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UBC Theses and Dissertations

Geology and genesis of the midway silver-lead-zinc deposit, north-central British Columbia Bradford, John Allan


The Midway Ag-Pb-Zn manto deposit is hosted in Middle Devonian McDame Group limestones of the Cassiar Terrane, a displaced segment of the North American miogeocline. In the map area (1040/16), platformal carbonates and siliciclastics of Cambrian to Devonian age are unconformably overlain by Devono-Mississipian basinal sediments, including exhalites (Earn Group). These are structurally overlain, above a flat lying decollement, by Upper Paleozoic marginal basin sediments, basaltic volcanics, intrusives and ultramafites (Sylvester allochthon). The decollement represents the boundary between Cassiar Terrane and Slide Mountain Terrane in the map area. Jurassic contraction of the North American margin and emplacement of the Sylvester allochthon produced southeasterly trending major and minor structures. Cretaceous - Tertiary extension, linked to transcurrent motion along the Kechika and related faults, produced the north trending high angle Tootsee River fault zone, which divides the map area. Two intrusive suites, of mid-Cretaceous (100 Ma) and Late Cretaceous (70 Ma) ages, are documented by K-Ar dating. The Late Cretaceous episode is evidenced by large sericite - pyrite alteration zones southeast of Midway, but only a few felsic porphyry dykes are exposed at current erosion levels. High angle faults in the Tootsee River fault zone confined intrusion driven hydrothermal fluid migration up plunge from an intrusive centre about 2 kilometres southeast of Midway. Fluids were ponded in a southeasterly plunging antiformal trap, and massive sulphides deposited in zones of pre-existing karst breccia porosity beneath a relatively impermeable shale cap. Fe and Cu rich pyrrhotitic assemblages occur at greater depth toward the intrusive centre, with sulphosalt rich pyritic assemblages at shallower depths to the north. Immiscible aqueous - carbonic liquid and vapour were trapped in quartz deposited with sulphides at 300-340OC. Later lower temperature inclusions record the progressive loss of CH₄ and CO₂ from the system. Salinities were about 7-10 weight % NaCl equivalent. Sulphur isotopes of sulphides span a narrow range, indicating a well mixed reservoir and H₂S dominated environment. Both sedimentary and igneous sources are indicated. Oxygen isotopes of carbonates demonstrate large ¹⁸O depletions due to extensive meteoric water interaction. In a regional context (Rancheria silver district), Midway lead isotopes suggest mixing of upper crustal lead from country rocks with intrusion derived lower crustal lead. Cassiar Terrane epigenetic deposits are distinguished from epigenetic deposits of Slide Mountain Terrane, which have significant mantle lead, consistent with derivation from oceanic and arc host rock assemblages.

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