Cross River sills, Rocky Mountains, southeastern British Columbia

Dioritic sills up to 40 m thick were discovered by G.M. Dawson along the Cross River in the 1880's (Dawson, 1886). The Cross River Sills intrude Middle Cambrian limestone, argillaceous limestone, calcareous argillite and argillite of the Mount Docking Formation (middle Chancellor Group). The sills have metmorphosed the adjacent strata and are themselves regionally metamorphosed, folded, and crosscut by numerous nearly perpendicular quartz veins that locally contain minor chalcopyrite. Recent mapping in the Kananaskis area (McMechan, 2011; McMechan and Leech, 2011) discovered similar sills in the Mount Docking Formation along the Albert River and at Fenwick Creek. These locally contain abundant pyrrhotite and are cut by numerous barren quartz veins. They are considered to be part of the Cross River suite on the basis of geological setting, general lithology and geochemistry. At Cross River a small zone of medium crystalline plagioclase-hornblende diorite was sampled for age dating. It contained no zircon. The hornblende had very low K content, excess Ar and produced highly disturbed spectra with non-interpretable results. Apatite fission track analysis indicates cooling in the early Tertiary and structural relationships require their intrusion by the Early Cretaceous. High quality multi-element chemical analyses are used to characterize the compositions of the Cross, Fenwick, and Albert sills, and dykes in the McKay Group from the Kananaskis area, the Fowler sill, Milford Group and Clachnacudain pluton from the Omineca Belt and ash beds in the Exshaw and Banff formations exposed in the Front Ranges. These are compared with published results for intrusions associated with diatreme breccias in the Kananaskis area. Standard geochemical variation diagrams portray homogeneous and variable major and minor, trace and rare earth element (REE) compositions and help indicate geochemical similarities and differences amongst these suites. Major element analyses indicate that many of the units are altered as indicated by their high loss on ignition values. Intrusive suites are mostly mafic with the exception of the dacite and rhyolite intrusive rocks in the Fowler suite and the trachyandesite and trachydacite of the Exshaw ash beds. The Albert sills are closest in major and minor element composition to the Cross and McKay intrusions. High field strength elements (HFSE) are less affected by alteration but similarly help group various suites and confirm the major element classifications. The Fowler and Cross suites have similar trace element compositions and compare closely with subalkaine, calc-alkaline, arc-type igneous rocks. The Fenwick, McKay, and Albert intrusions and alternatively Fowler and Cross sills overlap in compositional space on many of these plots but the Exshaw ash beds samples are distinct. All suites samples have continental affinities in K2O-TiO2-P2O5 space. Multi-element plots and normalized to primitive mantle compositions help confirm some similarities amongst suites and show effects of alteration processes. The Cross, McKay, and Albert suites have the closest similarities in overall multi-element patterns and the Fenwick and Laussedat dyke are also closely similar.

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