twitch

A twitch is a sudden and severe thinning of an ore vein, usually over a short distance along strike or dip. The vein may pinch from, say, 1.5 metres down to 0.3 metres or less, then thicken again. Unlike a gradual taper, a twitch happens abruptly, creating a section where ore grade drops sharply and working width becomes cramped.

Twitches occur in hard-rock mining, particularly in gold, copper, and tin extraction where underground stoping or open-pit benching follows persistent vein systems. A miner working a 1-metre-wide vein may suddenly encounter a pinch 2 to 10 metres along, recover to full width, then hit another. In inclined or sub-vertical veins, twitches can appear both horizontally and vertically within a single panel.

The geological cause usually lies in local stress relief, fault displacement, or mineralisation interruption during the original hydrothermal event. A small shear or joint may have offset the vein slightly, or silica and metal minerals may have precipitated unevenly. Twitches are common where multiple veins intersect or where host rock composition changes abruptly.

Why this matters on site

A twitch forces crews to adjust drilling patterns, blasting design, and support placement. If a vein narrows to 0.2 metres, drilling standard hole spacings becomes wasteful and dangerous; pillars and walls may become unstable. Planning must account for width variation, and crews need warning from geological mapping to avoid surprises underground.

The term is British and Australian mining vernacular. North American operations may call the same feature a pinch or roll, depending on whether the narrowing is symmetric or asymmetric. Unlike a dead (complete loss of vein), a twitch is temporary; ore resumes on either side. Recognising twitches early during exploration drilling helps reserve calculations and mine design.