Criteria for interpreting kimberlite as coherent: insights from the Muskox and Jericho kimberlites (Nunavut, Canada)

Abstract

The Jurassic Muskox and Jericho kimberlites (Northern Slave Province, Nunavut, Canada) contain a variety of facies exhibiting different geometries, contact relationships, internal organisation, country rock abundance and olivine shapes, although many have similar matrix/ groundmass mineralogies and textures. Five facies are examined that either have characteristics consistent with coherent rocks in general (i.e. intrusive and extrusive nonfragmental rocks) or are mineralogically and texturally similar to kimberlite described as coherent (or apparent coherent). Three facies are interpreted as coherent on the basis of: (1) geological setting, (2) apparent-porphyritic texture, (3) sharp contacts with fragmental kimberlite, (4) relative abundance of elongate and unbroken olivine crystals and (5) paucity of country rock xenoliths, while the remaining two facies are interpreted as fragmental on the basis of: (1) the gradational contacts with demonstrably fragmental kimberlite, (2) relative abundance and range of sizes of country rock lithic clasts and (3) numerous broken olivine crystals. Comparisons are made with coherent and apparent-coherent kimberlite from the literature. Our three coherent facies are similar to literature reported coherent kimberlite dykes hosted in country rock (CK d ) in terms of internal organisation, low abundance of country rock xenoliths, and apparent-porphyritic texture. Conversely, our two fragmental facies share attributes with previously described pipe-filling coherent and apparent-coherent kimberlite (CK pf ) in terms of geometry, internal organisation and abundance of country rock xenoliths. We conclude that CK d and most CK pf , although similar in matrix/groundmass mineralogy and texture, can be distinguished on the basis of internal organisation, country rock lithic clast abundance, texture (e.g. apparent-porphyritic texture) and possibly olivine crystal shapes and suggest that fragmental kimberlite is more common than reported.

Intensive (P-T-fO2) crystallization parameters of Alto Paranaíba kimberlites and diamond instability: Três Ranchos IV and Limeira I intrusions

“…These rocks result of direct crystallization from kimberlitic magmas prior to degassing and fluidization (Clement and Reid, 1989), being well qualified to determine primary kimberlite compositions. Compelling evidence indicates that many examples intrusive coherent kimberlites are pyroclastic in origin and may have formed via welding processes (Brown et al, 2008b(Brown et al, , 2008aCrawford et al, 2009;Buse et al, 2011;Hayman and Cas, 2011;van Straaten et al, 2011).…”