Geology of the K1 and K2 Kimberlite Pipes at Murowa, Zimbabwe

Abstract

The Murowa kimberlite field includes three kimberlite pipes and multiple kimberlite dykes that have been emplaced into the Archaen Chibi granite batholith north of the Limpopo Belt in south-central Zimbabwe. Here we describe the geology of the two largest kimberlite pipes: K1 and K2. Observations of drill core, thin section petrography, and mapping of exposed rocks at the Murowa Diamond Mine are used to describe the internal geology and these data form the basis for three-dimensional geological models of each body. The modeled pipe geometries are combined with observed cross-cutting relationships and textural variations among deposits to interpret the nature of emplacement. K1 is an irregular-shaped, multi-lobed kimberlite pipe occupied by volcaniclastic and coherent kimberlite, and enveloped by kimberlite-poor country-rock granite breccias and volcaniclastic rocks. K1 is interpreted to result from multiple emplacement events, involving kimberlite magmas with varying proportions of gas, liquid and solid phases encountering country rock with different amounts of previous fracturing and brecciation, and intruding/erupting over contrasting timescales. K2 is a steep-sided, sub-circular pipe which flares slightly with depth and is dominantly infilled by massive coherent kimberlite. The textural characteristics of the infill preserved in K2 indicate a less explosive emplacement style relative to K1. Observations of pipe shape, deposits along the pipe margins, and textural modification form the basis for interpreting the relative stage of volcanic development, or ‘maturity’, of K1 compared to some other kimberlites. Designations of pipe maturity acknowledge the uniqueness of kimberlite occurrences, and have implications for interpreting the explosivity and duration of kimberlite eruptions, and for resource modeling of kimberlite pipes.