24-MMP-A1 General Geology and Exploration · December 2018
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
EGBC National Exam — Mining and Mineral Processing Engineering, 09-MMP-A1 General Geology and Exploration, 2018-Dec. 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: Klein & Dutrow, Manual of Mineral Science, 23rd ed. (crystal systems, diagnostic physical properties, hand-specimen identification); 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); Telford, Geldart & Sheriff, Applied Geophysics, 2nd ed. (gravity, magnetic, electrical, EM and seismic methods); 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 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.
Given. The eight genetic categories named in the question are the standard divisions of Guilbert & Park's genetic classification of ore deposits, grouped by the process that concentrates the ore rather than by the host rock it ends up in.
| Genetic category | How such deposits form | Characteristic deposit type |
|---|---|---|
| (i) Magmatic | Ore minerals crystallize directly from, and remain part of, a cooling silicate magma – via fractional crystallization and gravity settling of dense early-formed crystals into cumulate layers. | Chromitite seams of the Bushveld Complex, South Africa (a layered mafic intrusion). |
| (ii) Pegmatitic | Volatile- and incompatible-element-enriched residual melt, left after a granitic magma has largely crystallized, itself crystallizes as an exceptionally coarse-grained rock enriched in rare elements. | Rare-element (Li–Cs–Ta) pegmatites such as the Tanco pegmatite, Manitoba. |
| (iii) Magmatic-hydrothermal | Aqueous fluids exsolved from a cooling, crystallizing intrusion carry dissolved metals outward and precipitate ore in fractures, veins, or by replacement of the surrounding rock as the fluid cools or reacts with wall rock. | Porphyry copper deposits centred on shallow felsic-to-intermediate stocks. |
| (iv) Supergene | Near-surface, descending oxidizing groundwater weathers and leaches metal from an already-mineralized zone, then reprecipitates it in a secondary, higher-grade blanket at or below the water table. | Supergene chalcocite enrichment blankets over low-grade porphyry copper deposits. |
| (v) Allochthonous sedimentary | Dense, resistant mineral grains are mechanically liberated by weathering, TRANSPORTED away from their primary source, and concentrated by hydraulic (gravity) sorting during that transport. | Placer gold and heavy-mineral (ilmenite–rutile–zircon) beach-sand deposits. |
| (vi) Autochthonous sedimentary | Ore constituents are chemically or biochemically precipitated essentially IN PLACE within the sedimentary basin, syngenetic with the enclosing sediment, with no significant lateral transport of the ore material itself. | Banded iron formation (BIF), e.g. the Precambrian iron ranges of the Lake Superior district. |
| (vii) Diagenetic-hydrothermal | After burial, warm basinal brines migrate through porous strata during diagenesis and precipitate ore minerals at a chemical or structural trap – well after, and independent of, the original sediment deposition. | Mississippi Valley Type (MVT) Pb–Zn deposits hosted in platform carbonate rock. |
| (viii) Metamorphosed | An ore deposit of any of the above genetic types is subsequently recrystallized, remobilized and/or structurally reworked by later regional or contact metamorphism. | Taconite – metamorphosed banded iron formation of the Lake Superior iron district. |
The eight categories fall into three natural groups: (i)–(iii) are all driven by an igneous process, differing only in HOW FAR the ore-forming material has travelled from the parent magma (retained in the rock itself, concentrated in residual melt, or carried off in an aqueous fluid); (iv)–(vii) are all surface-to-shallow-crustal processes acting on a sedimentary basin, differing in whether concentration happens by mechanical transport, in-place chemical precipitation, post-burial fluid flow, or post-depositional weathering; and (viii) is a single "wildcard" category that can be applied on top of any of the other seven, since almost any ore body can be subsequently metamorphosed if the terrane it sits in is later deformed and heated.
