Shaft Mining: This involves sinking a vertical tunnel (shaft) deep into the earth to reach ore veins that are too deep for surface mining. Elevators are used to transport miners, equipment, and extracted ore to the surface.
Slope and Drift Mining: These methods use inclined or horizontal tunnels to access ore bodies located in hillsides or at moderate depths. They are often more cost-effective than vertical shafts when the topography allows.
Subsurface Challenges: Underground mining requires complex infrastructure for ventilation, water drainage, and structural support (pillars or roof bolts) to prevent tunnel collapse and ensure worker safety.
Depth and Geometry: The physical location of the ore body is the primary factor; shallow, wide deposits favor surface mining, while deep, narrow veins require underground techniques. The 'stripping ratio' (overburden to ore) determines the economic limit of surface mining.
Economic Viability: The total cost of extraction, including labor, machinery, and processing, must be lower than the market value of the recovered metal. This is calculated using the formula: .
Environmental Impact: Surface mining causes significant land disturbance and habitat loss, requiring extensive reclamation plans. Underground mining has a smaller surface footprint but can lead to ground subsidence or acid mine drainage.
| Feature | Surface Mining | Underground Mining |
|---|---|---|
| Depth | Shallow (usually < 200m) | Deep (can exceed 3km) |
| Cost | Lower per tonne of material | Higher due to complex infrastructure |
| Safety | Generally safer; better ventilation | Higher risk of collapse and gas buildup |
| Scale | Large scale, high volume | Targeted, lower volume |
| Environment | High surface disturbance | Lower surface disturbance; risk of subsidence |
Check the Units: When calculating ore value, ensure the grade units match the price units (e.g., converting to ounces or percentages to kilograms). Misplacing a decimal point in grade calculations is a common source of error.
Stripping Ratio Logic: Always evaluate the stripping ratio (). If the ratio exceeds the economic limit, the mine must either switch to underground methods or cease operations.
Sustainability Context: In descriptive questions, always mention reclamation (restoring land after mining) and tailings management (storing waste) as critical components of the modern extraction process.
Sanity Check: If a calculation suggests a 90% grade for a base metal like Iron, it might be realistic, but for Gold, a 90% grade () is impossible in nature; always verify if the result makes geological sense.
