The Copper Pan is a tool used to gather ore (and other items) from rivers and lakes. You can get one only after the Glittering Boulder at the bridge just west (left) of the mine entrance is removed, revealing a waterfall in its place. This task is accomplished by completing the Fish Tank Bundles or purchasing “Panning” from the Joja Community Development Form for 20,000g.
Acquisition in Stardew Valley
Go to the waterfall any day after the Glittering Boulder is removed. The game plays a cutscene where Willy is pondering the disappearance of the boulder. He points out the shimmering lights in the water that indicate that the stream is carrying ore and minerals out of the mountain. He then gives you his spare Copper Pan from his backpack.
You can Use
Watch for the shimmering lights that appear in any of the rivers and lakes. When present, you can “use” the pan to gather Minerals and/or Metal Ores at that location. The position of the shimmering lights seems to be directed and does not appear randomly.