Early Late Cretaceous-early Tertiary gold mineralization; a galena lead isotope study of the Bridge River mining camp, southwestern British Columbia, Canada

Abstract

The Bridge River gold camp, 180 km north of Vancouver, Canada, produced more gold than any other camp in British Columbia over its 70 years of operation (130 metric tons or 4 million oz), mainly from the Bralorne and Pioneer deposits. A compilation of new and existing lead isotope ratios from 20 widespread and diverse deposits in the Bridge River camp shows a distinct clustering of data. This is interpreted to be the expression of a single protracted but episodic mineralizing event coinciding with the emplacement of the Coast Plutonic Complex during early Late Cretaceous to early Tertiary time (90-45 Ma). The timing of this event is constrained at Bralorne by a premineral albitite dike set (91.4 + or - 1.4 Ma by U-Pb on zircon) and a late intramineral hornblende porphyry dike set (85.7 + or - 3 Ma by K-Ar on hornblende). Late lamprophyre dikes that crosscut the mineralized veins, dated at 43.5 + or - 1.5 Ma by K-Ar on biotite, set a lower age limit for the mineralization. The host rocks, the Bralorne intrusions, have no genetic relation to the mineralization since they are Early Permian (270 + or - 5 Ma by U-Pb on zircon).The conclusion that all mineralization is related to a coherent event is in accord with recognition of a districtwide mineral zoning scheme by Woodsworth, Pearson, and Sinclair. In this scheme, a sequence from high-temperature Au-As-W-Mo through intermediate Sb-Ag-Au-As to low-temperature Sb-Hg vein assemblages as generated from southwest to northeast within the thermal aureole of the Coast Plutonic Complex. Stable isotope and temperature data from other workers support this northeastly decreasing gradient in mineralization temperature.Clustered lead isotope data define a trend that plots between model Pb isotope curves of Doe and Zartman for the upper crust and the mantle. This trend is interpreted to be a mixing line, between primitive mantle-type lead and more radiogenic lead, by one or a combination of two postulated processes: between two groups of rocks (more primitive oceanic volcanics and more radiogenic upper crustal volcanic arc-clastic rocks) by a meteoric thermal system generated by the emplacement of the Coast Plutonic Complex, or between less and more radiogenic magmas during the emplacement of the Mesozoic-Cenozoic volcanics and intrusion hosting the deposits. The accreted nature of the host oceanic and arc terranes is emphasized by the differences between the model proposed and the upper crustal shale curve model of Godwin and Sinclair, which represents much of the locally adjacent autochthonous portion of the Canadian Cordillera.