Hi Busho. The Otago hot rocks react just as badly to VLFs. In the hydraulic sluicing mines just outside of Bannockburn a VLF detector will react with them every few seconds. Most are below the surface and will need digging. Only very few are visible! These hot rocks are water worn and are a very dull black color. They are most noticeably, very heavy. There are a lot in the rivers as well as everywhere else. Anyone who detects for gold in the Lakes District will come across them as soon as they start detecting. They must be masking gold, although, most of the Lakes District gold is too deep for any current detector to detect! You have to detect bedrock to get Lakes District gold, and the only exposed bedrock is on stream banks where you dig out crevices, like nugget hunter nz does on the West Coast.