I couldn't give you a specific number of mining companies in any particular taxation year; their attributes may change from taxable to non-taxable. The measures with respect to accelerated capital cost allowance, though, and the change to the classification of pre-production would primarily affect intermediate or senior companies that are in the process of opening a mine or have an operating mine, as opposed to junior companies, which would not be affected.
On November 18th, 2013. See this statement in context.