With any modern machine ... you still have to be directly over the target.
If you are sure of the location, DIG, yes you heard right...tunnel into it.
Just make sure the top and sides are supported and you ALWAYS have a partner with you.
If it's gold or silver, I have a PI detector with a one meter coil that should nail it but at that depth and for its size, I don't think the ferrous/nonferrous discrimination could be relied upon which makes it easy to mistake for common steel.