Usually very difficult to separate the gold from heavily iron-cemented material. The old-timers called it "cement" and would often have to put it through a quartz-crushing battery to break it up before separating the gold. Often that made it uneconomic so they just left it unworked except where very rich in gold.
The reason for my questions is that the iron cementing process occurred once the gravel was uplifted and exposed to the atmosphere, the iron cementation occurring from the top downwards, so often more heavily cemented near the top. I find it often breaks into solid blocks at top, but although also iron-rich, crumbles into fragments more readily lower down. So there might be a case for separating lower wash from upper wash if you find that, as the lower wash may be easier to treat. In the 1980s/1990s some material of this type was successfully treated (eg south of Avoca where it produced hundreds of kg of gold).
And don't ignore the top few tens of cm of underlying bedrock - it often contained a third of the gold that was recovered, half as much as in the overlying gravel.