Funny that you mention Kalgoorlie. This is a very typical goldfields ore genesis model: pyrite (with gold) replacing magnetite in a dolerite (gabbro). I thought this could be the case. An iron-rich dolerite is a magnet for gold as the gold-bearing fluids percolate through.
http://bbxminerals.com.au/wp-content/uploads/2017/09/click-here-to-view.pdf
