Anatomy of a distal kimberlite indicator mineral (KIM) anomaly in the Central Kalahari, Botswana

摘要

Kimberlite and lamproite exploration programmes have traditionally been underpinned by systematic sampling programmes, aimed at identifying anomalous concentrations of kimberlite indicator minerals (KIMs) proximal to the host rocks. These "pathfinders", which typically include one or more of the phases ilmenite, eclogitic and peridotitic garnets, chromite, Cr-rich diopside, occasionally olivine, and sometimes diamond, tend to concentrate as a lag halo around the source rocks because of their relatively high densities compared to more abundant crustal minerals such as quartz and feldspar. Dispersion of the KIMs away from the parent rocks by processes such as fluvial and glacial transport, and longshore currents in lacustrine or marine environments, typically results in decreasing concentrations away from the source. However, distal re-concentration of pathfinder minerals may occur in geomorphic settings which act as traps or sinks for relatively dense phases. It is important to distinguish between proximal and distal pathfinder concentrations at an early stage, not only because of the time and cost associated with sampling programmes, but also with the view to planning ongoing exploration to locate the ultimate source rocks. This study highlights some of the typical characteristics of distal KIM anomalies within the Kalahari environment with reference to a major kimberlitic garnet anomaly located in the southwestern Makgadikgadi basin in northern Botswana.