Geoscience Map 1993-8: Geology of the Bridge River Map Area
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Geoscience Map 1993-8 presents the geology of the Bridge River map-area (92J/16) in southern British Columbia. The 1:50 000 scale geology map includes a legend and references.
The area is underlain by rocks of the Methow terrane, ophiolitic assemblages, Cadwallader and Bridge River terranes, and overlap assemblages. The Yalakom Mountain facies of the Methow terrane includes Jurassic and/or Cretaceous lithic arkosic sandstone, conglomerate, siltstone and shale; possible Lower and Middle Jurassic lithic-arkosic sandstone intercalated with lesser amounts of conglomerate, siltstone and shale; and Lower Cretaceous Jackass Mountain strata including a unit with lithic sandstone, conglomerate, and conglomeratic sandstone with lesser amounts of siltstone and shale, and a unit with arkosic sandstone, conglomeratic sandstone, siltstone, shale and conglomerate. Ophiolitic assemblages include the Shulaps Ultramafic complex and the Bralorne-East Liza complex. The Permian and younger and perhaps older Shulaps Ultramafic complex consists of a dominantly harzburgite unit and a serpentinite melange unit with knockers of gabbro, ultramafic rock, diorite, diabase, amphibolite, greenstone, roddingite, chert, phyllite, sandstone, conglomerate and limestone. The Permian Bralorne-East Liza complex contains pillowed and massive greenstones, greenstone breccia, diabase, gabbro and serpentinite. The oldest Cadwallader terrane rocks are of the Upper Triassic Cadwallader Group including a lower volcanic unit consisting mainly of greenstone, volcanic breccia and tuff; and the Hurley Formation sandstone, calcarenite, siltstone, and some conglomerate. There are also Lower to Middle Jurassic Last Creek Formation strata, mainly shale, siltstone, and siliceous argillite, and possibly Upper Jurassic to Lower Cretaceous siltstone and shale. Strata of the Bridge River complex of the Bridge River terrane range in age from Mississippian to Middle Jurassic. Units consist of biotite-quartz and biotite-chlorite-actinolite schist, calcareous actinolite schist, talc schist, metachert and marble; serpentinite with local slivers of all other Bridge River rock types; and ribbon chert, argillite, phyllite, quartz phyllite and pillowed to massive greenstone. Overlap assemblages include the Lower and/or Upper Cretaceous Silverquick Formation, with conglomerate, sandstone and siltstone; and the Tertiary Jones Creek volcanics, which are mainly porphyritic dacite and volcanic breccia.
Intrusive rocks in the area are of Eocene age. They include hornblende–biotite granodiorite, and hornblende-biotite-quartz-feldspar porphyry.
Deposition of the late Early Cretaceous rocks was coincident with the onset of regional compressional tectonism. Clasts within Silverquick conglomerate were derived from both the Bridge River complex and Cadwallader Group. The older Taylor Creek Group (not exposed in this map area) is the oldest unit within the Tyaughton basin and is documented to sit unconformably above Bridge River basement. This suggests the possibility that thrust imbrication (and accretion?) of the Bridge River and Cadwallader terranes was predominantly a late Early Cretaceous event.
The map area is cut by a northwest-trending system of dextral strike-slip faults that was active in Late Cretaceous time. Northerly trending splays of the Relay Creek-Marshall Creek fault system connect with the Yalakom fault system to define a large-scale extensional duplex structure. This fault system steps across and bounds the northwestern margin of the Shulaps ultramafic complex at its southeastern end. In contrast to the extensional zone to the northwest, the Shulaps complex may have been deformed and uplifted during Late Cretaceous movement on the bounding strike-slip fault systems.
Metallic mineral concentrations are within or adjacent to strike-slip faults or associated structures, and have a close spatial relationship with plutons or dykes. The age of mineralization seems closely tied to igneous activity between Late Cretaceous and Early Tertiary time. The protracted history of mineralization and plutonism combined with differential uplift across faults has led to the juxtaposition of deposits of contrasting structural level and local inconsistencies in the pattern of regional metal zoning.
The map area straddles the northern part of the Bridge River mining camp, which is British Columbia’s foremost historical gold producer from low sulphide mesothermal quartz veins. Other known and potential deposit types in the area are copper-molybdenum porphyries, pluton-associated high sulphide auriferous veins, skarns, scheelite-stibnite or stibnite-only veins, gold-silver polymetallic veins, epithermal gold-silver veins and cinnabar veins and disseminations. In addition, the ultramafic rocks have small chromite concentrations, and potential for jade, magnesite and chrysotile.
Cinnabar veinlets and disseminations occur locally near the Bridge River fault.
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