“It’s a gold mine!” is an idiom that means it is something very valuable. If you had a real gold mine, it would provide many years of gold before being depleted. In the same way, when people use the idiom, “It’s a gold mine,” they mean it is something that keeps giving value.
The website, VOA Learning English, is a gold mine! Published by Voice of America, it has English lessons, written, audio and video articles, stories, news and more. Articles have audio versions so you can read and listen at the same time. This can aid comprehension. Articles have lists of words and definitions to also help in understanding. Current events news articles, stories by famous Americans, lessons about words or grammar, are just a few of the “treasures” (things of value) that can be “mined” (discovered) in VOA Learning English.
Check out VOA Learning English! I’m sure I’ll be spotlighting a lesson or article every so often. When I do, I’ll use the tag VOA.