There are plenty of old photos of people using very basic gold mining techniques stretching back to the gold rushes of California and Colorado. Cut out images from these photos or drawings modeled after them will serve as a very useful diorama of gold mining's golden age.
While the picture of someone kneeling next to a river or stream panning for gold is an indelible image of the gold rush, the fact of the matter is, giant mines were also built. Building a diorama that actually shows what the mine looks like from a cross-section point of view can be quite informative and interesting. Showing the small tunnels that miners had to traverse along with the carts they used to haul their finds back to surface.
While some basic dioramas will use cardboard or paper cutouts to illustrate a diorama, plastic or clay models of the miners hard at work in the mines or by the rivers will also go a long way in demonstrating the gold rush activities and initiatives. These models can be as detailed as you would like, but to get the perfect diorama, being able to pick out the different tools that are being used is going to be a big plus.
The one detail that can set one gold mining diorama apart from another is the use of pyrite, or other substances that look like gold. By putting these materials in the actual diorama, it gives it the look and feel of a real gold mining operation in miniature. If you pair the pyrite with detailed models, it will be the top of the class.