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.
Three elements whose minerals commonly acquire a strong magnetic signature are iron (Fe), nickel (Ni) and chromium (Cr) – all transition metals whose cations retain unpaired electrons in their partially-filled 3d orbitals. Magnetism in a mineral arises from these unpaired electron spins: each cation behaves as a tiny magnetic dipole, and the mineral's bulk magnetic behaviour depends on how neighbouring dipoles interact across the crystal structure. In a mineral like magnetite (an inverse spinel), the dipoles on different crystallographic sites align antiparallel but with unequal magnitude, leaving a large NET magnetic moment (ferrimagnetism); in pyrrhotite, a similar imbalance arising from ordered iron vacancies in its NiAs-type structure produces a smaller but still significant net moment. Where the dipoles are few, weakly coupled, or fully cancel, the mineral is only weakly (paramagnetic) or negligibly (diamagnetic) magnetic.
Magnetic susceptibility (k) is the ratio of the magnetization induced in a material to the strength of the external magnetic field that induces it (M = kH) – it measures how readily a rock or mineral becomes magnetized when placed in the Earth's field. Two properties that determine a rock's bulk magnetic susceptibility are: (1) the VOLUME PERCENTAGE of strongly magnetic minerals present in the rock (chiefly magnetite and/or pyrrhotite content – a rock with 5% magnetite is far more susceptible than an otherwise identical rock with 0.5%); and (2) the COMPOSITION/CRYSTAL CHEMISTRY of those magnetic minerals themselves (e.g. the degree of titanium substitution in titanomagnetite, or which pyrrhotite polymorph is present), since susceptibility per unit volume of magnetic mineral is not a fixed constant but depends on its exact composition and grain size.
In order of lowest to highest magnetic susceptibility:
| Rank | Mineral | Magnetic behaviour |
|---|---|---|
| 1 (lowest) | Salt (halite) | Diamagnetic – a very small NEGATIVE susceptibility |
| 2 | Pyrite | Paramagnetic – a very small positive susceptibility, essentially non-magnetic in the field |
| 3 | Pyrrhotite | Ferrimagnetic – a moderate, geophysically significant susceptibility |
| 4 (highest) | Magnetite | Ferrimagnetic – the highest susceptibility of any common rock-forming mineral |
This ranking – salt < pyrite < pyrrhotite < magnetite – is worth memorizing directly: it is the standard order quoted in exploration-geophysics texts and explains why a massive sulphide body containing pyrite alone is magnetically almost invisible, while the same body would produce a strong anomaly if even a modest amount of pyrrhotite were also present.
Ground or airborne magnetic surveys measure small spatial variations in the strength (and sometimes vector components) of the Earth's total magnetic field. Because these variations are caused almost entirely by lateral changes in the abundance of magnetite and/or pyrrhotite in the underlying rock, the resulting map can be used to trace lithological contacts and structures, and to directly detect ore bodies that are themselves magnetic (e.g. a magnetite skarn) or that occur within a magnetic host (e.g. pyrrhotite-bearing massive sulphide in a magnetic mafic complex, per Question 3's rock types).
A rock that would typically produce a magnetic "high" is a mafic-to-ultramafic intrusive rock such as gabbro or norite, or a magnetite-rich banded iron formation – these rocks crystallize abundant primary magnetite. A rock that would typically produce a magnetic "low" is a reduced ("ilmenite-series") felsic granite, which crystallizes little or no magnetite; a magnetic low can also form within an otherwise magnetic host where hydrothermal alteration has destroyed pre-existing magnetite (converting it to non-magnetic pyrite or hematite), which is itself a useful vector toward some hydrothermal ore systems, since the destroyed-magnetite halo can outline the altered, potentially mineralized zone.
