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 kinds of materials typically sampled in a geochemical survey are soil, stream sediment and rock (bedrock/outcrop chip) – vegetation and glacial till are two further media used in specific settings (deep cover, glaciated terrain). The main purpose of geochemical sampling is to detect, at surface, an anomalous concentration of a pathfinder or ore-related element that points toward a buried or otherwise concealed mineral deposit at depth or up-slope/up-stream. This is achieved by systematically collecting samples of the chosen medium on a grid, traverse or drainage-network pattern, analyzing each for the element(s) of interest, and comparing the results statistically against a local BACKGROUND level established from unmineralized ground – a sample that is anomalously enriched relative to that background marks the presence, and helps locate the direction, of a geochemical dispersion halo (part b).
A dispersion halo is the zone of anomalously elevated element concentration that surrounds a mineral deposit, produced as metal is physically or chemically dispersed away from the source into the surrounding rock, soil, water or sediment. A PRIMARY halo forms in bedrock around the deposit at the time of ore formation (by diffusion/infiltration of hydrothermal fluid into the wall rock); a SECONDARY halo forms much later, at or near the present surface, as weathering liberates metal that is then dispersed mechanically (in soil creep or stream sediment) or hydromorphically (in solution, in groundwater or surface water).
Factors controlling a dispersion halo's extent include: the SIZE and GRADE of the source deposit (a larger, richer source produces a larger halo); the MOBILITY of the element itself under surface (oxidizing, near-neutral-to-acid) conditions – some elements (e.g. Zn) are far more mobile in solution than others (e.g. Au, which disperses mainly as mechanical detrital grains); CLIMATE (a wet, temperate climate promotes hydromorphic dispersion in solution, while an arid climate favours mechanical dispersion and can even destroy a halo through evaporative re-precipitation near surface); TOPOGRAPHY and DRAINAGE (steep terrain and active streams carry mechanically-dispersed metal much further downslope/downstream than flat, poorly-drained ground); TIME since the source was exposed to weathering (a longer exposure time allows a larger halo to develop); and the PERMEABILITY of the transporting medium (porous, permeable soil or sediment disperses metal further than dense, impermeable material).
| Station | Gold content | Reason |
|---|---|---|
| 1 | Zero | On the Ootus River, upstream of station 2, which is itself upstream of X – dispersion in a stream-sediment train travels DOWNSTREAM from its source, so nothing upstream of the deposit can be anomalous. |
| 2 | Zero | Still upstream of X (X lies in the river reach between stations 2 and 3) – station 2 sits at the top of the mineralized reach and receives no sediment from the deposit itself. |
| 3 | High | The first station downstream of X, at the head of the dispersion train where the anomaly is freshest and least diluted by additional barren sediment or tributary inflow. |
| 4 | Zero | On the Boobalie River, a separate catchment with no hydrological connection to X above the confluence – sediment here cannot have passed the deposit. |
| 5 | Zero | Also on the Boobalie River, downstream of station 4 but still entirely within the barren Boobalie catchment. |
| 6 | Moderate | Below the confluence of the two rivers: it receives the Ootus dispersion train (already carrying gold from X, past station 3) but that train is now DILUTED by the additional, gold-barren water and sediment volume contributed by the Boobalie River – still clearly anomalous above background, but lower than the undiluted signal at station 3. |
The optimum spacing for a geochemical stream (drainage) survey is set by drainage-basin geometry rather than a fixed distance: a sample should be collected immediately below EVERY significant tributary junction, so that each individual sub-catchment is tested independently and an anomaly can be traced, junction by junction, back toward the specific tributary (and ultimately the specific reach) that is contributing it. As a rule of thumb for reconnaissance-scale work, this typically works out to roughly one sample per 1–2 km² of drainage area, though the true target is "one sample per confluence," not a fixed grid interval.
The stations shown on the map are only PARTLY at this optimum spacing: station 6, immediately below the Ootus–Boobalie confluence, is correctly placed to test the combined catchment. However, there is no station bracketing the mouths of the minor tributaries implied by areas W, Y and Z, and – more importantly – no station sits immediately at the downstream edge of area X itself; station 3 is displaced some distance downstream of the actual mineralized reach. A tighter spacing, with a sample taken at the outlet of each labelled sub-area (W, X, Y, Z) as well as immediately above and below the confluence, would pin down the source to a specific short reach rather than the broader interval currently bracketed only by stations 2 and 3.
