Question

 could be inferred that the wordpitchblende in the paragraph means "a black mineral deposit."

Which context clue best helps confirm this meaning of the word pitchblende?

"the mine owners were happy to oblige"

"and dumped what was left over in a nearby pine forest"

"ore, it was just dirt"

"they had already taken out the valuable part—the metallic element"

Answer 1

"they had already taken out the valuable part--the metallic element."

Answer 2

The sentence "they had already taken out the metallic element which is the valuable part" confirms the meaning of pitchblende.

Explanation:

Pitchblende is a mineral that is a type of ''uraninite'' and it is mostly arise in ''veins'' that appears in a black color. They are affiliated with ''silver ''and it is described as the principal source of ''uranium'' and ''radium''. Due to its "black color and high density" it is known as pitchblende.

The formula for pitchblende is $UO_{2}$.Since the black mineral clue is given, the best choice would be "the metallic element which is a valuable part taken already".