Gold is typically mined from two different types of geologic 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 the 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-deposit mines have historically been the source of approximately 35 percent 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 accessible 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 rock and using chemicals to extract gold, the environmental impacts are generally less than those of mining lode-deposits. The primary impacts of placer-deposit mining are habitat destruction and sediment release. Habitat destruction occurs as a result of river diversions and disruptions of river bottoms and banks, and the large amounts 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 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-deposit 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.

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Gold is typically mined from two different types of geologic formation

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Question 2 sub question 1 I am confused about why the statement
“Each lode deposit is located on a river upstream from a placer deposit”
is marked Not justified.

The passage says that placer deposits come from preexisting lode deposits whose gold is released and transported by rivers, and that the gold collects when the stream slows. This seems to imply that whenever a placer deposit exists, it must have come from a lode deposit upstream along a river.

My confusion is: since the statement says “upstream from a placer deposit,” doesn’t that already restrict the lode deposits being referred to to ones that are associated with a placer?

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