stock of porphyritic quartz monzonite. The larger zoned body near the town of Endako has: (1) a western zone of porphyritic granite; (2) an intermediate zone of porphyritic quartz monzonite; and (3) an eastern zone of porphyritic granodiorite. A granodiorite zone is absent in the asymmetrically zoned Nithi Mountain intrusive body. All internal contact are gradational and all rock zones contain large rectangular perthite megacrysts which enclose oriented crystals of plagioclase and other minerals. The development of the rock zones can be accounted for by differentiation of a hornblende-biotite granodiorite magma at depth and successive intrusion of first granite, then quartz monzonite and finally granodiorite. Topley stages II to V represent a continuous period of epizonal intrusion following the emplacement of the more deep seated (mesozonal?) Simon Bay complex. Northwesterly and northeasterly trending fracture zones controlled the structural evolution of the Topley complex in the map area.

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