Ok, I can honestly say that I have no experience doing what you propose, but I can tell you that the Prospector mode is going to give you as much depth as you'll get as far as the machine goes, but nothing is goin to detect gold and discriminate iron. Resign yourself to digging nails. Iron Mask can cutout some of it, but running too much risks missing the smallest bits of gold.
Beyond that, search coils make a depth difference, but there it depends on what sort of compromises you choose. You'll of course want to stick with the 18.75kHz (HF) coils, and you already have the water resistant 5x10" eliptical. So here's where you get to decide what's important to you, and what you think might produce at that site. A smaller coil would be the waterproof 6" DD, that will find the smallest gold and separate in debris strewn areas well, but due to it's size is depth limited. Or you can use the coil that you have, which gives little better depth than the 6", but covers more ground and can't "pick" through heavy trash. Or there is a waterproof 9" Concentric coil, which goes fairly deep, covers ground fairly well, and might be a good choice. Or the water resistant 10.5" DD, which separates well, gets nearly the depth of the 9" with a bit better coverage, but starts to be big enough to miss the tiny gold. or if depth is the imperitive, and small gold isn't the goal, then the waterproof Coiltek 15" All-Terrain will provide all the depth you'll squeak out of the 705. In packed sand, the 15" in Prospector mode sees penny size coins @ 17"+, but I can't say what it will do onsomething as dense as foot thick quartz.
Tailoring your machine to the task can easily make the difference between success and dissappointment. Choose your goal and set up for it.
Good luck! And tell us about it afterward!
HH