1. Finding gneiss is neutral, as "gneiss" simply describes a metamorphic rock. The fact that the rock has been metamorphosed can mean that any metals in the precursor rocks have been remobilized and concentrated. or it may not.
2. A vein is a good thing to find if it has lots of metal in it and is wider than (say) 1m. Narrow veins are and always have been expensive to mine and imply a low productivity (cf economic work done on the Golden Mile that indicates that mines here were effectively breakeven, at best, from around 1914 to 1990). The word "stringer" implies a secondary and uneconomic (on its own) structure to the main vein, but where many mineralised stringers are concentrated, there may be potential for bulk ore that can be mined simply and productively, which means economically.
3. Footwall is simply the rock beneath a mineralized structure. Historically (and today) it's what the miner stands on to mine ore above him.
