Actually I don't believe it comes from gold mining - though the meaning is similar. If you google "flash in the pan origin" you'll see it comes from a type of gun used a couple of centuries ago. The gunpowder in one part (the pan) exploded, but the gunpowder that needed to explode to shoot the gun did not. So, a flash in the pan is an early sign that something will happen that is not followed by it actually happening.

This idiom comes from gold mining in the 1800s. Miners used a pan to scoop up sand from a shallow river and swirl it around to look for pieces of gold. A shiny object might be a gold nugget, but sometimes it was something else that just shined for a second but wasn’t gold. That little shining was a flash in the pan … it look like something great, but really was worthless. So nowadays a flash in the pan is a brief trend or fad that doesn’t last.