MAGNESITE DEPOSITS OF SOUTHEAST BRITISH COLUMBIA, CANADA

Despite economic slowdown, the prices of Mg metal and MgO compounds remain steady and large magnesite resources in BC remain undeveloped, with the exception of the Mount Brussilof deposit. Magnesite deposits are hosted by Precambrian to Cambrian sedimentary rocks deposited in shallow marine to evaporitic environments, affected by burial diagenesis or low grade metamorphism. Most of the deposits (e.g. Mount Brussilof, Jab, Driftwood Creek and Topaz Lake) are associated with paleotopographic highs and unconformities. They are carbonate-hosted. The Marysville deposit is the sandstone-hosted exception. Magnesite mineralization is stratabound, stratiform or lens-shaped, although contacts between the dolostone-host and the magnesite are locally discordant. Magnesite is coarse-grained, and characterized by granola-like appearance on weathered surface. Fresh magnesite is white or pale gray and displays zebra textures, antipolar growths and other features traditionally interpreted as replacement of dolomite by magnesite. The median MgO content of individual deposits varies from 42 to 47% before calcining. The median concentrations of CaO, SiO₂, FeO and MnO in these deposits are below 2.5%, 7.5%, 1.5% and 0.03%, respectively. High-grade portions of these deposits are nearly monomineralic (magnesite with minor dolomite). Depending on the deposit, calcite, quartz, Mg-rich chlorite, huntite, clay, talc, pyrite and iron oxides may be also present. Where the age relationship between the sparry (coarse) dolomite and magnesite can be established, the sparry dolomite (commonly considered as indirect indicator for the Mississippi Valley-type Pb-Zn deposits) post-dates magnesite. Three main competing hypotheses (sedimentary, diagenetic /hydrothermal, and hydrothermal) were presented in the past to explain the origin of these deposits, but until the controversy is solved, effective exploration and development programs can easily be designed to accommodate all of them. If the market for Mg metal and MgO compounds remains strong, these magnesite deposits represent good exploration /development targets for major mining companies and for North American and European consumers of magnesia products or Mg-metal.