Abstract:

The Whistle offset dyke comprises three syn-impact rock types: quartz diorite, inclusionbearing quartz diorite, and metabreccia, which were emplaced during or shortly after the ca. 1.85 Ga Sudbury event. Metabreccia, quartz diorite, and inclusion-bearing quartz diorite are mineralogicaily similar, consisting of plagioclase (32-49%), K-feldspar (10-30%), quartz (7-22%), chlorite (4-13%), epidote (0-8%), amphibole (2-15%), ± biotite, ± titanite, and ± apatite. All three syn-impact rocks have large irregular quartz grains surrounding acicular or tabular laths of plagioclase and smaller stout prismatic K-feldspar grains. On major element Harker diagrams, metabreccia invariably plots between felsic Levack gneiss and the more mafic Levack gneiss and metagabbro, and is similar in composition to monzogranite. Metabreccia is also similar in major element composition to quartz diorite and inclusion-bearing quartz diorite. Its trace-element composition overlaps with that of inclusion-bearing quartz diorite, which bridges the gap between metabreccia on one hand and quartz diorite and Sudbury Igneous complex (SIC) rocks on the other hand. Thus, combined petrographical, modal mineralogical, image analysis, mineral chemistry, and lithogeochemical observations imply that the three syn-impact rocks are similar.<br/>The syn-impact rocks have lower 207Pb/204Pb and 206Pb/204Pb values than the SIC Main Mass suggesting that they were derived, at least partially, from less radiogenic source rocks than the SIC Main Mass. Calculated model initial 207Pb/204Pb ratios suggest that quartz diorite assimilated lesser volumes of Archean basement rocks than inclusion-bearing quartz diorite and the even less radiogenic metabreccia. The three syn-impact rocks differ based on the volumes of Archean basement rocks that they incorporated or assimilated.<br/>Metabreccia likely formed as a product of quartz dioritic melts from the early impact melt sheet. These melts may have become contaminated by the incorporation and assimilation of basement rocks during injection of the melts along the offset. An equally possible interpretation is that quartz dioritic melts mixed with melted Archean basement rocks at the base of the impact melt sheet and later incorporated clasts of the basement rocks as these melts were injected in basement fractures that formed during isostatic readjustment of the crater floor. <br/>Metabreccia is one of the main hosts of Cu-Ni-PGE mineralization along the Whistle offset dyke. Petrographical, modal mineralogical, image analysis, mineral chemistry, and lithogeochemical observations do not suggest any significant differences<br/>between mineralized and unmineralized metabreccia. Hydrothermal activity associated with massive chalcopyrite veins resulted in a thin (~5 cm thick) chlorite-rich selvedge and the addition of quartz within 50 cm of the vein contacts. It did not significantly increase the footprint of the mineralizing system.