The Kimberlites and Related Rocks of the Kuruman Kimberlite Province, Kaapvaal Craton, South
Africa. (Donnelly et al.)
Electronic Supplementary Material 1 - Classification and Petrography of the
Kuruman Intrusions
Classification:
Group I and Group II ‘kimberlites’ have been given separate definitions (e.g.
Mitchell 1995; Woolley et al. 1996; Le Maitre 2002) reflecting the general consensus
that they are derived from different magmas and sources. However, an agreement has
yet to be reached on terminology. Mitchell and Bergman (1991) and Mitchell (1995)
have suggested that as Group II kimberlites are not actually ‘kimberlites’ they should
have separate terminology and have proposed the term “orangeite” in recognition of
the distinct character and occurrence of these intrusions within the Orange Free State
in South Africa. The current International Union of Geological Sciences (IUGS)
classification of igneous rocks adopted the Group II kimberlite terminology but
follows the orangeite definition given Mitchell (1995). Tappe et al. (2005) recently
proposed an update to the IUGS classification system due to the omission of
ultramafic lamprophyres (UML) from this scheme. This modification adopts the terms
kimberlite and orangeite in place of the current IUGS terminology of Group I and
Group II kimberlite, respectively. The proposed modification utilizes groundmass
mineral modal abundances and compositions to discriminate between the
macroscopically similar kimberlites, orangeites, ultramafic lamprophyres (UML) and
olivine lamproites. Following this scheme, nine of the intrusions from this study have
been classified as kimberlite, one as orangeite and two as ultramafic lamprophyres
(aillikites and mela-aillikites).
The Kuruman intrusions can be further subdivided according to texture into
macrocrystic and aphanitic varieties (Clement and Skinner 1979, 1985; Clement et al.
1984; Mitchell 1986, 1995). The non-genetic textural term ‘macrocrystic’ indicates
the presence of large crystals (>0.5 mm), predominantly anhedral olivine crystals and
broken and/or rounded phlogopite laths, that may have been derived from the
disaggregation of mantle xenoliths (e.g. Mitchell 1986, 1995; le Roex et al. 2003). In
this work samples containing >10 vol% macrocrysts are classified as macrocrystic.
More euhedral olivine and phlogopite crystals are termed phenocrysts (>0.5 mm) and
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microphenocrysts (<0.5 mm). The groundmass is characterized by small (typically
<0.1 mm in size) randomly oriented crystals set in a uniform-textured matrix, with
subordinate regions of segregationary-textured (non-uniform distribution of
groundmass and mesostasis) groundmass. Mineralogical abundances and rock
classifications are summarized in Table 1.
Kimberlites
The majority (nine) of the intrusions from this study are kimberlites (Table 1).
These intrusions are classified as kimberlites based on the absence of groundmass
clinopyroxene (Mitchell 1995; Tappe et al. 2005), the presence of groundmass
monticellite (Mitchell 1995) and Al- and Ba-enriched phlogopite (Mitchell 1995;
Tappe et al. 2005). The Kuruman kimberlites are predominantly macrocrystic
hypabyssal-facies kimberlites, although regions of Bathlaros are aphanitic. The
kimberlites are characterized by macrocrysts and phenocrysts/microphenocrysts of
olivine and phlogopite set in a matrix dominated by variable proportions of carbonate,
phlogopite and serpentine. Accessory groundmass phases include spinel, perovskite,
apatite, ilmenite, rutile and altered monticellite (Helpmekaar, Toxteth 02 and White
Ladies). The Kuruman kimberlites have unusually high modal abundances of primary
groundmass and phenocrystal phlogopite (up to 30 vol% in White Ladies), relative to
other kimberlites (e.g. Group I kimberlites; Skinner 1989).
Orangeites
Two of the Kuruman kimberlites, Cox’s Mine and X007 are mica-rich
orangeites (De Beers database, and the present study). No samples from Cox’s Mine
were available for this study. X007 was classified as orangeite based on the
abundance of micas (up to 60 vol%) from the tetraferriphlogopite trend (Mitchell,
1995; Tappe et al. 2005) and the presence of Cr-rich spinels (atomic Cr/(Cr+Al) >85).
X007 is a macrocrystic to aphanitic, hypabyssal-facies orangeite that is comprised of
macrocrysts of altered olivine and rare phlogopite and subhedral phenocrysts and
microphenocrysts of olivine and phlogopite set in a matrix dominated by small
(generally < 0.1 mm) stubby phlogopite laths with lesser carbonate and serpentine.
Additional matrix phases include fine-grained (0.01 to 0.03 mm) magnesiochromite,
perovskite and apatite.
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Ultramafic Lamprophyres
Aalwynkop and Clarksdale are classified as UML (aillikite end-member)
following Tappe et al. (2005) based on the absence of melillite, nepheline and alkalifeldspar, and the presence of mica following the tetraferriphlogopite evolution trend
(Mitchell 1995; Tappe et al. 2005), spinels from the titanomagnetite trend with Cr#
<85, and Al- and Ti-enriched clinopyroxene (Tappe et al. 2005). Aalwynkop and
Clarksdale are characterized by macrocrysts and phenocrysts/microphenocrysts of
olivine and phlogopite set in a matrix of phlogopite, carbonate, serpentine, spinel and
apatite. Aalwynkop dominantly has a macrocrystic texture, although it contains some
carbonate-rich aphanitic zones that are typically 1 to 5 cm wide. Aalwynkop also
contains macrocrystal and groundmass ilmenite, trace clinopyroxene phenocrysts and
zircon. Clarksdale is termed a mela-aillikite as it contains >70 vol% mafic silicate
minerals (Tappe et al. 2004, 2005), while the more carbonate-rich Aalwynkop dike is
an aillikite.
Groundmass Phases (corresponding mineral chemistry data presented in Electronic
Supplementary Material 2)
Olivine
Olivine is present in varying abundance in all of the Kuruman intrusions,
comprising up to 50 vol% (e.g. Elston). There are three populations of olivine;
anhedral to rounded macrocrysts (>0.5 mm) and more euhedral-to-subhedral
phenocrysts (>0.5 mm) and microphenocrysts (<0.5 mm). Most of the macrocrystal
olivine crystals are 2 to 3 mm in size, but reach a maximum length of 10 mm. The
proportion of macrocrystic olivine varies greatly between the different rock types
(Table 1) with kimberlites typically having higher olivine macrocryst contents (5 to
25 vol%) relative to orangeite (5 to 10 vol%) and the aillikites (2 to 5 vol%).
Variations in the olivine macrocryst content can also be considerable within intrusions
(e.g. 15 vol% variation in Elston). Olivine phenocrysts and microphenocrysts
(reported together in Table 1) are present in similar abundances in the kimberlites and
aillikites (15 to 40 vol%), with lower olivine phenocrystal content in the X007
orangeite (15 to 20 vol%).
The degree of alteration of olivine to serpentine and calcite varies between
intrusions, with the colour of the olivine crystals ranging from dark green, to pale
green, to greenish white. In some instances olivine is partially replaced by fine,
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needle-like opaque minerals along internal fractures. Complete pseudomorphing of
olivine crystals by serpentine is common. Partial pseudomorphs, where the outer
portion of the grain is replaced by serpentine and carbonate while core compositions
are preserved, are observed in Elston, Helpmekaar, and Zero.
Phlogopite
Phlogopite is present in all of the Kuruman intrusions but abundances are highly
variable with ~ 5 vol% in the Belle Isle kimberlite up to 35 vol% in the White Ladies
kimberlite, 60 vol% in the X007 orangeite and 30 to 40 vol% in the aillikites
(Table1). Phlogopite macrocrysts (>0.5 mm, maximum size of 4.2 mm) are relatively
rare in the kimberlites (<5 vol%) and orangeite (1 to 2 vol%), compared to the
aillikites (5 to 10 vol%). The phlogopite macrocryst population is characterized by
dark orange-brown irregular or fragmented laths, the majority of which are likely
derived from larger crystals. The macrocrysts are commonly weakly pleochroic and
often possess thin, irregular rims of later-stage phlogopite. The phlogopite
macrocrysts occasionally show evidence of strain deformation (e.g. kink banding),
undulose extinction, and are frequently replaced along cleavage planes by calcite and
chlorite.
Phlogopite phenocrysts and microphenocrysts are typically characterized by
weakly-pleochroic, randomly oriented, euhedral to subhedral poikilitic plates and
laths that are pale-yellow to light-brown in colour. The phlogopite phenocrysts and
microphenocrysts are late-stage phases that commonly contain inclusions of
groundmass spinel, perovskite and occasionally olivine microphenocrysts.
Groundmass phlogopites (typically <0.1 mm) in kimberlites generally occur as
colourless to light-brown to orange-brown subhedral to euhedral poikilitic laths that
range from long, slender laths to stubby laths. Groundmass phlogopites in the X007
orangeite are typically darker orange-brown euhedral to subhedral laths. Groundmass
phlogopites from the Aalwynkop aillikite are similar to those of the kimberlites, while
Clarksdale phlogopites are characterized by extremely slender needle-like laths
typically 0.1 to 0.2 mm in length. The type and intensity of phlogopite alteration is
variable and pipe-dependent, typically characterized by partial chloritization of
phlogopite (e.g. White Ladies, X007 and Clarksdale) and/or replacement of
phlogopite by carbonate along grain margins and cleavage planes. Many of the
phlogopites are colour-zoned, usually with darker yellow-brown cores and thin near4
colourless rims. The cores typically exhibit normal pleochroism from colourless to
pinkish-orange, while the rims often show reversed pleochroism from colourless to
red-brown.
Apatite
Apatite is a late-crystallizing groundmass phase present in all of the Kuruman
intrusions, except Helpmekaar. Apatite ranges in modal abundance from <1 vol% up
to ca 5 vol% in Bathlaros (Table 1). Apatite is occasionally resorbed and replaced by
calcite, but many unaltered grains are present. Apatite typically occurs as euhedral,
prismatic crystals or more rarely as thin (<0.01 mm) needle-like laths. Acicular,
radiating aggregates of apatite are observed in the Elston kimberlite.
Spinel
Spinel is ubiquitous in the Kuruman kimberlites, orangeite and aillikites. In the
kimberlites spinel abundances range from approximately 1 to 2 vol% in Exit up to 15
vol% in Toxteth 02 and White Ladies. The X007 orangeite and the Clarksdale and
Aalwynkop aillikites have much lower abundances of spinel at 1 to 2 vol%. Spinel is
present in all size ranges from minute groundmass crystals to rare macrocrysts
reaching a maximum size of 2.1 mm. Spinel macrocrysts occur only within the
kimberlites and are present in trace amounts (<1 vol%) in all kimberlites, except
Bathlaros in which they were absent. Spinel macrocrysts are typically rounded partial
fragments of larger crystals and are often mantled by later groundmass spinel or, more
rarely, they are surrounded by very fine-grained euhedral groundmass spinel crystals.
Most groundmass spinels are homogenous euhedral to subhedral crystals that are
typically 0.03 to 0.05 mm in size in the kimberlites and aillikites but are smaller (0.01
to 0.03 mm) in the orangeite. Spinels are occasionally found as “necklaces” around
olivine crystals and as inclusions in late-stage phenocrystal phlogopite. Atoll-textured
spinel, where chromite cores are surrounded by atolls of resorbed spinel, (e.g.
Mitchell and Clarke 1976) is observed rarely and only in the kimberlites.
Perovskite
Groundmass perovskite is found in five kimberlites (e.g. Bathlaros, Elston,
Helpmekaar, White Ladies and Zero) and in the X007 orangeite. The size and the
morphology of the perovskite grains vary significantly between pipes. In X007
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perovskite is rare and occurs as small (typically 0.01 to 0.03 mm) rounded crystals,
while in Bathlaros it comprises up to 10 vol% (typically 0.2 to 0.4 mm). Pervoskite
occurs as well-preserved euhedral grains that are 0.04 to 0.07 mm across in
Helpmekaar and Elston and 0.02 to 0.04 mm across in the Zero pipe. The White
Ladies kimberlite contains rounded grains, typically 0.03 to 0.05 mm across.
Perovskite occurs as discrete crystals set in the uniform-textured matrix and smaller
grains commonly form “necklaces” around olivine macrocrysts.
Ilmenite
Ilmenite is present in the Aalwynkop aillikite, dominating the opaque-mineral
population (5 to 10 vol%), and in four kimberlites (Bathlaros, White Ladies, Toxteth
01 and Toxteth 02) where it comprises < 1 vol%. In kimberlites, ilmenite typically
occurs as small (<0.02 mm) anhedral-shaped crystals that possibly represent
fragments of larger magnesian ilmenites from the megacryst suite. In the Aalwynkop
aillikite ilmenite macrocrysts are variably abundant (1 to 5 vol%) and typically occur
as anhedral crystals that have a continuous range of grain-sizes from 0.5 mm up to 3
mm. Groundmass ilmenites are common (5 to 8 vol %) in Aalwynkop and generally
do not have well-formed crystal shapes and are typically <0.1 mm in size.
Additional Groundmass Phases (without corresponding mineral chemistry data)
Monticellite
The presence of monticellite (altered completely to carbonate) in the
groundmass of three Kuruman kimberlites (e.g. Helpmekaar, Toxteth 02 and White
Ladies) is suggested by the characteristic relict ‘sugary’ texture produced by small
~0.02 mm high-relief colourless crystals.
Carbonate
Carbonate is present as a late-stage groundmass phase in all of the Kuruman
intrusions. It modally dominates the matrix of many of the kimberlites, varying in
abundance from 5 to 35 vol% (Table 1). Carbonate is less abundant in the X007
orangeite (5 to 10 vol%). Considerable variation exists between the Aalwynkop
aillikite (20 to 35 vol%) and the Clarksdale mela-aillikite (10 to 15 vol%). Carbonate
is present in the uniform-textured matrix as inter-locking euhedral to anhedral crystals
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that are typically <0.1 mm in size. Calcite also occurs within the segregationarytextured regions of the matrix, commonly occurring as intergrowths with serpentine.
Serpentine
Serpentine is present in all of the Kuruman intrusions and varies considerably
in abundance from 1 to 5 vol% (e.g. White Ladies) to 15 to 30 vol% (e.g. Toxteth 01
and Toxteth 02) in the kimberlites, with lower abundances in the X007 orangeite (5 to
10 vol%) and the Clarksdale and Aalwynkop aillikites (5 to 15 vol%). In all of the
intrusion types, primary serpentine occurs as late-stage, green-brown, irregularlyshaped segregations which are commonly intergrown with carbonate.
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