UBC Theses and Dissertations
The nature of carbonate alteration in basalt at Erickson Gold Mine, Cassiar, north-central British Columbia Sketchley, Dale Albert
The Erickson mine is within the McDame lode gold camp, aproximately 12 km southeast of Cassiar, British Columbia. The mine is in mafic volcanic, ultramafic and sedimentary rocks of the Upper Paleozoic to Lower Mesozoic Sylvester Allochthon. Gold-silver mineralization occurs in white quartz veins that are mostly within mafic volcanic rocks. K-Ar dates from sericite in gold-silver-bearing white quartz veins indicate that mineralization occurred in the Early Cretaceous at about 130 Ma. Thus, these veins predate the mid-Cretaceous Cassiar batholith and Lower Cretaceous and Early Tertiary plutons in the immediate area. However, the Early Cretaceous date probably represents a thermal event, precursor to emplacement of the Cassiar batholith, which may have caused circulation of the meteoric fluids responsible for the veins. Rocks within the Erickson mine were affected by four types of alteration: synvolcanic hydrotherma1, regional metamorphic, epigenetic hydrothermal and contact metamorphic. The most common type is epigenetic carbonatization of basalt, which occurs as we11-developed envelopes around gold-silver-bearing white quartz veins and carbon veins. Dolomite veins are surrounded by similar carbonate alteration envelopes; these veins postdate white quartz veins and carbon veins. Disseminated and fracture-controlled carbon may be present around white quartz veins and carbon veins. An idealized model of carbonate alteration envelopes is presented based on field observations and mineralogical studies. 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 was also developed. Mass balance calculations, using Gresens' (1967) metasomatic equation, indicate the following compositional changes around a gold-silver-bearing white quartz vein: gain of K₂O and SiO₂ ; loss of Na₂O, Fe₂O₃ and MgO; 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 at least CO₂ , H₂O and S. Geochemistry of carbonate alteration envelopes was investigated using an aqua regia digestion with multi-element inductively coupled plasma (ICP) analyses and gold-silver fire assays. The ICP analyses were compared to X-ray fluorescence (XRF) analyses to determine the range of extraction and to assist in interpretation. The comparison indicates that if there is strong enrichment in carbonate alteration envelopes surrounding gold-silver-bearing white quartz veins, the ICP analyses will show these patterns well. Strong enrichment in potassium, barium, boron and arsenic, and sporadic enrichment in gold, silver, copper, lead, zinc and antimony occur in carbonatized basalt around gold-silver-bearing white quartz veins.
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