ASBOG - Association of State Boards of Geology Economic Geology and Energy Questions and Answers 2 — Questions and Answers
Question 1: Which type of ore deposit forms when hydrothermal fluids precipitate minerals along faults and fractures in host rock?
- Placer deposit
- Vein deposit (Correct answer)
- Evaporite deposit
- Residual deposit
Correct answer: Vein deposit
Vein deposits form when hydrothermal fluids migrate through fractures and precipitate ore minerals as the fluids cool or chemically react with host rock.
Vein deposits are a classic type of hydrothermal ore deposit formed when hot, mineral-laden fluids circulate through fault zones and fractures. As the fluids cool or interact with surrounding rock, dissolved metals precipitate and fill the void spaces. Gold, silver, and base metal sulfides are commonly found in vein deposits.
Question 2: What is the primary driving force behind the formation of sedimentary exhalative (SEDEX) deposits?
- Magmatic intrusion heating basin brines (Correct answer)
- Glacial transport and deposition
- Wind erosion concentrating heavy minerals
- Biological activity fixing metals in sediments
Correct answer: Magmatic intrusion heating basin brines
SEDEX deposits form when magmatic or geothermal heat drives basin brines upward through faults to exhalate on the seafloor, depositing metal sulfides.
Sedimentary exhalative (SEDEX) deposits are major sources of lead, zinc, and silver. They form in marine basins where hydrothermal brines, often driven by subsurface heat from magmatic sources or deep burial, migrate through fault systems and exhale on the seafloor or within sediments. The metals precipitate as sulfides upon contact with cold seawater or reduced sediment pore waters.
Question 3: Which energy resource is primarily associated with organic-rich black shales deposited in anoxic marine basins?
- Coal
- Uranium
- Shale gas and oil (Correct answer)
- Geothermal energy
Correct answer: Shale gas and oil
Organic-rich black shales deposited under anoxic conditions preserve large quantities of organic matter that thermally mature into shale gas and oil over geologic time.
Anoxic marine basins inhibit bacterial decomposition of organic matter, allowing it to accumulate in black shales. Over millions of years and with increasing burial depth and temperature, this organic material (kerogen) matures into liquid and gaseous hydrocarbons. Hydraulic fracturing (fracking) is commonly used to extract these tight hydrocarbon resources from low-permeability shales.
Question 4: A geologist identifies a porphyry copper deposit. What is the most common tectonic setting for this deposit type?
- Mid-ocean ridge
- Continental rift zone
- Convergent plate margin above a subduction zone (Correct answer)
- Transform fault boundary
Correct answer: Convergent plate margin above a subduction zone
Porphyry copper deposits form above subduction zones where magmas derived from the mantle wedge carry copper-rich fluids that concentrate in shallow intrusions.
Porphyry copper deposits are the world's largest source of copper and are intrinsically linked to convergent plate margins. Subduction of oceanic crust releases water and other volatiles into the overlying mantle wedge, generating hydrous, oxidized calc-alkaline magmas rich in copper and sulfur. These magmas rise into the crust and form porphyritic intrusions, and the exsolved magmatic-hydrothermal fluids deposit copper sulfide minerals in stockwork veins.
Question 5: In petroleum geology, what is the term for impermeable rock that prevents upward migration of hydrocarbons, trapping them in a reservoir?
- Source rock
- Seal rock (cap rock) (Correct answer)
- Reservoir rock
- Basement rock
Correct answer: Seal rock (cap rock)
Seal rock, also called cap rock, is an impermeable layer (commonly shale, evaporite, or tight carbonate) that overlies a reservoir and prevents hydrocarbons from escaping upward.
A petroleum system requires four essential components: source rock (generating hydrocarbons), reservoir rock (storing hydrocarbons in pore spaces), seal rock (preventing escape), and a trap geometry. Seal rocks must have very low permeability and capillary entry pressure to hold back buoyant oil and gas. Evaporites (salt, anhydrite) and organic-rich shales are among the most effective seals in the geologic record.
Question 6: Which geochemical process is responsible for the supergene enrichment of copper ore deposits?
- Metamorphic recrystallization of primary sulfides
- Downward percolation of oxidizing groundwater dissolving and re-precipitating copper (Correct answer)
- Magmatic differentiation concentrating copper in late-stage melts
- Evaporation of saline lake waters depositing copper salts
Correct answer: Downward percolation of oxidizing groundwater dissolving and re-precipitating copper
Supergene enrichment occurs when oxidizing groundwater descends through a primary sulfide deposit, dissolves copper, and re-precipitates high-grade chalcocite and covellite in a zone below the water table.
In the supergene enrichment process, meteoric water infiltrates a primary porphyry copper deposit, oxidizing primary chalcopyrite and pyrite in the leached cap zone. The resulting acidic, copper-bearing solutions migrate downward to the water table, where they react with primary sulfides to deposit secondary copper sulfide minerals such as chalcocite (Cu2S). This process can dramatically increase copper grades in the enriched zone compared to primary ore below.
ASBOG - Association of State Boards of Geology Economic Geology and Energy Questions and Answers 2
Which type of ore deposit forms when hydrothermal fluids precipitate minerals along faults and fractures in host rock?