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24-MMP-A1 General Geology and Exploration · May 2018

Question 3 of 7: Structural Geology and Ore Deposits

Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)

Notes on this paper

EGBC National Exam — Mining and Mineral Processing Engineering, 09-MMP-A1 General Geology and Exploration, 2018-May. Closed book; only a Casio or Sharp approved calculator permitted. Questions 1–4 are compulsory; a candidate then completes ONE more question chosen from Questions 5, 6 or 7.

Reference texts: Guilbert & Park, The Geology of Ore Deposits (genetic classification, deposit-type descriptions throughout); Evans, Ore Geology and Industrial Minerals, 3rd ed. (deposit classification, structural controls on ore); Klein & Dutrow, Manual of Mineral Science, 23rd ed. (crystal systems, diagnostic physical properties, hand-specimen identification); Telford, Geldart & Sheriff, Applied Geophysics, 2nd ed. (gravity, magnetic, electrical, EM and seismic methods); Kearey, Brooks & Hill, An Introduction to Geophysical Exploration, 3rd ed. (survey design and method selection); Rose, Hawkes & Webb, Geochemistry in Mineral Exploration, 2nd ed. (sample-medium selection, dispersion patterns); Peters, Exploration and Mining Geology, 2nd ed. (drilling methods and sampling).

Question 3: Structural Geology and Ore Deposits (20 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

a) Anticline and syncline

An anticline is an upward-convex (arch-shaped) fold in which the oldest beds occupy the fold's core, and the two limbs dip away from the fold axis in opposite directions. A syncline is a downward-concave (trough-shaped) fold in which the youngest beds occupy the core, and the limbs dip toward the fold axis. The two are complementary structures that alternate along a single, continuously folded sequence: the down-dip continuation of an anticline's limb is the up-dip limb of the adjacent syncline.

ANTICLINE limbs dip AWAY from axis; oldest bed in core axial trace SYNCLINE limbs dip TOWARD axis; youngest bed in core axial trace
Fig. 3a – Anticline (arch, oldest beds in the core, limbs dipping outward) and syncline (trough, youngest beds in the core, limbs dipping inward), each with its axial trace and limb dip arrows.

b) The schematic cross-section

[Figure not reproduced: Fig. 3b – Redrawn schematic reconstruction of the exam's cross-section, preserving the original unit labels (E, A, U, I), the fault B and the tabular intrusive body O. Units E (oldest), A and U are cut and vertically offset (down to the right) by the normal fault B; the tabular dark body O is . See the official exam paper.]

(i) Structure B. B is a fault – specifically a normal fault. Tracing the A/E and U/A contacts across the structure shows the block on the right-hand side of B has been displaced downward relative to the block on the left, which is the defining geometry of a normal (extensional, dip-slip) fault.

(ii) Rock unit O. O is a dike (an igneous intrusive body): it is tabular, discordant (it cuts directly across the bedding of units E and A rather than running parallel to it), and follows the same steep, planar trend as fault B, showing that the fault plane acted as the permeable pathway the magma exploited to reach a shallower level.

(iii) Geologic history, oldest to youngest. Reading the cross-cutting relationships (Steno's law of superposition and the principle that a structure/intrusion must be younger than everything it cuts) gives the following sequence:

  1. Deposition of unit E. The lowermost, and therefore oldest, unit in the section is laid down first.
  2. Deposition of unit A. A accumulates conformably above E.
  3. Deposition of unit U. U accumulates conformably above A, completing the layered sequence later affected by faulting.
  4. Normal faulting along B. B cuts and vertically offsets all three units (E, A and U), downthrown to the right, so faulting must post-date deposition of the youngest unit it displaces (U).
  5. Intrusion of dike O. The dike cuts across E and A (and the fault plane itself), so the intrusion post-dates both deposition of its host rocks and the faulting event – magma exploited the fault as a ready-made conduit.
  6. Uplift and erosion. The top of the faulted, intruded sequence (U, A, the fault B and the dike O) is bevelled flat by erosion, producing the unconformity surface seen truncating all of the older structures.
  7. Deposition of unit I. The youngest unit in the section is deposited unconformably across the erosion surface, sealing over the fault, the dike and the eroded tops of E, A and U.
the redrawn figure above preserves the labelled units, the fault, the dike and the unconformity relationship exactly as printed, but exact bed thicknesses and the precise fault dip are schematic, not to scale.

c) Joints and their role in ore formation

A joint is a naturally occurring fracture in rock across which there has been no significant displacement parallel to the fracture surface (in contrast to a fault, where measurable displacement has occurred). Joints form by brittle failure of rock in response to tensile or differential stress – common causes include cooling and contraction of igneous rock, unloading and expansion as overlying rock is removed by erosion (exhumation), regional tectonic stress, and desiccation of fine sediment. Joints typically occur in systematic, sub-parallel sets, and two or more intersecting sets divide a rock mass into a regular network of blocks.

Joints play a major role in ore formation because they are the principal permeability pathways along which hydrothermal ore-forming fluids migrate through otherwise low-permeability rock. Open joints and joint intersections provide the high-permeability conduits that focus fluid flow, and it is within these openings that ore minerals precipitate directly as fissure-fill or "sheeted" veins when the fluid cools, boils, or mixes with a second fluid and becomes supersaturated. Densely jointed rock (a "stockwork") can host disseminated, low-grade-but-large-tonnage mineralisation distributed through thousands of individual micro-veinlets, as seen in porphyry copper deposits, while dilational jogs and intersections between two joint sets are preferred sites of the highest fluid flux and consequently the highest-grade ore shoots within a vein system. Joint networks also increase the surface area and permeability available for supergene (near-surface, weathering-related) enrichment, allowing oxidising, metal-bearing groundwater to penetrate deep along joints and redeposit metal at and below the water table.