Oxygen Isotope Evolution of Volcanic Rocks at the Sturgeon Lake Volcanic Complex, Ontario

King, E.M., Valley, J.W., and Davis, D.W. (2000) Oxygen isotope evolution of volcanic rocks at the Sturgeon Lake volcanic complex, Ontario. Canadian Journal of Earth Science, v. 37, p. 39-50.

Abstract

Igneous zircons from the Sturgeon Lake volcanic complex, host to several massive sulphide deposits in the Superior Province, Canada, have an average d¹⁸O(Zircon) of 5.4±0.3‰ VSMOW (n=9 rocks). These zircons are from units differing in age by 18 m.y. in the 2.7 Ga complex. There is no detectable interaction of high d¹⁸O, supracrustal lithologies in the magma. Quartz from volcanic units beneath the largest ore body, the Mattabi deposit, has an average d¹⁸O of 9.3±0.6‰. Quartz phenocrysts from the Mattabi unit and overlying volcanics have elevated and heterogeneous d¹⁸O values averaging 13.8±0.9‰ and are not in magmatic equilibrium with zircons. The d¹⁸O values of whole-rock powders range from 5.6 to 14.3‰ and follow the trend observed in the d¹⁸O values of quartz. Healed microcracks are visible in cathodoluminescence images (but are not obvious optically) of quartz phenocrysts from units with high d¹⁸O values and disequilibrium D (Quartz-Zircon) suggesting that recrystallization facilitates the elevation of d¹⁸O. Quartz phenocrysts from volcanic units with D (Qtz-Zrc) values near equilibrium at magmatic temperatures do not display healed microcracks in cathodoluminescence. The elevated d¹⁸O(Qtz) values are not restricted to units hosting ore bodies, but are seen in all rocks in the volcanic stratigraphy that postdate eruption of the Mattabi unit. Oxygen isotope ratios combined with physical volcanology studies suggest that impermeable volcanic layers control the size and location of the many hydrothermal systems that may have occurred in the Sturgeon Lake complex.

Figure 2: Histogram of average d18O values for igneous zircons from Sturgeon Lake volcanic complex, Kidd Creek mine, Superior Province plutons (King et al. 1998b) and Superior Province volcanics (King 1997). The unenriched intrusives are tonalite to granodiorite plutons.

Figure 3: Plot of d18O values for igneous zircons, quartz phenocrysts, multi-grain and bulk quartz analyses. "Calculated quartz" values represent equilibrium with d18O(Zrc) at 800 C. Samples are arranged oldest to youngest (left to right). BB, Beidelman Bay; BR, Bell River; BRL, Bell River Lake; D, Darkwater; HLL, High Level Lake; JL, Jackpot Lake; LC, Lyon Creek; LL, Lower L Succession; LLA, Lyon Lake andesite; PLG, Pike Lake gabbro; ML, Middle L succession; NNL, No Name Lake; SL, Swamp Lake; StL, Sturgeon Lake; TL, Tailings Lake. The solid vertical lines correspond to divisions of the volcanic complex, subvolcanic volcanic intrusions, pre-caldera rocks, early caldera rocks, late caldera rocks and post-caldera rocks. The stars above unit names indicate units that host ore deposits. For duplicated analyses, all data and associated error bars fall within symbol.