Geology of the Indian River area, southwestern British Columbia
Creators: name: Reddy, Douglas Gerald
Publisher: name: University of British Columbia
Year: 2010

description: The Britannia - Indian River pendant is a composite of volcanic and sedimentary units within the Coast Plutonic Complex, southwestern British Columbia. Geology of the Indian River valley consists of a rhyolitic to basaltic calc-alkaline suite of volcanic flows and tuffs interbedded with shallow marine sedimentary rocks. The pendant is within Wrangellia and has been assigned to the Gambier Group of Late Jurassic to Early Cretaceous age. K-Ar analyses indicate three major thermal events took place in the Britannia -Indian River pendant: (1) a late Early Cretaceous contact metamorphism (108 ± 4 Ma), (2) an early Late Cretaceous regional metamorphic reset associated with emplacement of granitoid plutons (96.1 ± 3.0 Ma, 95.6 ± 3.3 Ma), and (3) a Late Cretaceous (83.5 ± 3.0 Ma, 84.2 ± 2.9 Ma) metamorphic reset due to a deformational and/or intrusive event. A poorly defined whole rock Rb-Sr isochron from seven fresh-looking volcanic units indicates a 102 ± 10 Ma age that also probably reflects metamorphic reset. An internal Rb-Sr isochron comprising partial mineral separates from one sample yielded 93 ± 3 Ma, which supports the regional metamorphic reset. Younger dykes and sills are dated as Early Oligocene (36.1 ± 1.3 Ma). These Tertiary intrusives are the same age as dykes in the city of Vancouver and indicate a more widespread magmatic event than previously recognized. The stratigraphic section in the Indian River and Stawamus River valleys consists of more than 2,850 metres, and comprises seven units that trend northwesterly and dip moderately south or southwest. A change in the overall strike from northwest in the Indian River valley to west in the Stawamus Valley suggests either: (1) an angular unconformity within unit 4a, (2) the existence of a major shear zone in the Stawamus River valley, or (3) warping of the strata due to emplacement of the plutonic bodies. The stratigraphy in the Indian River area forms the western limb of a broad northwesterly trending antiform, overturned to the northeast. Along the Indian River a smaller anticline has been disrupted by several faults. These northwest trending faults are the northern extension of the Indian River shear zone. The stratigraphic units are mainly subaqueous felsic to intermediate pyroclastic rocks, felsic and intermediate to mafic flows, and sedimentary rocks including cherts, argillites and greywackes. Major and trace element chemistry of volcanic units indicates the calc-alkaline rocks are dominantly rhyolite and basaltic andesite. Mafic units on Sky Pilot Mountain have a "borderline" tholeiitic - calc-alkaline character.Late Cretaceous lower greenschist facies metamorphism is related to emplacement of Coast Plutonic Intrusives. Intense cordierite-biotite contact metamorphism post-dates mineralization in the Slumach zone: a polymetallic quartz-chlorite vein with anomalous gold values. The War Eagle zone is a low grade volcanogenic system containing remobilized sulphides. Galena lead isotopic analyses of volcanogenic prospects in the Indian River valley are uniform and are less radiogenic than those of the Britannia volcanogenic ore bodies 10 kilometres to the west. The Indian River portion of the Britannia - Indian River pendant is proposed to be Late Jurassic in age while the Britannia area is Late Jurassic to Early Cretaceous.