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UBC Theses and Dissertations

The timing and genesis of the Blackwater gold-silver deposit, central British Columbia : constraints from geology, geochronology and stable isotopes Looby, Erin Lynn


The Blackwater Au-Ag deposit of central British Columbia is hosted by a lithic-rich, latite lapilli tuff and felsic volcanic rock sequence. U/Pb zircon geochronology shows that these two volcanic rock sequences are Late-Cretaceous, and were deposited at ca. 73.5 and 73.1 Ma, respectively. These host rocks overlie plagioclase to hornblende porphyritic andesite lava flows, mudstone, chert and conglomerate. A monzonite intruded the western portion of the deposit. Felsic dykes or sills are inferred to have intruded felsic volcanic rocks at ca. 68 Ma. A post-mineralization, dacitic sill intruded contacts between sedimentary rocks and lithic-rich, latite tuffs in the south of the Blackwater deposit at ca. 53 Ma. High temperature (>300⁰C), biotite-sericite-sulfide, actinolite-sulfide and garnet-sulfide assemblages alter rocks at the deposit’s perimeter. Low temperature (250-350⁰C), Au-mineralizing fluids precipitated the early main-stage green sericite-quartz-pyrite and the late main-stage green sericite-chlorite-quartz-base metal sulfide assemblages located in the deposit’s centre. ⁴⁰Ar/³⁹Ar geochronology suggests that high temperature alteration minerals precipitated prior to Au-mineralization, between ~68-64 Ma, and that Au-mineralization took place at ca. 65.6-63.9 Ma. The ⁴⁰Ar/³⁹Ar systematics of biotite were reset at ca. 59 Ma during a thermal event. High temperature, barren alteration is attributed to fluids with stable isotope compositions distinct from those that precipitated low temperature alteration. High temperature alteration minerals are enriched in ¹⁸O relative to primary magmatic waters (δ¹⁸OH₂O = 8.9-12.6‰) and have variable δD values (δD =-93.9 to -66.1‰). This, in conjunction with depleted, sedimentary δ³⁴S values (δ³⁴Smineral =-17.7 to -7.7‰) suggests a combination of exchanged meteoric waters, sedimentary formation and/or magmatic fluid sources. A component of magmatic fluids ± evolved meteoric fluids is inferred for low temperature, Au mineralizing fluids. Calculated stable isotope compositions of fluids in equilibrium with green sericite (δ¹⁸O =6.6 - 7.1‰, δD = -79.2 to -62.9‰) plot within the primary magmatic water field while isotopic compositions of associated sulfides (δ³⁴Smineral =-1.8 - 0.3‰) provide evidence for igneous derived sulfur. Blackwater’s host rocks, tectonic environment, metal signature and alteration match those of an intermediate-sulfidation epithermal system with both low- and intermediate sulfidation state sulfide minerals.

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