what factors determine the location of metallic gold deposition relative to magma flows?

If a magma plume (within the Earth, *not* external -- which would be a lava flow, not a magma flow) encounters or contains water, *and* the magma has a reasonably high gold content (Au), *and* it's lucky enough to be taken up into solution, *and* the local rock is fractured, then these hydrothermal fluids can move through the cracks. *If* you're lucky, this gold in solution can be deposited in these fractures, concentrating as the hydrothermal fluids disappear, leaving behind metallic gold veins.