Question 1 of 19: Fault Types and Their Effect on Mining

Question 1.1: Fault Types and Their Effect on Mining (8 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.

1.1.1 — The three major fault types. A normal fault forms under extensional (tensional) stress: the hanging wall moves down relative to the footwall along a steeply dipping (typically 45–70°) plane, thinning the stratigraphic section. A reverse fault (thrust when the dip is shallow, <30°) forms under compression: the hanging wall moves up relative to the footwall, thickening or repeating the section. A strike-slip fault forms under shear stress with predominantly horizontal, lateral movement (dextral or sinistral) and near-vertical dip; offset is measured along strike rather than dip. The five associated terms an examiner expects sketched: hanging wall (the block lying above an inclined fault plane), footwall (the block below), throw (the vertical component of displacement), heave (the horizontal component of displacement), and strike/dip of the fault plane (its orientation, described the same way as any planar geological surface). A normal fault sketch shows the hanging-wall block dropped and the strata offset downward on the hanging-wall side; a reverse/thrust fault sketch shows the hanging wall pushed up and over the footwall, with older rock potentially thrust onto younger; a strike-slip sketch is a plan view showing a marker bed offset laterally with no vertical throw.

1.1.2 — Effect on mining costs and control. In underground operations a fault is a structural discontinuity that can host water inflow, loosened/broken ground requiring extra support (shotcrete, rock bolts, cable bolts) and altered ground stress around the fault plane, all raising development and ground-control cost; a fault crossing a planned drift or stope also forces re-design of the mining sequence and can sterilize ore left as a fault-crossing pillar. In open pit operations a fault plane, especially one dipping out of a slope (a "daylighting" structure), is frequently the controlling surface for a structurally-controlled slope failure, so pit slope angles must be de-rated wherever a fault is mapped, increasing the waste-to-ore stripping ratio; faults also displace ore blocks between drill sections, complicating grade continuity and short-term mine planning.

1.1.3 — Effect on resource quantity and quality. A normal fault can either juxtapose barren hanging-wall rock against ore (truncating and reducing apparent tonnage) or, where extension has thinned overburden, bring a deeper ore horizon closer to surface (locally improving strip ratio). A reverse fault can structurally repeat or thicken a mineralized horizon (increasing tonnage where the same bed is duplicated), but can also thrust barren rock over ore, sterilizing it beneath a false footwall contact. A strike-slip fault offsets an orebody laterally along strike with no change in the true thickness or grade of the displaced segments, but it can dramatically complicate exploration drilling and interpretation if the offset is not recognized — drill programs designed on the wrong side of an unmapped strike-slip fault can entirely miss the offset continuation of the deposit. In all three cases, fault gouge/breccia zones can also locally upgrade grade (structurally-focused fluid flow depositing secondary mineralization) or downgrade it (dilution by fault gouge and comminuted wall rock).

1.1.4 — En-echelon folding. "En-echelon" describes a series of parallel or sub-parallel structures (here, fold hinges, though the term applies equally to en-echelon veins or faults) that are offset from one another in a staggered, overlapping arrangement transverse to their own trend — like a row of roof shingles or the steps of a staircase seen in plan view, rather than one continuous linear structure. En-echelon folds typically develop under a component of shear (transpression) superimposed on the main compressive/extensional stress, so the individual fold segments are consistently oblique (commonly ~15–45°) to the overall trend of the fold belt or shear zone. For mineral resource estimation this matters because an orebody hosted in an en-echelon fold train is not one continuous fold limb but a series of discrete, overlapping segments — correlating drill intersections between adjacent segments as if they were one continuous structure over-connects the resource model and overstates continuity and tonnage.