24-MMP-A1 General Geology and Exploration · May 2015
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, 2015-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, concordant/stratiform vs stratabound terminology); 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. (stream-sediment dispersion, survey design parameters); 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.
Ore genesis is most usefully organized as a spectrum running from deposits formed directly within a crystallizing magma, through fluid-mediated (hydrothermal) processes at progressively lower temperature and greater distance from the heat source, to surface and near-surface sedimentary and weathering processes, with metamorphism able to act on a deposit of any prior origin. Each category below is described in that framework.
Magmatic deposits form by direct crystallization and physical concentration of ore minerals from a silicate magma, with no separate aqueous fluid phase involved. Concentration mechanisms include fractional crystallization and gravitational settling of dense early-formed crystals (chromite, magnetite) to the base of a layered intrusion, and liquid immiscibility, in which an immiscible sulphide melt (Fe-Ni-Cu-S) separates from the silicate magma and settles or is injected into structural traps. Characteristic deposit type: magmatic Ni-Cu-PGE sulphide deposits, e.g. the Sudbury Igneous Complex, Ontario, and Voisey's Bay, Labrador.
Pegmatitic deposits crystallize from the final, most hydrous and volatile-rich residual melt left after a granitic magma has fractionally crystallized. This residual liquid is strongly enriched in incompatible elements (Li, Be, Cs, Ta, Nb, Sn, REE) that could not be accommodated in the earlier rock-forming minerals, and its high volatile content depresses the liquidus and promotes very rapid crystal growth, producing the coarse, often zoned, dike- or lens-shaped pegmatite bodies at the margins of the parent pluton. Characteristic deposit type: rare-element (Li-Cs-Ta, "LCT") pegmatites, e.g. the Tanco pegmatite, Manitoba.
Magmatic-hydrothermal deposits form from an aqueous, metal- and volatile-bearing fluid that exsolves from a crystallizing, commonly porphyritic intermediate-to-felsic intrusion once the melt becomes saturated in an aqueous phase. The fluid rises into the carapace of the intrusion and surrounding country rock, depositing metals as it cools, mixes with meteoric water, and reacts with the wall rock, producing zoned hydrothermal alteration. Characteristic deposit type: porphyry copper (±Mo, Au) deposits, e.g. Highland Valley Copper, British Columbia.
Supergene deposits form at and below the water table by weathering: descending, oxygenated meteoric water oxidizes and leaches metals from an exposed primary (hypogene) sulphide deposit, and the metal-charged solution re-precipitates the metal at or just below the water table, where it encounters reducing conditions. This produces a zone of secondary enrichment with grades several times the original hypogene grade. Characteristic deposit type: supergene chalcocite blankets over porphyry copper deposits, e.g. Chuquicamata, Chile.
Allochthonous ("from elsewhere") sedimentary deposits form when ore-mineral grains are mechanically eroded from a primary source, transported by surface processes (rivers, waves, wind) and then concentrated by density sorting at a site physically separate from where the minerals originally formed. Because the ore constituents themselves were transported, this is a mechanical, not a chemical, concentration process. Characteristic deposit type: placer deposits, e.g. placer gold of the Klondike goldfields, Yukon.
Autochthonous ("in place") sedimentary deposits form by chemical or biochemical precipitation of the ore minerals directly within the sedimentary basin, with no significant transport of the ore-forming constituents themselves – they are precipitated in place from basin water (seawater, lake water or evaporating brine). Characteristic deposit type: evaporite potash deposits, e.g. the Prairie Evaporite Formation potash deposits of Saskatchewan, precipitated in place as Devonian seawater evaporated.
Diagenetic-hydrothermal deposits form during diagenesis (basin burial, prior to metamorphism), when warm basinal brines – heated and mobilized by burial and tectonic loading rather than by a magma – migrate along permeable strata or faults and precipitate ore minerals on encountering a chemical or physical trap (a reactive carbonate host, a redox boundary, a structural/stratigraphic seal). Characteristic deposit type: Mississippi Valley-type (MVT) Pb-Zn deposits, e.g. Pine Point, Northwest Territories, hosted in Devonian carbonate.
Metamorphosed deposits are pre-existing ore deposits, of any of the origins above, that have subsequently been subjected to regional or contact metamorphism. Metamorphism recrystallizes the ore and gangue minerals, can locally remobilize metals into new structural sites (fold hinges, boudin necks, shear zones), and typically upgrades textural coarseness and mineral assemblage without necessarily changing the total metal endowment. Characteristic deposit type: metamorphosed SEDEX Pb-Zn-Ag deposits, e.g. Broken Hill, New South Wales, Australia, whose original diagenetic-to-syngenetic sulphide layers were recrystallized and structurally remobilized during granulite-facies metamorphism.
