Bralorne: a Mesothermal, shield-type vein gold deposit of Cretaceous age in southwestern British Columbia

CIM Bulletin, Vol. 83, No. 941, 1990

C.H.B. Leitch, Department of Geological Sciences The University of British Columbia

The Bralorne-Pioneer deposit produced more gold than any other deposit in British Columbia over its 70 years of operation (130 kg or 4 million oz of Au, from 7 million tonnes of 18 g/t ore). Mineralization is related to a suite of late Cretaceous albi-tite dykes of 85 to 90 Ma age. It thus occurred long after and is genetically unrelated to the Early Permian (275 Ma) Bralorne diorite and soda granite. The major veins strike about 110 degrees and dip north at 70 degrees, with slickensides plunging 45 degrees east that indicate last movement was reverse. Major ore shoots in the veins occupy somewhat less than 20% of the vein and plunge steeply west, roughly perpendicular to the slickensides. The structures that contain the veins probably formed at around 95 to 90 Ma during a strike-slip episode with easterly directed maximum compression. Disseminated pyrite, pyrrhotite, and minor chalcopyrite occur within envelopes up to several metres from the veins. Ar-senopyrite is confined to vein selvages. Local sphalerite and galena appear to correlate with gold-rich portions of the veins. Traces of tetrahedrite and stibnite have been observed but tellurides have not. Gold is found principally as thin smeared flakes of the native metal in the black sulphidic septae of the strongly ribboned veins. Hydrothermal alteration envelopes around the veins grade outwards from intensely foliated quartz - ankerite - sercite (±fuchsite) to less sheared calcite - chlorite - albite to unsheared epidote -calcite.

Keywords: Economic geology, Gold deposits, Geology, Bralorne-Pioneer deposit.