Distinct kimberlite pipe classes with contrasting eruption 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)
The emplacement of some diatreme-facies kimberlites
Model of a kimberlite pipe
Premier mine
Energy release in subvolcanic environments: Implications for breccia formation
Econ. Geol.
(1985)- et al.
Recent Canadian kimberlite discoveries
- 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...
- et al.
The origin of kimberlite pipes: An interpretation based on a synthesis of geological features displayed by southern African occurrences
- et al.
A textural-genetic classification of kimberlites
Trans. Geol. Soc. S. Afr.
(1985) Degassing of alkalic basalts
Am. Min.
(1997)