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.
BIF forms in shallow, relatively quiescent Precambrian marine basins (dominantly Archean to Paleoproterozoic) from iron- and silica-charged seawater. Before the atmosphere and oceans were oxygenated, hydrothermal venting and continental weathering could keep large quantities of soluble ferrous iron (Fe²⁺) dissolved in seawater; the onset of oxygenic photosynthesis by cyanobacteria (leading toward the Great Oxidation Event, ≈2.4 Ga) periodically raised dissolved oxygen enough to oxidize Fe²⁺ to insoluble ferric oxyhydroxide, which settled out chemically, alternating with silica-rich chert layers to produce the characteristic banding – a rhythm most often attributed to cyclical or seasonal fluctuation in the oxidation front or in silica/iron supply.
SEDEX deposits form in extensional (rifted), fault-bounded sedimentary basins where deep-circulating basinal brines are heated by the elevated geothermal gradient typical of active rifting and leach base metals (Pb, Zn, Ag) from underlying and adjacent basin-fill sediments and basement. These metalliferous brines rise along syn-sedimentary growth faults and vent onto, or are injected just below, the contemporaneous seafloor into anoxic basinal muds, precipitating stratiform to sub-seafloor-replacement sulphide layers interbedded with fine basinal sediments (black shale, chert, carbonate).
Orogenic (mesothermal) lode gold deposits form during regional metamorphism and orogenesis, when progressive devolatilization of subducting or thickening metamorphosed crust (particularly greenschist-to-amphibolite facies transition) releases large volumes of low-salinity, CO₂-H₂O-bearing metamorphic fluid. This fluid migrates upward and outward along crustal-scale fault and shear zones, and gold (with associated pyrite, arsenopyrite, and quartz-carbonate vein gangue) is deposited where structural dilation (fault jogs, fold hinges, splays) and physicochemical traps (fluid/wall-rock sulphidation reactions, pressure fluctuation and fluid immiscibility/boiling) cause rapid destabilization of the gold-transporting complex.
Porphyry copper deposits form around shallow (roughly 1–5 km depth), volatile-rich, intermediate-to-felsic porphyritic stocks emplaced in subduction-related magmatic arcs. As the intrusion crystallizes, it becomes saturated in an aqueous fluid that exsolves and separates from the melt, carrying copper (and commonly molybdenum, gold) upward into the cupola and surrounding country rock, where it deposits metal in pervasive stockwork veinlets accompanied by zoned hydrothermal alteration (potassic core, grading outward through phyllic, argillic and propylitic zones), producing large-tonnage, low-grade, disseminated/stockwork mineralization in both the intrusion and its wall rock.
Marine phosphorite forms on continental shelves and upper slopes beneath zones of strong oceanic upwelling, where cold, nutrient- and phosphate-rich deep water is drawn onto the shelf, fuelling high biological productivity and phosphate release from decaying organic matter. Where clastic sediment input is low (so the phosphate signal is not diluted), phosphate is authigenically precipitated as fine-grained carbonate-fluorapatite, commonly microbially mediated, within organic-rich sediment; subsequent wave and current reworking can further concentrate the phosphate into higher-grade nodular, pelletal or oolitic beds.
