Question 24 of 27

Question 5.1 mine planning and scheduling — ore schedule (7 marks)

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. Ore reserve by elevation and phase (Figure 5.1.1, million tonnes); annual ore schedule 7 Mt/yr starting Year 1; NW-corner method (mine the highest un-mined elevation down to quota, then continue to the next elevation/phase).

Find. The completed year-by-year ore schedule (5.1.1), then the qualitative planning discussion (5.1.2–5.1.5).

Approach. Sweep the NW-corner method top-to-bottom through Phase 1’s non-zero elevations first, then continue seamlessly into Phase 2’s non-zero elevations once Phase 1 is exhausted, filling each year to exactly 7 Mt (the final year taking whatever remains).

1. 5.1.1 — completed ore schedule. Applying the 7 Mt/yr NW-corner sweep to the 50 Mt combined reserve:

$$\boxed{\text{Ore mining spans 8 years, with a final partial Year 8 of 1 Mt}}$$ Phase 1 (24 Mt, elevations 1970–1910) is fully consumed during Year 4, at which point Phase 2 mining (elevations 1970–1880) begins seamlessly within that same year and continues through Year 8.

5.1.2 — the Phase 1 to Phase 2 transition. The transition happens mid-Year 4: the last Phase 1 ore (2 Mt at el. 1925, 1 Mt at el. 1910 — the deepest Phase 1 benches) is mined in the same year that the first Phase 2 ore (2 Mt at el. 1970, 2 Mt at el. 1955 — Phase 2’s HIGHEST benches) begins, i.e. Phase 2 mining starts near the ORIGINAL SURFACE while Phase 1 is finishing near its floor. Because large electric cable/rope shovels are not readily relocated day-to-day (slow, high-cost tram moves, trailing cable re-routing, substation relocation), the mine plans the changeover as a SCHEDULED, single tram move: the shovel finishing the last Phase 1 benches trams directly to the Phase 2 collar once its own bench is complete, timed against the blast/drill crew having a Phase 2 bench already prepared and ready to load — avoiding an idle shovel waiting on drill/blast readiness at the new location.

5.1.3 — stockpiling. Because porphyry/epithermal grade is spatially variable and blending is needed to hold a steady mill feed grade, ore mined ABOVE the mill’s target cut-off in a given period (when the schedule’s bench sequence happens to expose richer ore faster than the mill can absorb, e.g. a high-grade bench like Year 2’s full 1955 Phase-1 bench) is stockpiled rather than milled immediately, and later reclaimed to SUPPLEMENT lower-grade periods (e.g. the transition Year 4–5 benches, which blend Phase 1 tail-grade with Phase 2 near-surface grade) so the mill always sees a blended feed near its design grade. Relative grade: ore stockpiled is typically ABOVE the current cut-off but below the very highest-grade material sent direct to the mill; pit-run grades mined would be LOWER than stockpiled grades whenever the schedule is deliberately blending a rich bench down with lower-grade material to protect metallurgical recovery or avoid over-feeding the mill’s design capacity for a high-grade ore type.

5.1.4 — head grade over deposit life. Copper porphyry/epithermal deposits typically show a HIGH near-surface grade (supergene-enriched cap, often the Phase 1 upper benches here) declining toward a lower, more uniform primary (hypogene) grade at depth, then possibly a secondary bump if a second higher-grade zone (Phase 2’s deeper benches) is intersected later in life.

The mill and concentrate load-out must be designed with BLENDING flexibility (stockpile reclaim capacity, variable reagent dosing for flotation, and load-out/shipping capacity sized for the PEAK grade period, not the average) so recovery and concentrate quality stay on-spec through both the high-grade cap years and the leaner primary-ore years.

5.1.5 — effect of reduced shovel availability near Phase 1 completion. If poor blasting (oversize, toe problems) cuts shovel availability/productivity as Phase 1 nears its final benches (Year 4), the mine risks MISSING the 7 Mt/yr ore quota in that transition year precisely when Phase 2 must simultaneously start up — a double burden on the shovel fleet at the busiest scheduling point. The likely consequence is a delayed or compressed Phase 1→Phase 2 transition, forcing either a temporary ore shortfall to the mill or an accelerated (and less well-prepared) start to Phase 2 benches, both of which increase near-term operating risk exactly where the schedule already has the least slack.