10th International Kimberlite Conference, Bangalore - 2012 10IKC-246 FACTORS CONTROLLING THE INTERNAL FACIES ARCHITECTURE OF KIMBERLITE PIPES Kurszlaukis, S., Fulop, A. De Beers Canada Inc., Toronto, Canada Under close examination, most if not all diatreme and maar crater with several lobes. kimberlite pipes show a multitude of facies types For our study we are investigating kimberlite suggesting that the pipes were emplaced under pipes of the Jurassic Attawapiskat Kimberlite an episodic reoccurrence of eruptive phases, often Field in northern Ontario, Canada. The Field is with intermittent phases of volcanic quiescence. comprised of 23 known kimberlite pipes, one of The majority of these facies can be related to either which is Victor Mine. Next to Victor, 8 pipes a change in the fragmentation behavior of the (Bravo-1, Delta-1, India-1, Tango Extension, magma during emplacement or are a result of Whiskey, X-Ray, Yankee and Zulu) were drilled changing conditions during sedimentation of sufficiently in order to understand the internal volcaniclastic deposits, as well as their alteration facies architecture that is controlling diamond and compaction after deposition. distribution within a specific pipe. Victor itself is An additional factor controlling pipe facies actually comprised of several pipes (Webb et al., architecture is the degree of mobility of the loci 2004; van Straaten et al., 2009, 2011): Victor of explosions in the explosion chambers of the Southwest, Southeast and Victor North. The latter root zone or root zones at the base of the maar- occurrence is a product of several amalgamated diatreme volcano (Ort and Núñez-Carrasco, pipes: Victor Northwest and Victor Main, all of 2009). In a growing pipe the root zone moves which have a complex internal facies architecture downwards and with that movement the overlying that resulted from the emplacement of different diatreme enlarges both in size and diameter phases of kimberlite over an extended span of (Lorenz and Kurszlaukis, 2007). However, during time. the life span of the volcano, the explosion chamber During the study of the maar diatreme can also move upwards, back into the lower volcanoes we realized that the facies architecture diatreme, where renewed explosions result in the of a number of pipes showed repetitive patterns. destruction of older deposits and their structures These patterns are not controlled by a change in (White and Ross, 2011). the nature of the country rocks, since all pipes Next to vertical shifts of explosion belong to the same kimberlite field and the country chambers, the loci of explosions can also move rock strata is a Paleozoic sedimentary cover laterally along the feeder dyke or dyke swarm. sequence that had a thickness of about 500 m at This mobility of explosion chambers results in a the time of emplacement and that shows very little highly complex facies architecture where a pipe lateral lithological variation throughout the field. can be comprised of several separate root zones In the case of the Attawapiskat kimberlite pipes, that are overlain by an amalgamated, crosscutting the distribution of volcanic facies within a Extended Abstract 1 10th International Kimberlite Conference, Bangalore - 2012 kimberlite pipe is rather controlled by the degree the energy released in a subsequent emplacement of shifting of the explosion chambers and their cycle, earlier tephra that filled the pipe may be respective feeder vents during the emplacement completely ejected, leaving little or even no history of a pipe. A variation of pipe sizes also evidence of its existence. However, if late eruptive allows us to investigate the facies architecture of cycles start with relative low energy explosions, different structural levels of a pipe, straddling from then earlier tephra in the pipe can survive this lower diatreme to deposits that filled the syn- and event and serves as a witness to earlier volcanic post-emplacement crater. Hence, our study activity in the pipe. Depending on the explosive includes processes that are both primary and force position of the explosion chamber in the pipe resedimented. By having an understanding of the and vent geometry, the existing tephra becomes processes, we could also distinguish deposits that variably affected by the renewed volcanic activity. were modified in their original structure during In steep-sided and small pipes it is rather the emplacement of younger phases. easy for a subsequent eruption to either fully eject The examples from the Jurassic earlier tephra deposits filling the pipe or Attawapiskat kimberlite field show that some completely homogenize and mix it. The latter case kimberlite pipes were excavated during initial is indicated by massive deposits and grain-in-grain strong explosions forming several 100 m deep mixing of different types of juvenile pyroclasts. craters. Towards the end of the emplacement, the The Bravo-1 pipe is such an example in the eruptive style became less explosive, resulting in Attawapiskat kimberlite field. a rather low-energy fragmentation behavior of the In larger pipes or pipes that are to some magma with a very low ability to fragment country extent flaring towards the surface, it is more rock. During this phase the deep craters are filled difficult for a renewed eruption to fully eject with hot pyroclastic deposits that often have a earlier tephra from the pipe. The stronger an remarkable low content of country rock xenoliths. explosion is the more overlying tephra will be The final phase of an eruptive cycle is often affected. Less energetic explosions will result in marked by low-energy spatter fragmentation that eruptions that pierce in the form of a feeder vent produces welded pyroclastic deposits or even lava through the overlying tephra pile to exit to the lakes. Late intrusions of coherent kimberlite sheets surface or, in weak explosions, will not be able to into older kimberlite tephra filling the pipe are a leave the explosion chamber in the root zone or common theme in the Attawapiskat kimberlite may get stuck in the lower portion of the diatreme pipes. This emplacement behavior can be repeated as failed tephra jets. In the latter case these weak if a new pulse of magma enters the pipe at its explosions will still alter the original structure in bottom. Depending on the time gap in between the lower diatreme, where they destroy bedding the ascending kimberlite pulses, older tephra in and homogenize the pipe infill. These failed tephra the pipe is entirely, partly or not at all jets will mix with the surrounding tephra already consoli­dated. In either case mixing of cognate filling the pipe and may, in some cases, be still volcaniclastic pyroclasts or juvenile pyroclasts of traceable by their diagnostic petrographic features different nature testify the emplacement of and variations in diamond content. kimberlite within a pipe as repetitive, cyclic If the explosion sites in the root zone are events. laterally immobile then older deposits, which were It is apparent that these eruptive cycles, from originally forming mostly horizontal layers on the highly explosive to low energy fragmentation and respective crater floors, become tilted and down coherent intrusion can be repeated several times rafted towards the centre of the pipe. This is firstly during the emplacement of a pipe. Depending on a function of simple compaction of the tephra, Extended Abstract 2 10th International Kimberlite Conference, Bangalore - 2012 which will be more pronounced in the central If the explosion sites are stagnant, for portion of a pipe due to its larger depth extent of example due to the strong influx of external water the underlying tephra column compared to the from an aquifer into the root zone, then the margins of the pipe. In addition, the consumption ongoing explosions will widen the root zone. The of existing tephra in the lower portion of a pipe wider a root zone the less impact an explosion due to renewed eruptions originating in the root has on the country rocks, since the root zone is zone also causes tilting and down rafting of filled by down sagged kimberlite tephra. Hence formerly horizontal beds towards the deep central these explosions will not further grow the pipe part of a kimberlite pipe. by the excavation of country rock but will rather This cyclic emplacement behavior combined mix and recycle tephra of the overlying diatreme with tilting due to compaction and down rafting with newly fragmented tephra from kimberlite forms an internal symmetric facies architecture intruding the bottom of the root zone. This with a cone-in-cone like arrangement of volcanic scenario of a stagnant root zone leads to a pipe deposits. The Victor NW pipe is an example where that flares upwards from the bottom of the pipe these emplacement sequences are repeated over with a tephra infill that is typically low in xenolith time. content and is highly homogenous in nature. Vertical shifts of the explosion sites within Victor Main, which is to some extent flaring from the root zone can also add complexity to the an artesian aquifer at the interface between internal pipe architecture. Pipe growth occurs basement and overlying Paleozoic sediments, is when the explosion chambers move downwards an example for such a pipe. and the overlying diatreme expands both towards If aquifers or hydrologically active faults depth and laterally. In basaltic maar-diatreme become depleted during volcanic activity, volcanoes this continuous downward growth intruding magma can pass the site of earlier process is typically achieved when a vertical fragmentation and forms intrusive sheets higher hydrologically active fault is cut by the feeder within the diatreme or forms spatter that may dyke of the volcano resulting in thermohydraulic reconstitute to lava on the maar crater floor (as explosions. In the Attawapiskat area the seen in Victor NW, Delta and India kimberlite hydrological situation is different in that pipes).