Elsevier

Lithos

Volume 76, Issues 1–4, September 2004, Pages 183-200
Lithos

Distinct kimberlite pipe classes with contrasting eruption processes

https://doi.org/10.1016/j.lithos.2004.03.044Get rights and content

Abstract

Field and Scott Smith [Field, M., Scott Smith, B.H., 1999. Contrasting geology and near-surface emplacement of kimberlite pipes in southern Africa and Canada. Proc. 7th Int. Kimb. Conf. (Eds. Gurney et al.) 1, 214–237.] propose that kimberlite pipes can be grouped into three types or classes. Classical or Class 1 pipes are the only class with characteristic low temperature, diatreme-facies kimberlite in addition to hypabyssal- and crater-facies kimberlite. Class 2 and 3 pipes are characterized only by hypabyssal-and crater-facies kimberlite. In an increasing number of Class 1 pipes a new kimberlite facies, transitional-facies kimberlite, is being found. In most cases this facies forms a zone several metres wide at the interface between the hypabyssal- and diatreme-facies. The transitional-facies exhibits textural and mineralogical features, which are continuously gradational between the hypabyssal and the diatreme types. The textural gradations are from a coherent magmatic texture to one where the rock becomes increasingly magmaclastic and this is accompanied by concomitant mineralogical gradations involving the decline and eventual elimination of primary calcite at the expense of microlitic diopside. Both transitional- and diatreme-facies kimberlites are considered to have formed in situ from intruding hypabyssal kimberlite magma as a consequence of exsolution of initially CO2-rich volatiles from the volatile-rich kimberlite magma. The transitional-facies is initiated by volatile exsolution at depths of about 3 km below the original surface. With subsequent cracking through to the surface and resultant rapid decompression, the further catastrophic exsolution of volatiles and their expansion leads to the formation of the diatreme facies. Thus diatreme-facies kimberlite and Class 1 pipes are emplaced by essentially magmatic processes rather than by phreatomagmatism.

Distinctly different petrographic features characterize crater-facies kimberlite in each of the three pipe classes. In crater-facies kimberlites of Class 1 pipes, small pelletal magmaclasts and abundant microlitic diopside are characteristic. These features appear to reflect the derivation of the crater-facies material from the underlying diatreme zone. Most Class 2 pipes have shallow craters and the crater-facies rocks are predominantly pyroclastic kimberlites with diagnostic amoeboid lapilli, which are sometimes welded and have vesicles as well as glass. Possible kimberlite lava also occurs at two Class 2 pipes in N Angola. The possible presence of lava as well as the features of the pyroclastic kimberlite is indicative of hot kimberlite magma being able to rise to levels close to the surface to form Class 2 pipes. Most Class 3 kimberlites have very steep craters and crater-facies rocks are predominantly resedimented volcaniclastic kimberlites, in some cases characterized by the presence of abundant angular magmaclasts, which are petrographically very similar to typical hypabyssal-facies kimberlite found in Class 1 pipes. The differences in crater-facies kimberlite of the three classes of pipe reflect different formation and depositional processes as well as differences in kimberlite composition, specifically volatile composition. Kimberlite forming pipe Classes 1 and 3 is thought to be relatively water-rich and is emplaced by processes involving magmatic exsolution of volatiles. The kimberlite magma forming Class 2 pipes is CO2-rich, can rise to shallow levels, and can initiate phreatomagmatic emplacement processes.

Introduction

Field and Scott Smith (1999) recognize at least three distinct types of kimberlite pipe based on pipe shape and internal geology and propose that the different types formed by different emplacement processes. The first type or class of pipe (see terminology below) is the so-called classical pipe first described by Hawthorne (1975) based on examples from southern Africa. These pipes are characterized by root, diatreme and crater zones, but their origin is controversial with both magmatic and phreatomagmatic emplacement processes being advocated Field and Scott Smith, 1999, Lorenz, 1993. The second type or Class 2 pipes are mainly characterized by shallow crater zones as epitomized by the Canadian Prairies, Fort a la Corne kimberlite clusters. Emplacement processes are believed to include phreatomagmatic excavation with subsequent magmatic or sedimentary infilling of the crater Field and Scott Smith, 1999, Webb et al., 2003. Class 3 pipes include many of the Ekati pipes at Lac de Gras in Canada as well as the Jwaneng pipe in Botswana. These pipes are steep-sided and are infilled with abundant resedimented volcaniclastic kimberlite and lesser pyroclastic kimberlite. Machin (2000) advocates dominant magmatic and lesser phreatomagmatic processes for the Jwaneng pipe. An important aspect to recognition of the three classes is that the diatreme zone is unique to Class 1 pipes.

In this contribution we present new petrographic and geological information, which bears on the emplacement processes for each of the three types or classes of kimberlite pipe. We also describe hitherto unrecognized in-situ transition zones, found between diatreme-and hypabyssal-facies kimberlite in Class 1 pipes, which supports a magmatic emplacement model for this class of pipe.

Section snippets

Terminology

Use of the designator ‘type’ is widely used in the kimberlite literature—for example Type 1 and 2 eclogites; Type 1 and 2 (or Group 1 and 2) kimberlites. To avoid confusion we propose to use the term ‘class’ for the three types of kimberlite pipe identified by Field and Scott Smith (1999). In describing kimberlite rock varieties we continue to follow the terminology of Clement and Skinner (1985) with respect to hypabyssal-facies kimberlite (HFK), diatreme-facies kimberlite (DFK) and

Overview of pipe classes

Class 1 pipes are deep (up to 3 km) and comprise three zones: (1) the uppermost, often flared, crater zone containing a variety of volcaniclastic materials or CFK; (2) the intermediate, steep-sided, diatreme zone containing DFK, and (3) the lowermost, irregular, root zone characterized by HFK (see Hawthorne, 1975, p. 10). The crater zones of Class 1 pipes may be up to 680 m deep (e.g. Mwadui). All Class 1 pipes have well-developed diatreme zones characterized by walls with steep slopes of

Class 1 pipes

On the basis of petrography and location within the pipe, kimberlites have previously been classified into HFK, DFK and CFK (Clement and Skinner, 1985) but there is little information on the details of the petrographic changes accompanying the transition from HFK to DFK. In the section below we review the petrographic features of HFK to DFK, with particular attention to the character of the transition zones between them.

Class 2 pipes

Class 2 Pipes are predominantly characterized by CFK. In this study we have investigated samples from Jubilee (Siberia) and several Angolan kimberlites. Included in these samples are possible lavas from two pipes in northeast Angola as well as pyroclastic and resedimented volcaniclastic rocks. PK dominates the CFK in Class 2 pipes. Regardless of location, the PKs contain magmaclasts that are of lapilli size, may be amoeboid in shape, contain olivine that is often relatively unaltered, are

Class 3 pipes

Crater-facies kimberlite in Class 3 pipes is dominated by RVK varieties but PKs may also be abundant. In this study we have investigated samples from Jwaneng, Botswana, several Angolan kimberlites including Camafuca Camazambo, and the Sytkanskaya and Aichal pipes in Siberia. The proximal RVKs (i.e. those that are hydraulically slumped, such as those at Jwaneng, Machin, 2000) consist of olivine macrocrysts, magmaclasts and country rock xenoliths and xenocrysts, all set in a matrix of mud. Distal

Discussion

The main aspects arising from the descriptions presented above can be summarized as follows:

  • (a)

    We confirm the existence of the three classes of kimberlite pipe as suggested by Field and Scott Smith (1999).

  • (b)

    We describe further examples of these classes and expand on the criteria, particularly petrographic criteria, for the recognition of each pipe class.

  • (c)

    We describe the occurrence of a newly recognized transitional-facies kimberlite (TFK) between the diatreme-facies and hypabyssal-facies in the Class

Class 1 pipes

It is envisaged that within a Class 1-pipe complex, an individual pipe-forming event is an essentially magmatic process involving volatile exsolution through first and second boiling. This process leads initially to the formation of in situ TFK and later to redistributed DFK, which is generated in a large explosion. This explosion is initiated from about 700 m below the original surface and is instantaneously propagated downwards through the magma column to depths of just over 2 km. The

Conclusion

Kimberlite pipes can be classified into three pipe classes. Eruption processes that create these three pipe classes must be different. Only Class 1 pipes contain DFK as well as the newly recognized TFK, which has petrographic and mineralogical features that are intermediate and gradational between HFK and DFK. In all cases where HFK is found in association with DFK, transition-facies kimberlite occurs at the interface between the two. TFK represents a kimberlite magma in which volatile

Acknowledgements

This paper benefited greatly from the input of the sub-editor Barbara Scott Smith and two referees, particularly Casey Hetman.

References (30)

  • C.R. Clement

    The emplacement of some diatreme-facies kimberlites

  • J.B. Hawthorne

    Model of a kimberlite pipe

  • P.J. Bartlett

    Premier mine

  • C.W. Burnham

    Energy release in subvolcanic environments: Implications for breccia formation

    Econ. Geol.

    (1985)
  • J.A. Carlson et al.

    Recent Canadian kimberlite discoveries

  • R.A.F. Cas et al.

    Volcanic Successions, Modern and Ancient

    (1987)
  • Clement, C.R., 1982. A comparative geological study of some major kimberlite pipes in the Northern Cape and Orange Free...
  • C.R. Clement et al.

    The origin of kimberlite pipes: An interpretation based on a synthesis of geological features displayed by southern African occurrences

  • C.R Clement et al.

    A textural-genetic classification of kimberlites

    Trans. Geol. Soc. S. Afr.

    (1985)
  • J.E. Dixon

    Degassing of alkalic basalts

    Am. Min.

    (1997)
  • M. Field et al.

    Textural and genetic classification schemes of kimberlites: a new perspective

  • M. Field et al.

    Contrasting geology and near-surface emplacement of kimberlite pipes in southern Africa and Canada

  • M. Field et al.

    The geology of the Orapa A/K1 kimberlite Botswana: Further insight into the emplacement of kimberlite pipes

  • C.M. Hetman et al.

    Geology of the Gahcho Kue kimberlite pipes, NWT, Canada: root to diatreme transition zones

  • Hetman, C.M., Scott Smith, B.H., Paul, J.L., Winter, F.W., this volume. Geology of the Gahcho Kue kimberlite pipes,...
  • Cited by (0)

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