The mineral that could have been deposited by black smokers on the seafloor is iron sulfide. Black smokers are hydrothermal vents found in the deep ocean that release hot, mineral-rich water. One of the primary minerals produced by these vents is iron sulfide, which is a combination of iron and sulfur. When the hot water released by the black smokers comes into contact with the cold seawater, the iron sulfide minerals precipitate and form chimney-like structures known as black smokers. Therefore, the correct answer is iron sulfide.
