Yeah rhino, the records are available from the Qld Gov websites on mining, like Dept of Mines and Geoscience. It's real archive stuff and you have to go searching for it. Gov never makes anything easy. I know I downloaded heaps of stuff and used up a lot of my download allowance.
Yabba and Bella Creeks flow into the dam. When I say where they rise, I mean where they start up in the mountains/hills. I think they are generally called the Brisbane Ranges or maybe the Blackbutt Ranges but not sure. The Brisbane river starts on the southern side of them so look at any decent map and you will get an idea. Also on the southern side is a place called Jimna. They mined for gold there and actually dug a tunnel through a tongue of land that jutted out into the Jimna Creek.
On the western side of the ranges is the town of Nanango. Gold was mined in Nanango and at a place called The 7 Mile Diggings directly south of the town. You should be able to Google 7 Mile Diggings with no worries, it was quite well known. But if you have a look at a good map of the area, you will find Jimna and Lake Borumba easily enough. Not far north of Imbil you've got Gympie of course but also Kilkivan which also had gold mines.
Like I said mate, they were not big fields around there, not like Gympie, but lots of miners found gold there. There's a lot of ground out there, but most of it is now private, so you have to be careful where you go. Also a lot of state forests that might give you some hope.
Looks like you got a lot of learning to do Rhino, but that can be half the fun. You have to research the areas and that can take a lot of work. But if you don't, you can do a lot of searching out there for nothing, and that will really pee you off. I can tell you that me and a mate have found gold in that area, but like any prospector, I'm not going to tell you my secret spots, it doesn't work that way. But in the boundaries of the areas I've mentioned, there is gold to be found. Cheers mate.