Questions: Mineral Resources and Ore Deposits

Question 1

Gold flakes and nuggets are found mixed with gravel in stream beds downstream from gold-bearing veins. What type of deposit is this, and what concentrated the gold?

• A. A hydrothermal vein, concentrated by hot fluids in the stream
• B. A placer deposit, concentrated because flowing water carries away lighter grains and leaves dense gold behind
• C. An evaporite deposit, formed as the stream dried up
• D. A magmatic deposit, formed as gold crystallized in lava

Answer: (B)

Gold is very dense and chemically resistant. When water slows, dense grains settle first, while lighter quartz and clay are carried on. Over time this sorting concentrates gold in places such as the inside of bends and cracks in the stream bed.

Question 2

Explain how hydrothermal fluids form ore deposits.

Model answer: Hot water, often heated by a nearby magma body and sometimes released from the magma itself, dissolves metals as it moves through rock. When the fluid cools, boils, changes pressure, or reacts with surrounding rock, the metals come out of solution and precipitate as minerals, often sulfides, in veins and fractures or spread through the rock.

Hydrothermal processes form many of the world's copper, lead, zinc, gold, and silver deposits. Porphyry copper deposits, found above subduction zones such as the Andes, are a major example.

Question 3

Bauxite, the main ore of aluminum, forms by intense chemical weathering in warm, wet climates, which removes more soluble elements and leaves aluminum-rich minerals behind.

• T. True
• F. False

Answer: True

This kind of residual deposit forms where heavy rainfall and warm temperatures leach silica and other soluble components from the soil over long periods. Aluminum oxides and hydroxides, which dissolve very little, remain and become concentrated.

Question 4

A copper deposit was not mined in 1990, but a company opens a mine on it in 2020 with no new geologic discoveries. What is the most likely reason?

• A. The copper grew in concentration over thirty years
• B. A rise in copper prices or a cheaper extraction method made the deposit profitable, so it became ore
• C. Laws now require every copper deposit to be mined
• D. The rock changed from igneous to sedimentary

Answer: (B)

Ore is an economic definition as well as a geologic one. When prices rise or technology lowers costs, lower-grade rock can be mined at a profit, and reserves grow without any change in the rock.