Answer: Coke ovens.
Next question: What is a coke oven?
Answer: A furnace-like device used to "cook" the impurities from coal.
Last question: Why is a coke oven needed?
Answer: Gold and silver weren't the only minerals mined in Colorado. As you've undoubtedly guessed from this post, coal was also mined there. And for some uses, coal needed to be converted to coke.
That's the short version of the story. For a more detailed explanation, click here.