Bridge River Mining Camp Geology and Mineral Deposits
Paper 1995 - 3
Paper 1995-3 discusses the geology and mineral deposits of the Bridge River Mining camp (NTS 092/J, O). The area is located 185 kilometres north of Vancouver at Gold Bridge, and the accompanying map covers an area of 1500 square kilometres of mountainous terrain bounded by the Coast Range on the west and southwest, and the Shulaps Range on the northeast. The camp has more than 50 mineral localities including the Bralorne-Pioneer mining complex which retains the status as the foremost gold producer in British Columbia and the sixth largest producer in Canada. This study re-evaluates the geology of the camp, fits the numerous mineral occurrences to modern geological interpretations, and provides a synthesis of mineral potential and a framework for further exploration.
The rocks of the Bridge River mining camp comprise a variety of Paleozoic, Mesozoic and Tertiary volcanic and sedimentary strata and igneous intrusions. The Bralorne intrusions and Pioneer volcanic rocks are the most consistently mineralized rocks in the area and the granitic rocks of the Coast Plutonic Complex appear to have been the principal source of mineralizing solutions.
The geology of the camp records repeated cycles of deformation. The oldest rocks are strongly fragmented and intricately folded; spilitic greenschist metamorphism is common. Numerous slices and wedges of Cadwallader and Bridge River metamorphic rocks are found throughout the area testifying to a complicated tectonic history. The youngest units are weakly metamorphosed and block faulted.
It is believed that the imbrication of rocks from Cadwallader (Stikinia) and Bridge River (Cache Creek) terranes occurred at the time of plate collision. Faults and folds disrupt all the units and the general lack of stratigraphic markers makes it difficult to fully evaluate the structures. Although current studies allow tentative restoration of the ancient terranes the details remain controversial. The present map pattern mainly reflects Cretaceous and Tertiary tectonic activity. A relatively young 'slice fabric' dominates the region. This consists of panels of diverse rocks (including ramped blocks of older rocks) bounded by major northwest and north-trending faults of the Cadwallader and Yalakom fault systems, which mark the boundaries of the principal structural domains that have persisted through the emplacement of the late Cretaceous to Early Tertiary granitic plutons.
That parts of the Cadwallader and Bridge River suites were deposited penecontemporaneously in adjacent terranes is suggested by similar fossil assemblages and similar geochemical signatures of the volcanic rocks. These volcanic rocks are MORB-like tholeiites generated from rising mantle diapirs, possibly in a back-arc setting.
The Bralorne intrusions are small gabbro and diorite stocks mostly aligned along the Cadwallader break. Zircon from a coarse-grained phase near Gold Bridge yields a U-Pb date of 293±13 Ma, indicating that the intrusions are among the oldest rocks in the area. These rocks have silica contents in the range 45 to 55% (averaging 50.8%), similar to the Pioneer volcanics, but relatively high in magnesia and low in titania and iron oxides. The geochemistry is similar to that of rocks of ophiolitic affinity in the Thetford area of Quebec and in a general way to that of magmas of oceanic arc tholeiite association.
The Bridge River mining camp is known principally for mesothermal gold-quartz vein mineralization. An intricate system of fractures is thought to have controlled the movement of the ore-solutions; the most profound crustal breaks being the main solution channelways.
Mineralizing solutions in the Bridge River camp were originally considered to be magmatic, the result of differentiation of Bralorne gabbro and diorite that produced the soda granite (plagiogranite). However, it is now known that the Bralorne intrusions and associated ophiolite complex are Paleozoic and much older than the ore veins. Indeed, the age of mineralization at the Bralorne mine, determined by K-Ar dating of wallrock alteration, is 85.1 Ma. This is similar to the age of the nearby Gwyneth Lake satellitic stock, dated 85.9 Ma, and within the 69.5 to 98.4 Ma-zircon-dating age range of the adjacent Bendor pluton.
It is speculated that the stresses caused by intrusion of these granitic plutons resulted in shearing and the development of fissure veins. It is believed that an important part of this movement is manifest in reactivation of the Cadwallader fault zone, a pre-existing major break. The evidence suggests that emplacement of the Coast Plutonic Complex provided both the structural controls and the necessary thermal engine to drive the mineralizing solutions, which were of mixed connate and juvenile origin.
All printed publications of the BC Geological Survey are available digitally, free of charge, from this website.
For questions or more information on geology and minerals in British Columbia contact BCGS Mailbox or call toll free (BC Residents only).