Carbon-rich material associated with gold-bearing quartz veins in the Erickson gold mine, Cassiar district of northern British Columbia, has been analyzed geochemically, petrographically, and isotopically. Isotopic compositions of carbon in veins and associated siliciclastic and carbonate rocks suggest that the organic matter in adjacent siliciclastic sediments is the source of carbon. Field relations and petrographic evidence suggest that carbon emplacement took place during early phases of quartz vein formation, after the metamorphic event responsible for the maturation of the carbon in the source rocks and before the emplacement of the Cassiar batholith. The results suggest that carbon was mobilized from carbonaceous material in siliciclastic rocks as CO 2 , and to a lesser extent CH 4 , and forced its way through fractures joining hydrothermal fluids. Precipitation of carbon occurred, probably due to reduction of CO 2 by H 2 . Vein carbon is associated with gold-bearing quartz veins and it is possible that organic matter played a significant role in gold precipitation.