Based on studies of numerous kimberlite deposits, geologists have divided kimberlites into 3 distinct units based on their morphology and petrology. These units are:
Clickable Image Map
From Mitchell 1986
From Mitchell 1986
The surface morphology of an unweathered kimberlite is characterised by a crater, up to 2 kilometers in diameter, whose floor may be several hundred meters below ground level. The crater is generally deepest in the middle. Around the crater is a tuff ring which is relatively small, generally less than 30 meters, when compared to the diameter of the crater. Two main categories of rocks are found in crater facies kimberlite: pyroclastic, those deposited by eruptive forces; and epiclastic, which are rocks reworked by water. See the page on Clement's Classification for Kimberlite.
A. Pyroclastic Rocks: These rocks are found preserved in tuff rings around the crater and within the crater.
Tuff rings have small height:crater diameter ratios and are preserved in very few kimberlites. Igwissi Hills in Tanzania and Kasami in Mali are the only pipes with well preserved tuff rings. Heights range from 1-4 metres on one pipe, and 15-50 metres in one kimberlite field. Deposits are commonly bedded, vesicular and carbonatised.
Tuff deposits preserved within the crater are also rare, however, the Igwissi Hills pipes in Tanzania have been examined and revealed three distinct units. From top to bottom, they are:
1. well-stratified tuffs - layers defined by lapilli and ash size particles. Graded bedding and depositional features appear absent. Believed to be products of air fall and possibly settling through water.
2. poorly stratified coarse pyroclastics - recognized by deposits of complex folding and slumping. Shards of glass, scoriaceous materials, cauliflower bombs and pelletal lapilli were not observed.
3. basal breccia
From Mitchell 1986B. Epiclastic Rocks - These sediments represent fluvial reworking of pyroclastic material from the tuff ring in the crater lake formed on top of the diatreme. They are complex and resemble a series of overlapping alluvial fans mixed in with lacustrine deposits. They coarsen with distance from the wall rock and become better sorted towards the center. Fossils may be found in these sediments. Some epiclastic deposits have been replaced with chalcedony - evidence for late-stage volcanic hot-spring activity.
C. Lavas - no concrete evidence for this.
Considering how few kimberlites exist with well preserved crater facies it is difficult to develop a model with any certainty that all kimberlites will conform to the observed features above.
Crater facies kimberlite is difficult to distinguish from diatreme facies kimberlite. The most distinguishing feature is visible bedding.
Kimberlite diatremes are 1-2 kilometer deep, generally carrot-shaped bodies which are circular to elliptical at surface and taper with depth. The dip contact with the host rocks is usually 80-85 degrees. The zone is characterized by fragmented volcaniclastic kimberlitic material and xenoliths plucked from various levels in the Earths crust during the kimberlites journey to surface.
Some Textural features of Diatreme Facies Kimberlite:
i. Country rock fragments - angular
ii. Cognate fragments (juvenile) - rounded to angular
iii. Country rock xenoliths found 1000 meters below depositional unit - it is clear that sinking occurs in the pipe.
iv. Local stratigraphy is crudely preserved by floating reefs in diatreme.
v. Pelletal lapilli - appear to have formed by the rapid crystallization of a volatile poor magma containing phenocrysts. They are characterised by a crystal nucleus surrounded by microphenocrysts which align themselves tangentially to the central crystal.
vi. Nucleated autoliths - similar to pelletal lapilli but lacking microphenocryst orientation. Kernel grain usually country rock. Magmatic nucleation about a nucleating center.
vii. Matrix composed almost entirely of fine-grained diopside, serpentine and phlogopite.
viii. Hardly any calcite found in matrix (whereas lots of calcite is found in hypabyssal matrix). This suggests that the magma has already degassed.
ix. Crystallisation in diatreme occurs at low temperatures based on the lack of thermal effects seen in intruded limestones.
x. Contact metasomatic/metamorphic effects with the country rock are few.
xi. Upwarping and fractures associated with the intrusive body are absent.Photograph of diatreme facies kimberlite core. Click on the fragment for a close up. These fragments with halos of crystallised kimberlite magma are characteristic of diatreme facies rocks.
3) Hypabyssal Facies Kimberlite
These rocks are formed by the crystallization of hot, volatile-rich kimberlite magma. Generally, they lack fragmentation features and appear igneous.
Some Textural features:
i. Calcite-serpentine segregations in matrix.
ii. Globular segregations of kimberlite in a carbonate-rich matrix.
iii. Rock fragments have been metamorphosed or exhibit concentric zoning.
iv. Inequigranular texture creates a pseudoporphyritic texture.
Photograph of hypabyssal facies kimberlite. Note the segregationary texture seen. The white matrix material is calcite while the green segregations are serpentine. Rounded fragments are unknown. Click on the image for a close up.
Introduction
1.0 Definition of Kimberlite
3.0 Kimberlite Classification Model
4.0 Kimberlite emplacement models