Gold is typically mined from two different types of geologic formation

Official Explanation

1. The Grasberg gold mine in Papua Indonesia is the largest gold mine in the world, producing over 57,000 kilograms of pure gold per year. For each of the following, select Implied if the information provided implies that, at the time the passages were written, it was likely true of the Grasberg mine. Otherwise select Not implied.

According to the environmental scientist, unlike placer-deposit mines, modern commercial lode-deposit mines are massive operations. Given that the Grasberg mine is the largest in the world, it is more likely to be a lode-deposit mine than a placer-deposit mine. Again according to the environmental scientist, unlike mining lode deposits—mining placer deposits does not usually involve crushing rock and using chemicals to extract gold, which implies that these are typical practices for mining lode deposits.

The correct answer is Implied.

As discussed in the analysis above, the environmental scientist’s statements suggest that the Grasberg mine is most likely a lode-deposit mine. Although the environmental scientist states that on average, such mining operations process approximately 130 kilograms of raw ore to produce 1 gram of pure gold, there is no information given to suggest that production at the Grasberg mine approximates that average.

The correct answer is Not implied.

As discussed in the analysis above, the environmental scientist’s passage implies that the Grasberg mine is most likely a lode-deposit mine. But the prospector’s passage implies only about placer deposits that they are always associated with rivers. There is no mention of a necessary relationship between lode deposits and rivers. Therefore, the information provided does not imply that the Grasberg mine was located at or adjacent to a river.

The correct answer is Not implied.

2. For each of the following, select Justified if it is a justified inference on the basis of the information provided. Otherwise select Not justified.

The prospector’s statements indicate that placer deposits are created when exposed lode deposits are weathered and the gold released is transported down rivers—thus indicating that all placer deposits form downstream of lode deposits. But there is no mention of a necessary relationship between lode deposits and rivers.

The correct answer is Not justified.

As noted in the analysis above, placer deposits are created by the weathering of lode deposits. According to the prospector’s passage, lode deposits are created by mineral-bearing solutions that carried the gold up from the earth’s interior. There is no other mechanism given for the formation of lode deposits.

The correct answer is Not justified.

According to the prospector’s statements, placer deposits form where gold from weathered lode deposits is washed down rivers and deposited in sand as the stream slows. Hence, placer deposits are likely to be located in current or former riverbeds.

The correct answer is Justified.

3. For which one of the following questions concerning the time the passages were written does the information provided supply the clearest answer?

The average ratio of ore to pure gold is given, but there is insufficient information to determine the average amount of ore processed per mine per day (ruling out A). The percentage of total gold production in the US that comes from placer deposits is given, but there is insufficient information to determine what that quantity is, let alone what percentage of the world total comes from placer deposits (ruling out E and B). Chemicals used to extract gold are claimed to cause environmental damage, but the specific forms of that damage are not given (ruling out C). However, the environmental scientist’s passage clearly lists habitat destruction and sediment release as the principal forms of environmental damage attributable to mining placer deposits (D).

The correct answer is D.

4. For each of the following statements, select Yes if, according to the information provided, it was accurate at the time the passages were written. Otherwise select No.

The prospector’s statements describe lode deposits as large and indicate that they account for the majority of the gold mined. The environmental scientist draws a contrast between placer and lode mines, indicating that lode mines have a much larger environmental impact by virtue of their size, and referring to them as massive operations. Therefore, there is substantial information to suggest that placer deposits are, on average, smaller than lode deposits.

The correct answer is No.

That the environmental scientist describes habitat destruction and sediment release as the primary impacts of placer-deposit mining suggests that these do not constitute an exhaustive list of impacts. Therefore, it is reasonable to infer that mining placer deposits sometimes has impacts other than habitat destruction and sediment release.

The correct answer is Yes.

According to the prospector’s statements, placer deposits account for only a small percentage of total gold mined in the US because of a decrease in the number of such deposits that are readily available. There is no information given to suggest that an increase in production from lode deposits is the cause of this decrease either in the US or globally.

The correct answer is No.

5. Each of the following claims appears in the environmental scientist’s passage. For each, select Supported if the scientist cites specific evidence to support the claim. Otherwise select Not supported.

The environmental scientist makes this claim in the third sentence of the second paragraph but does not provide evidence anywhere to support this claim.

The correct answer is Not supported.

The environmental scientist makes this claim in the final sentence of the first paragraph but does not provide evidence anywhere to support this claim.

The correct answer is Not supported.

The environmental scientist makes this claim in the final sentence of the second paragraph and then indicates that modern commercial lode-deposit mines process up to 180,000 metric tons (1 metric ton = 1,000 kilograms) of raw ore per day, helping to illustrate the magnitude of such operations. Therefore, the passage offers specific evidence to support the claim that modern commercial lode-deposit mines are massive operations.

The correct answer is Supported.

6. Suppose that a mine is established on a lode deposit that contains ore with 20 times the average proportion of gold to ore. Which one of the following describes the number of days it would take for such an operation, working at the fastest rate described in the passages, to produce 100 metric tons of pure gold?

The environmental scientist indicates that, on average, lode-deposit mines process 130 kilograms of raw ore to produce 1 gram of pure gold. If this lode deposit has 20 times the average concentration of gold, it will take 130/20, or 6.5, kilograms of raw ore to produce 1 gram of pure gold. The fastest rate of processing listed in the passage is 180,000 metric tons (or 180,000,000 kilograms) of ore per day. At that rate, this mine would be able to produce 180,000,000/6.5 grams, which is approximately 27,680,000 grams (27.68 metric tons), of pure gold per day. Therefore, it would take 100/27.68 days, which is approximately 3.61 days, for this mine to produce 100 metric tons of pure gold working at this rate, which is greater than 3 but less than 4 days.

The correct answer is C.