Carbonate alteration in basalt, Total Erickson gold mine, Cassiar, northern British Columbia, Canada

Abstract

The Total Erickson gold mine is in a sequence of upper Paleozoic to lower Mesozoic mafic volcanic, ultramafic, and sedimentary rocks of the Sylvester allochthon in the McDame lode gold camp, approximately 12 km southeast of Cassiar, British Columbia. Gold and silver occur in quartz veins that are mostly within mafic volcanic rocks. Rocks within the mine were affected by four types of alteration: synvolcanic hydrothermal, regional metamorphic, epigenetic hydrothermal, and contact metamorphic. The most common alteration type is epigenetic carbonation of basalt, which occurs as well-developed envelopes around gold-silver-bearing white quartz veins and carbon-rich veins. Younger dolomite veins also are surrounded by carbonate alteration envelopes. Disseminated and fracture-controlled elemental carbon is present locally in white quartz veins and locally is sufficiently abundant to form carbon veins.Rocks within carbonate alteration envelopes are composed of ankerite, siderite, dolomite, quartz, sericite, kaolinite, pyrite, carbon, and minor titanium oxides. A generalized model of mineral species distribution throughout the envelopes around white quartz veins, carbon veins, and dolomite veins is developed. Mass balance calculations indicate the following compositional changes around a gold-silver-bearing white quartz vein: gain of K 2 O and SiO 2 ; loss of Na 2 O, FeO, and MgO; and loss of CaO from the outer portion of the envelope and gain in the inner. Loss on ignition and the alteration mineral assemblage indicate gain of volatile components including, at least, CO 2 , H 2 O, and S.This redistribution of elements during alteration processes that are genetically related to gold quartz vein formation defines a primary dispersion halo, the documentation of which is a sound basis for confident interpretation of exploration lithogeochemical data.