PEREGRINE DIAMONDS LTD. MINERAL SERVICES An MS Group Business M.C. Baumgartner M. Harder C.M. Hetman J. Pell REFERENCES Sweet, A.R., Stasiuk, L.D., Nassichuk, W.W., Catuneanu, O., McIntyre, D.J., 1998. Paleontology and diamonds: geological environments associated with kimberlite emplacement, Lac de Gras, Northwest Territories, Canada. In: ExtendedAbstracts of the VIIIth International Kimberlite Conference, Victoria.Abstract FLA_0086. Doyle, B.J., Kivi, K., Scott Smith, B.H., 1999. The Tli Kwi Cho (DO27 and DO18) Diamondiferous Kimberlite Complex, Northwest Territories, Canada. In: Proceedings of the VIIth International Kimberlite Conference, CapeTown. Vol 1, 194-204. Harder, M., Hetman, C.M., Scott Smith, B.H., Pell, J., Baumgartner, M.C., This Volume. The evolution of geological models for the DO-27 kimberlite, NWT, Canada: implications for evaluation. ACKNOWLEDGEMENTS The authors would like to thank Meilani Smith and Deanna Hanchar for their drafting assistance in preparing this contribution. PIPE FORMATION AND DEPOSITIONAL PROCESSES Legend Mud Cover Sediments Mud-rich Kimberlite Water Glacier Eruption Column Granitoid Pyroclastic Kimberlite Coherent Kimberlite Fractured granite 7 tContinental glaciers cover the area, removing the Cretacous/Paleocene mudstones and likely the uppermost portion of the pipe. 6 t t Following cessation of the eruption, the crater rim erodes by sedimentary processes A crater lake forms, and minor sedimentation into the crater lake occurs. 5 t t The crater rim continues to grow and to erode into the crater. Zones dominated by resedimented kimberlite become more common and larger. These zones are concentrated near the pipe walls. 4 t t As the crater rim grows, this material, including juvenile and mud-rich kimberlite and surface mud, locally falls into the crater by mass wasting. The pipe is progressively infilled by pyroclastic kimberlite, likely by column collapse events into the pipe. 3 t t A crater rim starts to develop around the vent. Pyroclastic fall and flows are likely the dominant volcanic deposition mechanisms. Zones of fractured/pulverized country-rock locally fall into the pipe, increasing the pipe diameter. 2 t t The kimberlite breaches surface creating a pyroclastic cloud comprising kimberlitic ash, granite fragments, and mud. The marginal country-rock continues to fracture as the pipe develops. 1 t t Pre-pipe fracturing of country-rock granite; intrusive coherent kimberlite is present to the south of the pipe. The surface at the time of eruption was terrestrial with a thin veneer of Cretaceous mudstones. DISCUSSION t t t t Although DO-18 is similar to most Lac de Gras kimberlites in pipe morphology and the presence of mud-rich resedimented kimberlite, DO-18 is atypical in the high proportion of accessory granite and locally developed country-rock breccias. DO-18 is further characterised by common unbroken olivine grains and locally common kimberlitic ash. The energy of volcanic explosions producing and infilling the DO-18 pipe was insufficient to effectively break all olivine grains. The eruption column was likely not high enough or sustained for long enough to allow for dispersal of the country-rock granitic material and kimberlitic ash to significant distances from the vent. The marginal breccias likely represent the remnant products of pipe formation and excavation processes that did not efficiently ream out the pipe prior to infilling. DO-18 is interpreted to have been excavated and infilled by a relatively low-energy, short lived volcanic eruption dominated by one kimberlite phase (pulse of magma from the mantle). Mineral Abundances 653 55 664 468 141 0 522 0 0 200 400 600 800 1000 Garnet Chromite Ilmenite Cr Diopside Mineral Type Pyroclastic Kimberlite Resedimented Kimberlite 300-425um grains per kg All Garnet CaO (wt%) 0 2 4 6 8 10 12 0 2 4 6 8 10 12 14 16 Pyroclastic Kimberlite Resedimented Kimberlite G10D G10 / G10D G12 G9 / G11 G3 / G4 Cr O (wt%) 2 3 t t Mantle indicator mineral chemistry indicates that the pyroclastic and resedimented kimberlite sampled the same mantle material. Any differences in the indicator mineral, and therefore diamond, distribution between these two rock types is therefore likely dependent on the volcanic processes involved in deposition of the kimberlite rock types. Pyroclastic Kimberlite Relative Abundance 9% 86% 2% 3% <1% G10D Peridotite Megacryst Low-Cr Low-Cr D Garnet Paragenesis Resedimented Kimberlite Relative Abundance 7% 89% 2% 2% <1% MINERAL CHEMISTRY 300 m 0m VOLCANICLASTIC KIMBERLITE North South 200 m Lower Confidence Solid Below 200 m ? ? ? ? 0 50 100 m A A’ PYROCLASTIC KIMBERLITE RESEDIMENTED VOLCANICLASTIC KIMBERLITE 0 50 100 m B B’ RESEDIMENTED VOLCANICLASTIC KIMBERLITE PYROCLASTIC KIMBERLITE GEOLOGICAL MODEL t t The DO-18 kimberlite is oval in plan view with inward-tapering, steep-sided pipe walls. Pyroclastic kimberlite is the volumetrically dominant rock type, with lenses of resedimented volcaniclastic kimberlite. 4 hectare DO-18 pipe outline Drill Trace Drill Collar TILL GRANITE TILL GRANITE 1.7 mm C) olivine granite xenocrysts mud 1 cm A) PK RVK 1.7 mm B) olivine granite xenocryst mud 1.7 mm E) olivine granite xenocrysts juvenile magmaclast mud 1 cm C) mud granite 1.7 mm F) olivine granite xenocryst mud mud 1.7 mm D) olivine granite xenocrysts juvenile magmaclast 1 cm B) 1 cm A) mud granite OTHER RESEDIMENTED KIMBERLITE tHighly variable, very mud-rich kimberlite is present as lenses within the PK and as late-stage crater infill. Figure 3: A) Drill core photograph showing mud-rich RVK; soft sediment mixing of different types of mud is common; B) Photomicrograph comprising common brown mud clasts and fresh granitic xenocrysts. 1.7 mm B) olivine granite xenocrysts mud mud 1 cm A) mud RVK very mud- rich RVK granite RESEDIMENTED VOLCANICLASTIC KIMBERLITE t t Resedimented volcaniclastic kimberlite (RVK) containing a high proportion of unbroken olivines and kimberlitic ash. This domain is variable and comprises coarser-grained beds as well as quartz, mud, or kimberlitic ash-rich beds. Figure 2: A) Drill core photograph comparing PK (top two core slabs) with mud-rich RVK (bottom two core slabs); B) Photomicrograph showing common unbroken olivines; C) Photomicrograph showing common fresh granitic xenocrysts. PYROCLASTIC KIMBERLITE t t Dominant rock type infilling the DO-18 pipe, characterised by a high proportion of accessory granite (<25%, locally >75%). Olivine-rich, juvenile magmaclast-bearing, generally massive pyroclastic kimberlite (PK). Figure 1: A, B, C) Drill core photographs showing common accessory granite and mud clasts; D, E, F) Photomicrographs comprising olivine, magmaclasts, granitic xenocrysts, and mud clasts. t t t DO-18 was emplaced into Archean granitoids and an overlying veneer of now eroded mudstones. Palynology dating of these mudstones indicates DO-18 to be Late Cretaceous to Paleocene in age. DO-18 is atypical of other Lac de Gras kimberlites due to the high abundance of accessory country-rock granite xenoliths and locally preserved marginal country-rock breccias. INTRODUCTION The Preliminary Geology of the DO-18 Kimberlite, Lac de Gras Kimberlite Province, Canada The Preliminary Geology of the DO-18 Kimberlite, Lac de Gras Kimberlite Province, Canada M. Harder , C.M. Hetman , M.C. Baumgartner , J. Pell 1 1 1 2 1 2 Mineral Services Canada Inc., North Vancouver, BC; Peregrine Diamonds Ltd., Vancouver, BC, Canada