Practice Problems
Problem W18.1. Profitability of a Gold Mine. In the United States today, ore that
contains at least 1 ppm gold generally is economically feasible to mine by some methods.
But realize what that means: You have to mine 1 million pounds of ore from the ground,
and process it, to extract only 1 pound of gold!
Q18.1. Imagine a gold mine in Nevada. One day, 1450 metric tonnes of ore is removed
from the mine. The ore is run through a “heap-leach” process with cyanide, and 222 troy
ounces of gold are removed from it. Assuming that 1 ppm is the lowest gold
concentration that is profitable to mine, how did the mine perform on this day?
___the mine showed a profit
___the mine showed a loss
___the mine broke even
Problem W18.2. Calculating Gold and Silver Production. Some mines produce more
than one metal in economically valuable quantities. A certain gold mine, for example,
may mine ore that contains 2.2 ppm gold and 15.5 ppm silver.
Q18.2. If the mine produces 70,000 metric tonnes of ore per day, how many troy ounces
of gold and how many troy ounces of silver are produced each day?
Problem W18.3. Interpreting a cross section. Study the cross section in Figure WS18.1.
Circle areas of the cross section where crude oil could accumulate.
Q18.3. For each area you circle, enter in Table 18.1 your reason for thinking the area
could be a crude oil trap. You may or may not need all of the blanks provided.
Figure WS18.1 Cross section.
Table 18.1 Oil traps and causes.
Circled oil traps
Reason for trap
Now, study the surface profile of the cross section and its relationship to the underlying
geology.
Q18.4. What single feature of the subsurface geology appears to offer the best
explanation for the stream location?
___ different rock types erode at different rates
___ the fault weakened the rocks next to it, making them more susceptible to erosion
___ the limbs of anticlines always are weakened structurally, making them more
susceptible to erosion
___ it is pure chance because most streams could be anywhere on the map
Problem W18.4. The Largest Skarn in the World
Skarn is an old Swedish mining term for silicate gangue that contains iron ore and sulfide
deposits. Skarn forms when magma pushes up into overlying limestone rocks. Chemical
differences between the silica-rich magma and calcite-rich limestone create a wonderful
array of rare minerals. Metals sometimes are concentrated in the radically altered
limestone.
An enormous skarn deposit called the Big Gossan lies in southeast Asia, just north of
Australia, in Irian Jaya. Geologists believe the total ore available in the Big Gossan is
about 37 million metric tonnes. The average grade of the skarn ore is 2.7% copper, 1 ppm
gold, and 16 ppm silver.
Q18.5. As a first step toward understanding this remarkable ore deposit, calculate how
much copper, gold, and silver the mine can be expected to produce. Use the units shown:
copper = ___________ pounds
gold = _____________ troy ounces
silver = ____________ troy ounces
Q18.6. Check the current market prices of copper, gold, and silver in a newspaper or on
the Web. Using these prices, calculate the gross (total) value of the ore in the Big Gossan
skarn:
$____________
By now you probably are convinced that the Big Gossan is valuable property! Geologists
who work there, and consulting geologists who visit, have had plenty of incentive to
understand the ore body. The following features have helped them:
1. Skarn deposits lie between an igneous intrusive (plutonic) rock and limestone. The
heat and fluids from the original magma alter the limestone into marble. It is this
marbleized zone in which the ore lies.
2. Copper-gold skarns, like the Big Gossan, show strong zonation within the ore. Near the
core of the pluton that is responsible for forming the ore, the skarn has abundant dark redbrown garnets with pale, iron-poor pyroxenes. Farther from the core of the pluton, the ore
contains garnets that are pale green to green-brown and abundant pyroxenes that are dark
green and iron-rich.
Geologists and mine managers at the Big Gossan want to predict where the richest ore
lies and where there might be even more. The geology does not simply consist of one
stock (pluton) intruded into limestone. There are several stocks, including one younger
than the ore across which it cuts.
This exercise presents a simplified version of what geologists face at the Big
Gossan. As you can see in Figure WS18.2, five drill cores have been made in one
cross section of the area. Table W18.2 is a record of what each drill core
intercepted at 50-meter intervals. To grasp the geometry of what the drill cores
encountered, transfer the information from the table to the figure, using colored
pencils for different rock types. Note that the figure also gives information on
surface rocks.
Now imagine you are a consulting geologist, brought to Big Gossan to give your opinion
of proposed drilling plans. The figure shows three options: drill holes X, Y, and Z.
Q18.7. What is your rationale for or against proceeding with proposed drill core X?
___________________________________________________________
___________________________________________________________
Q18.8. What is your rationale for or against proceeding with proposed drill core Y?
___________________________________________________________
___________________________________________________________
Q18.9. What is your rationale for or against proceeding with proposed drill core Z?
___________________________________________________________
___________________________________________________________
Figure WS18.2 Big Gossan—a simplified version.
Table W18.2 Drill Core Records.
Drill
hole
1a
1b
1c
1d
2
Depth
(meters)
50
100
150
200
250
300
350
50
100
150
200
50
100
150
200
250
300
350
400
450
50
100
150
200
250
300
50
100
150
200
250
300
350
400
450
500
550
600
650
700
750
800
Rock/ore present
marble
marble
quartzite
granodiorite with large phenocrysts
granodiorite with large phenocrysts
granodiorite with large phenocrysts
quartzite
marble
skarn: abundant, dark-red garnets
skarn: abundant, dark-red garnets
granodiorite with large phenocrysts
marble
skarn: abundant, dark green pyroxene
skarn: abundant, dark green pyroxene
skarn: abundant, dark-red garnets
skarn: abundant, dark green pyroxene
skarn: abundant, dark green pyroxene
granodiorite with no phenocrysts
granodiorite with no phenocrysts
granodiorite with no phenocrysts
marble
skarn: abundant, dark green phenocrysts
skarn: abundant, dark green phenocrysts
marble
marble
marble
marble
marble
marble
marble
marble
marble
marble
granodiorite with no phenocrysts
granodiorite with no phenocrysts
granodiorite with no phenocrysts
granodiorite with no phenocrysts
granodiorite with no phenocrysts
granodiorite with no phenocrysts
granodiorite with no phenocrysts
granodiorite with no phenocrysts
granodiorite with no phenocrysts