Geology of the Snap Lake kimberlite intrusion, Northwest Territories, Canada: field observations and their interpretation

Abstract

8The Cambrian (523 Ma) Snap Lake hypabyssal kimberlite intrusion, Northwest Territories, 9 Canada, is a complex segmented diamond-bearing ore-body. Detailed geological investigations 10 suggest that the kimberlite is a multi-phase intrusion with at least four different magmatic 11 lithofacies. In particular, olivine-rich (ORK) and olivine-poor (OPK) varieties of hypabyssal 12 kimberlite have been identified. Key observations are that the olivine-rich lithofacies has a strong 13 tendency to be located where the intrusion is thickest and that there is a good correlation between 14 intrusion thickness, olivine crystal size and crystal content. Heterogeneities in the lithofacies are 15 attributed to variations in intrusion thickness and structural complexities. The geometry and 16 distribution of lithofacies points to magmatic co-intrusion, and flow segregation driven by 17 fundamental rheological differences between the two phases. We envisage that the low viscosity 18 OPK magma acted as a lubricant for the highly viscous ORK magma. The presence of such low 19 viscosity, crystal-poor magmas may explain how crystal-laden kimberlite magmas (> 60 vol.%) 20 2 are able to reach the surface during kimberlite eruptions. We also document the absence of 21 crystal settling and the development of an unusual sub-vertical fabric of elongate olivine crystals, 22 which are explained by rapid degassing-induced quench crystallisation of the magmas during and 23 after intrusion. 24 KEY WORDS: kimberlite; magma; intrusion; Snap Lake; emplacement processes 25 26 This paper presents a field and petrographic description of an exceptionally well-exposed 2 to 3 27 m-thick hypabyssal kimberlite intrusion at Snap Lake Diamond Mine, NW Territories, Canada. 28 We present data on intrusion thickness, lithofacies relationships, crystal size distributions, crystal 29 fabric, crystal content and crystal concentration profiles. The Snap Lake intrusion provides 30 important new insights into the architecture and emplacement processes of high-level magmatic 31 plumbing systems. 32Many processes can occur during magma transport or intrusion, including melting and 33 solidification (Delaney & Pollard, 1982; Huppert & Sparks, 1988; Marsh, 1996), crystallisation 34 and gravity-driven crystal fractionation (Shaw, 1965; Martin & Nokes, 1988; Blundy & 35 Cashman, 2001, in-situ differentiation (Sparks et al., 1984; Mitchell, 2008), assimilation 36 of wall-rock and contamination (Philpotts & Asher, 1993; Sparks et al., 2009), injection of 37 different magma batches (Eichelberger, 1980; Eichelberger et al., 2000), magma mixing or 38 unmixing (Sparks et al., 1977; McBirney, 1980;Spera, 2000; Couch et al., 2001), and flow 39 differentiation (Komar, 1972a-b). Another key magmatic process is viscous segregation, which 40 can occur during the co-intrusion of discrete magma batches (Carrigan, 2000). Where present in 41 kimberlite intrusions, rheological segregation may have economic implications in that t...

Physical characteristics of kimberlite and basaltic intraplate volcanism and implications of a biased kimberlite record

“…There are few detailed studies of kimberlite dikes and sills (Dawson and Hawthorne, 1973;Gurney and Kirkley, 1996;Basson and Viola, 2003;Brown et al, 2007;Kavanagh and Sparks, 2011;Gernon et al, 2012;White et al, 2012). Kimberlite dikes occur in regional swarms (Nixon, 1973;Kresten and Dempster, 1973).…”

The mechanics of sill inception, propagation and growth: Experimental evidence for rapid reduction in magmatic overpressure

“…Volcanic plumbing systems comprise a series of dykes, cone sheets and sills that connect source magma reservoirs with volcanic conduits to feed eruptions at the surface (e.g. Geshi et al, 2010). Placing constraints on the factors that influence dyke and sill formation and growth may improve our understanding of eruption there are rigidity (Kavanagh et al, 2006;Rivalta et al, 2005) or strength (Gudmundsson, 2011) contrasts between layers.…”

Dynamics of kimberlite magma ascent, intrusion and eruption

“…Magma mixing between these units is also common. Examples of these phenomena include Snap Lake (Gernon et al, 2011), AK6 in Orapa district, du Toitspan (Olgivie-Harris et al, 2009) the Wesselton sills (White et al, 2012) and Orapa (Sarkar et al, 2012). Mutiple pusles with clear breaks in activity are inconsistent with the idea that kimberlite eruptions are single event dramatic explosions.…”