Oxygen isotope evolution of volcanic rocks at the Sturgeon Lake volcanic complex, Ontario

Igneous zircons from the Sturgeon Lake volcanic complex, host to several ma ssive sulphide deposits in the Superior Province, Canada, have an average d elta(18)O(zircon) of 5.4 +/- 0.3 parts per thousand VSMOW (n = 9 rocks). Th ese zircons are from units differing in age by 18 million years in the 2.7 Ga complex. There is no detectable interaction of high delta(18)O, supracru stal lithologies in the magma. Quartz from volcanic units beneath the large st ore body, the Mattabi deposit, has an average delta(18)O of 9.3 +/- 0.6 parts per thousand. Quartz phenocrysts from the Mattabi unit and overlying volcanics have elevated and heterogeneous delta(18)O values averaging 13.8 +/- 0.9 parts per thousand and are not in magmatic equilibrium with zircons . The delta(18)O values of whole-rock powders range from 5.6 parts per thou sand to 14.3 parts per thousand and follow the trend observed in the delta( 18)O values of quartz. Healed microcracks are visible in cathodoluminescenc e images (but are not obvious optically) of quartz phenocrysts from units w ith high delta(18)O values and disequilibrium Delta(quartz-zircon) suggesti ng that recrystallization facilitates the elevation of delta(18)O. Quartz p henocrysts from volcanic units with Delta(quartz-zircon) values near equili brium at magmatic temperatures do not display healed microcracks in cathodo luminescence. The elevated delta(18)O(quartz) values are not restricted to units hosting orebodies, but are seen in all rocks in the volcanic stratigr aphy that postdate eruption of the Mattabi unit. Oxygen isotope ratios comb ined with physical volcanology studies suggest that impermeable volcanic la yers control the size and location of the many hydrothermal systems that ma y have occurred in the Sturgeon Lake complex.