Comminution of indicator minerals in a tumbling mill: implications for mineral exploration

Abstract

Heavy mineral dispersal trains are commonly used during mineral exploration to locate ore bodies buried at depth. Heavy minerals tend to physically break down (comminute) as they are transported away from their bedrock sources, causing them to become smaller, less frequent and commonly rounder. The size, roundness and concentration (number) of heavy mineral grains are therefore commonly used as indications of proximity to source. However, few studies have investigated the rate at which different heavy minerals break down during transport. To provide quantitative insight into this, we studied the comminution of several heavy minerals used in diamond exploration, termed kimberlite indicator minerals (KIMs), in a tumbling mill. In the experiment, pyrope garnet grains lost mass the fastest, followed by chrome diopside grains, which generally lost mass much slower, and, finally, ilmenite grains, which hardly lost any mass at all. The pyrope grains lost mass the fastest because each grain broke into tens to hundreds of angular fragments, producing abundant sand--?sized particles, in addition to abundant mud (i.e. silt and clay). By contrast, the chrome diopside and ilmenite grains remained relatively intact and lost mass primarily by edge rounding, which produced a comparatively small amount of mud and little to no sand. These results suggest that in situations where grain comminution occurs during transport, the sand and gravel (>0.063 mm) and mud (<0.063 mm) fractions of kimberlite dispersal trains have the potential to continuously change in composition downflow-specifically the mud and finer sand fractions may become progressively enriched in pyrope garnet fragments relative to fragments of other indicator minerals moving away from the source.