Geological Structures

Faults, folds, joints and veins control where ore is found and how rock behaves. Learn strike and dip, apparent dip, true thickness from drill intersections, types of faults and folds, stereonets, and why structures matter for grade continuity and slope/tunnel stability.

Ore bodies are rarely simple blocks: they are offset by faults, bent by folds and cut by joints. Mining engineers must read structural geology to predict where ore continues and where walls may fail.

An analogy

A folded and faulted orebody is like a book that has been bent and then cut and shifted — to find a page you need to know how the book was deformed.

Strike and dip

Strike is the compass direction of a horizontal line on a plane; dip is the steepest angle below horizontal, measured perpendicular to strike. Written e.g. 045/60 SE or as dip/dip direction 60/135. A geological compass (Brunton, Clar) measures them.

Faults and folds

Normal faults (extension, hanging wall down), reverse/thrust (compression), strike-slip (horizontal movement). Folds: anticlines (arch), synclines (trough); hinge and limbs. Faults can offset ore by hundreds of metres or act as fluid pathways that created the ore.

Joints and discontinuities

Joints, bedding and foliation create blocks in the rock mass. Their orientation, spacing, persistence and roughness control slope and tunnel stability and blast fragmentation — the basis of rock-mass classification.

Stereonets and 3D modelling

Lower-hemisphere stereonets plot planes as great circles and poles; clusters define joint sets and kinematic failure modes (planar, wedge, toppling). Implicit 3D modelling (Leapfrog) builds fault networks and veins from drill data.

Formulas

• Apparent dip: tan α = tan δ × sin β
• True thickness: t = L × sin γ (γ = angle between hole and plane)
• Dip direction: Dip direction = strike + 90° (right-hand rule)

Worked example: Apparent dip in a section

Given: Vein dips 40°; Cross-section at 30° to strike

Find: Apparent dip.

• tan α = tan 40° × sin 30° = 0.839 × 0.5 = 0.420
• α = 22.8°

Answer: 22.8° — the vein looks much flatter on this section

Worked example: True thickness

Given: Hole intersects 20 m of ore; Hole meets the vein plane at 60°

Find: True thickness.

• t = 20 × sin 60° = 17.3 m

Answer: 17.3 m

Common mistakes

• Wrong: Reporting downhole length as true width. — Right: Convert intersections to true thickness.
• Wrong: Reading dip on an oblique section as true dip. — Right: Correct with tan α = tan δ sin β.

Applications

• Drill planning: Holes designed to cut veins at high angles.
• Slope design: Joint orientations define failure modes.
• Ore continuity: Faults limit or offset ore zones.

Key takeaways

• Strike/dip define plane orientation.
• tan α = tan δ sin β.
• t = L sin γ for true thickness.
• Structures control ore and stability.

Going further

• Kinematic analysis: Markland test for planar, wedge and toppling failure.
• Oriented core: Measuring structures in drill core with orientation tools.
• Implicit modelling: RBF interpolation of geological surfaces.