Internal geology and emplacement processes of Kimberlites, Case Study from the Tsabong Kimberlite Field, Botswana

Abstract: The Crateceous (85 Ma) MK1 pipe is part of the Tsabong kimberlite cluster located approximately550km south west of Gaborone and geologically found within the Kapvaal Craton. The Tsabongkimberlite field consists of 86 known kimberlites. The MK1 pipe is the largest of this cluster at 140ha,1.7km long axis and 750m short axis and it is considered the largest diamondiferous kimberlite pipe inthe world. The kimberlite intrudes the Karoo supergroup that ranges in age from Late Carboniferousto Early Jurassic. The Karoo supergroup sequence is made of mostly glacial, siliciclastic rocks andrecently unconsolidated to semi-consolidated material. The Tsabong area consists dominantly oftillites, mudstones, siltstones, sandstones and quartzites. Only limited geological information isavailable from MK1 since the kimberlite pipe is not exposed to the surface and the exploration projectis not yet in the advanced. Because of this limited data available for understanding the morphology ofthe pipe an assumption has been made that the MK1 pipe belongs to the common type of SouthernAfrican pipes. The classical Southern African pipes consist of three pipe zones, the crater, diatreme androot zone. Furthermore, MK1 pipe displays minimal erosion and as such the current geology depictsthe results of the processes during emplacement. This study involves the investigation of the MK1emplacement process by studying its internal geology based on four boreholes by detailedsedimentological and volcanological analysis of the volcaniclastic material filling the pipe. A downholemagnetic susceptibility measurement was also conducted from the four boreholes to furtherdistinguish the internal lithofacies of the MK1 pipe. Three distinctive lithofacies have been identified,these principal lithofacies are: RVKA-resedimented volcanoclastic kimberlite A , RVKB-resedimentedvolcanoclastic kimberlite B that is divided into two sub-lithofacies RVKB C (clast supported) andRVKB m (matrix supported) and finally the PVK-pyroclastic volcaniclastic kimberlite (lower most facie).The RVKA and RVKB are formed as crater infill by re-deposition of unconsolidated pyroclastic and othersurface material. These mechanisms are normally dominated by lacustrine, fluvial and mass flows(talus and debris) processes. The post emplacement related hydrothermal alteration andmetamorphism, mainly serpentinization forming secondary magnetite is believed to have been thelargest contributor to high magnetic susceptibility readings of the MK1 facies. Contaminationprocesses such as wall rock collapse also contributed to a wide variation within the differentlithofacies. The PVK has higher average magnetic susceptibility and variation which is mostly attributedto high content of kimberlitic material and strong serpentinization of olivines resulting in high contentsof magnetite. Most of the country rock within the Tsabong area are siliciclastic sediments and occur asan important component in the RVK-resedimented volcaniclastic kimberlite resulting in generally lowmagnetic susceptibility in the RVKA and more dynamic variations within the RVKB. The emplacementof the MK1 pipe is similar to the Class 1 kimberlites. The pipe excavation is related to the initial nearsurface emplacement process characterized by violent explosive activities caused by exsolution ofjuvenile volatiles from the uprising magma. The PVK of the MK1 pipe depicts the speedy primarypyroclastic deposition processes as a result of the explosive volcanic eruptions, the PVK is dominatedby mostly pyroclastic fall material and to a lesser extent both flow and surge pyroclastic process. ThePVK is mostly medium to coarse grained, massive and homogenous unit with poorly sorted material asa result of low energy settling during eruption. The RVKB represents the pipe wall collapse duringsubsequent crater enlargement stage. This unit is made up of two sub-lithofacies with differentsedimentation processes. RVKB C is related to gravitational sidewall failure producing high energytalus mass flow deposits of poorly consolidated angular macro-clasts of mainly Permian mudstone andsiltstones of the Ecca group. The RVKB m represents debris flow dominated mass flows and ischaracterized by normal grading and lacking large country rock clasts. The RVKB m clasts are mostlysand sizes and well-rounded indicating longer transport and distal settling from the pipe wall. The RVKAconsist of tuff ring material that was subsequently filled into the remaining crater space. The RVKA  represents fluvial and lacustrine depositional environments, as the unit is dominated by finer grainedand bedded material of both juvenile kimberlitic material and country rock sediments. The beddedunits indicate low energy sedimentation environments as they show normal grading of clasts. Some ofthe RVKA have laminated beds that show micro faulting which may be attributed to subsidence of thevolcaniclastic infill related to gravity induced compaction of epiclastic sediments. A second eruptionmay have occurred that resulted in the displacement of the PVK unit found within the RVKA unit inborehole BH004.This also may be attributed to the characteristics of juvenile magma clasts foundwithin the volcaniclastic kimberlites that suggests an earlier crystallization and devolatisation ofprimitive kimberlite magma at depth before the vigorous fragmentation of country rock (embryonicpipe development).The MK1 also shows that there must have been a phreatomagmatic eruptioninfluence to a certain degree in the emplacement process. This is evident by the presence of somearmoured lapilli and accretionary clasts within the RVKA and RVKB facies. These phreatic explosionsmay have been during initial magma eruption or maybe synchronous with RVK resulting from nearsurface interaction of magma with groundwater which are common within the Karoo sedimentary rocks.