Hi, I just read your posting and checked my best references, Field Guide to Recreational Gold Panning in Montana, and Gold Placers of Montana, and neither of them show any discoveries in the area that you will be in. the closest is S.W. of Billings on the Clark's Fork of the Yellowstone River from the Wyoming border North to the town of Bridger. The gold there is what they call "flood gold", that means that "the flakes are very small, wide and thin, it can float on the surface of the water, a gold pan must be used carefully to recover it".
Farther West, some discoveries were made in Park County, one is Emigrant Creek just South of the town of Emigrant and another was Bear Creek between Gardiner and Jardine. The Field Guide also shows that flood gold is found in the Yellowstone River. Here is a quote from the Field Guide "The Yellowstone River from Emigrant to Columbus is better known for it's Agate, Petrified Wood, and for its fishing than for it's gold content. Most of the gold here is 'flood gold' and while there is a lot of it, it takes an awful lot of colors to make an ounce. 'Flood gold' is extremely thin and it takes 200,000 to 800,000 flakes to make an ounce. Look in the grass roots and in the sand piles behind logjams; the gravel bars shouldn't be forgotten either". Most of the gold discoveries were in the mountain areas of the South Western part of the state. Most of the good areas that produce gold are private property or have claims filed on them. Most claim and property owners take trespassing very serious and permission should be acquired before entering any private land or claims.
I don't know of any place in Mt. that will rent detectors. There are two prospecting/metal detector shops in the Helena area, one is The Prospectors Shop and the other is The Modern Prospector. I don't have their phone No's handy but if you want to check with them I am sure you can find them on the Yellow Page phone No look up on the web. Helena will be nearly 300 miles west of Lame Deer.
What kind of work will you be doing on the Reservation? Have you have ever been to a reservation in the west before?
Good luck with the job and the prospecting.
RAD in MT