Gold is typically mined from two different types of geological formations (known as deposits): lode deposits and placer deposits. Lode deposits are what prospectors dream of finding: large deposits of nearly pure gold. Such deposits are located where they were originally deposited by the mineral-bearing solutions that carried the gold up from the earth's interior. Placer deposits, on the other hand, come from preexisting lode deposits that are exposed at the surface of the earth. These lode deposits' exposure to the weather causes gold to be released from surrounding rock and transported by rivers in the form of dust or flakes. When a stream carrying the gold slows, the gold collects in pockets of sand. Placer-deposite mines have historically been the source of approximately the source of 35% of the total gold mined in the US. However, in recent years, the quantity of gold mined from such deposits has decreased as the readily accesible deposits have been exhausted. Thus, despite an increase in net gold mined, placer-deposit mining now accounts for only a few percent of total gold mined in the US.
Because -unlike mining lode deposits- mining placer deposits does not usually involve crushing rocks and using chemicals to extract gold, the environmental impact are generally less than those of mining lode-deposits. The primary impacts of placer-deposits mining are habitat destruction and sediment release. Habitat destruction occurs as a result of river diversions and disruption of river bottoms and banks, and the large amount of silt and sediment released can severely impact water quality. Modern commercial operations tend to use settling ponds to prevent this discharge.
mining lode deposits has a much larger environmental impact by virtue of the size of such operations, the generation of waste material, and the use of toxic chemicals. The gold comes out of the ground as a raw ore -gold aggregated with other minerals. On average, such mining operations process approximately 130 kilograms of raw ore to produce 1 gram of pure gold. Unlike placer-deposits mines, modern commercial lode-deposit mines are massive operations, some displacing and processing up to 180,000 metric tons (1 metric ton = 1,000 kilograms) of raw ore per day.
For which one of the following questions concerning the time the passages were written does the information provided supply the clearest answer?
a) How much raw ore did the average lode-deposit mine process daily?
b) What was the total annual quantity of pure gold produced globally by placer-deposit mines?
c) What were the principal forms of environmental damage attributable to chemicals used to extract gold from raw ore?
d) What were the principal forms of environmental damage attributable to mining placer deposits?
e) What was the total annual quantity of gold mined in the US?
Answer is D
Sincerely, I didn't even understand the question. I'd love some explanation on this question, and if someone knows this one should be an easy-medium-hard type of question.
Thanks a lot!