ASBOG Economic Geology and Energy 5 — Questions and Answers
Question 1: Sedimentary exhalative (SEDEX) deposits form when:
- Glacial meltwater leaches metals from tills and deposits them in glaciolacustrine sediments
- Metal-rich hydrothermal brines vent onto the seafloor and precipitate sulfides within sediments (Correct answer)
- Evaporating seawater concentrates metals in sabkha environments
- Subduction-related fluids deposit metals in accretionary prisms
Correct answer: Metal-rich hydrothermal brines vent onto the seafloor and precipitate sulfides within sediments
SEDEX deposits form when hydrothermal brines exhale onto the seafloor and precipitate stratiform Pb-Zn-Ag sulfides within marine sediments.
Question 2: Which factor MOST directly controls the economic viability of a phosphate rock deposit?
- Depth to water table
- P2O5 content and proximity to agricultural markets (Correct answer)
- Structural complexity of the host formation
- Radioactivity levels in associated uranium
Correct answer: P2O5 content and proximity to agricultural markets
The P2O5 grade of the ore and transportation costs to fertilizer markets are the primary economic factors for phosphate mining.
Question 3: In the context of oil sands (tar sands), the hydrocarbon extracted is called bitumen, which differs from conventional crude oil primarily because it:
- Has an API gravity above 40°
- Is too viscous to flow without heating or dilution (Correct answer)
- Contains no sulfur impurities
- Is found in carbonate rather than clastic reservoirs
Correct answer: Is too viscous to flow without heating or dilution
Bitumen has very high viscosity (API gravity <10°) and cannot flow under reservoir conditions without thermal treatment or dilution with lighter hydrocarbons.
Question 4: The 'critical zone' of a Bushveld Complex-type layered mafic intrusion is significant because it hosts:
- Gold-bearing quartz veins formed by late-stage hydrothermal fluids
- Platinum group element (PGE)-bearing reefs such as the Merensky Reef (Correct answer)
- Massive sulfide bodies of copper and nickel in the basal contact zone
- Chromite seams and iron-titanium-vanadium deposits
Correct answer: Platinum group element (PGE)-bearing reefs such as the Merensky Reef
The Critical Zone of the Bushveld Complex contains the Merensky Reef and UG2 Reef, which are the world's dominant sources of platinum group elements.
Question 5: Carbon capture and storage (CCS) for CO2 sequestration is BEST suited to which geologic setting?
- Fractured crystalline basement at shallow depths
- Deep saline aquifers or depleted oil and gas reservoirs with impermeable caprock (Correct answer)
- Active volcanic terranes with high permeability
- Near-surface unconfined aquifers used for drinking water
Correct answer: Deep saline aquifers or depleted oil and gas reservoirs with impermeable caprock
Deep saline aquifers and depleted hydrocarbon reservoirs are preferred CCS targets because they have proven trapping capacity and seal integrity.
Question 6: A skarn deposit forms when:
- Magmatic fluids react metasomatically with carbonate country rock adjacent to an intrusion (Correct answer)
- Seafloor hydrothermal vents deposit sulfides in sedimentary sequences
- Residual weathering concentrates iron oxides from mafic rocks
- Evaporite dissolution creates breccia pipes that localize ore fluids
Correct answer: Magmatic fluids react metasomatically with carbonate country rock adjacent to an intrusion
Skarns form by metasomatic replacement of carbonate rocks (limestone, dolomite) by calc-silicate minerals and associated ore minerals introduced by magmatic fluids.
Question 7: Which renewable energy resource directly utilizes the thermal gradient within the Earth's crust rather than solar or gravitational energy?
- Tidal power
- Geothermal energy (Correct answer)
- Hydroelectric power
- Wind energy
Correct answer: Geothermal energy
Geothermal energy taps the Earth's internal heat from radioactive decay and residual heat of formation, independent of solar or tidal forcing.
ASBOG Economic Geology and Energy 5
Sedimentary exhalative (SEDEX) deposits form when: