The petrology and petrogenesis of lamprophyric dykes in the Bushveld Complex, and their possible role in causing gas outbursts

Abstract

A suite of Jurassic-Cretaceous lamprophyric dykes cross-cuts the Bushveld Complex. The lamprophyric dykes, not observed at surface due to their friable nature, have frequently been documented underground in platinum mines due to their association with gas outbursts that have taken place in the Western Limb over the past decade. The dykes are steeply dipping, have a thickness of 0.5 to 3 m and occur in a dyke swarm consisting of irregular, subparallel segmented and anastomosing dyke sets. The dykes are composed of macrocrysts of olivine that have been serpentinised to variable degrees, phenocrysts of phlogopite and a groundmass composed of fine-grained phlogopite, pyroxene and apatite with interstitial calcite. Some dykes have voids and veins that cross-cut the dykes themselves and are infilled with secondary minerals. Upon outburst in 2007, one dyke, composed of phlogopite and pyroxene phenocrysts and set in a glassy groundmass of halite and natrolite, contained a geode of halite and natrolite, indicating that voids (in excess of 10 cm size) were present within the lamprophyre dyke. Lamprophyric dykes associated with outbursts have no structural, textural or mineralogical differences to those that have not been involved in outbursts. The halite-natrolite geode sample contains fluid inclusions of three different types, threephase liquid + vapour + solid, two-phase liquid + vapour and single-phase liquid-only inclusions, but shows very few phase-changes during microthermometry, preventing compositional characterisation by this method. ‘In-line’ gas chromatography shows that gases are trapped within the lamprophyric dykes within pore spaces or in the form of adsorbed gases, or fluid inclusions within secondary minerals. The dominant gases trapped in the dykes are CO2+CO, CH4 and C2H6 and occur in average abundances of 5.05×10-9 mol/g, 1.66×10-8 mol/g and 6.86×10-10 mol/g, respectively. Four boreholes drilled underground through the face of the dykes were tested for gas by in-situ gas analysis. CH4, CO and H2 were the prevalent gases detected by the gas sensor and these gases were found to be constantly streaming out of the dykes at maximum levels of 7500 ppm, 24 ppm and 135 ppm, respectively. Active gas emission from the dykes implies that there is a continuous source of gas originating, not from the dykes themselves, but elsewhere in the system. Therefore, the dykes are merely acting as a conduit through which these gases are passing. The present mechanism of outbursts in platinum mines of the western Bushveld appears to be the overpressurisation of the aforementioned gases (likely to be sourced externally from the dykes – e.g., deeper or laterally distal) within a conduit system, comprising lamprophyre dykes and possibly other permeable pathways (e.g. faults and fractures). Where present, voids of various sizes (but probably mostly microscale, although the geode would indicate that some decimetre scale examples exist) within the lamprophyre dykes also act to accumulate these gases, forming pressurised gas pockets. Upon rapid depressurisation due to mining activities such as blasting or drilling, outbursts (at a variety of scales and magnitudes) take place causing damage to underground equipment and injuries to mine workers.