You might be thinking of GST. In certain circumstances the supply of precious metails is GST-free (reference is below).
Precious metals is defined in the GST Act and includes "gold (in investment form) of at least 99.5% fineness"
Other than that and off the top of my head, I am unaware of any reason as to why a gold miner would not be subject to income tax on profits (ignoring the fact that they may have significant amounts of tax losses that they may be able to recoup, as this is not specific to gold miners).
