Halito Brother,
The first thing that "jumps out" to me from your post is; your coil. You're kinda battling Borgs with a coil that small when you're in the field, or in this case, hills. (Detecting is futile). (lol)
I could let you know the next time I'm heading for Mariposa to do a little 'tecting. I would be happy to have you go out with me on one of my little forays. I won't promise you gold, but I can show you some areas where you do have have a chance of finding some nice nuggets.
Also, you mentioned you found a mine shaft. In my nomenclature, a 'shaft' is a tunnel that goes straight down into the ground. An addit is a tunnel that has been driven (horizonally) into the side of a hill. The difference between the two would determine the right place to metal detect. (At least theoretically).
Generally, if it's a shaft, it would have been dug on a quartz 'blow-out', in the hopes of striking gold somewhere in the quartz. If it's an addit, chances are that the miner found gold further down the hill and followed it up the hill to its source.
Of course, this is not a set rule, but just something to keep in mind while out prospecting.